8 - Meteorology and Weather Recognition Flashcards
What is an isobar?
An isobar is a line on a meteorologic chart that joins places of equal sea
level pressure.
What is an isotherm?
An isotherm is a line joining places of the same mean temperature.
Heat and Temperature
What is heat?
Heat is a form of energy that is measured in calories. One calorie represents the energy required to raise the temperature of 1 g of water 1°C, so the energy required to raise the temperature of 2g of water by 1°C
would be 2 calories.
Heat and Temperature
What provides the earth’s heat energy?
The sun in the form of solar radiation provides the earth’s heat and light.
The solar radiation’s wavelengths are such that only about half the radiation that hits the upper atmosphere finally reaches the earth’s surface (the rest is either reflected back into space or absorbed into the
upper layers of the atmosphere).
This short-wave radiation is absorbed
by the earth’s surface, causing its temperature to increase.
This is called insulation. Energy is then re radiated out from the ground as long-wave radiation , and it is this radiation that heats up the lower atmosphere, near the ground, with the result that any parcel of air that is warmer than the surrounding air will rise.
(See Q: What are the different ways of transferring heat energy
into the atmosphere? page 222.)
Heat and Temperature
How does cloud cover affect the heating of the earth’s surface?
By day:
Cloud cover stops the incoming solar radiation from penetrating
to the earth’s surface by reflecting it back into space, causing
the heating of the earth’s surface and thereby the lower layers of
the atmosphere next to the ground to be reduced.
This results in
lower temperatures and less atmospheric convective movement.
By night:
Cloud cover causes the opposite effect by trapping the
heat energy in the lower atmosphere on and near the ground and
reflecting it back to the surface, in effect, recirculating the heat.
This results in the lower atmosphere, beneath the clouds, maintaining
a higher temperature because it experiences less cooling than on a clear night
Heat and Temperature
What is specific heat capacity?
Specific heat capacity is the ability of a material to hold thermal / heat energy.
(See Q: What is heat? page 221.)
Heat and Temperature
What is latent heat?
Latent heat is the heat energy, measured in calories, absorbed or released when water changes from one state to another.
Note: There are three states of water:
- Water vapor (gas)
- Water liquid (cloud, mist, fog, rain, etc.)
- Water solids (ice)
When water changes to a higher energy state, i.e., from ice to liquid to vapor, it absorbs/uses latent heat energy (from the surrounding atmosphere/properties)
Heat and Temperature
What is temperature?
Temperature is a measure of molecule agitation in a substance, which is represented as the heat of a body.
Therefore, temperature can be
thought of as a measure of the heat of a body.
Heat and Temperature
What factors determine the temperature at the earth’s surface (i.e., why is it
hot in the tropics and cold at the poles)?
The temperature at the earth’s surface and the lower atmosphere
where most of the weather is found depends on two factors:
- How much heat energy in the form of short-wave energy (solar radiation)
reaches the earth’s surface.
This depends on the following factors:
(a) Latitude - The more directly the sun’s rays hit the earth’s surface,
the greater is the quantity of heat energy transferred and therefore the greater is its temperature.
The sun is directly
overhead in the tropics, and this is where the intensity of insulation is greatest.
(b). Season - The earth's axis of rotation is tilted with reference to its orbit. This tilting creates the same effect as a change of latitude and is the primary cause of the different seasons and associated temperature changes throughout the year. (c) Time - The time of day detennines the amount of the sun's heating on the earth's surface.
The highest temperature of the day is not experienced until later in the afternoon, at approximately 1500 hours during the summer months.
- The energy absorption (and retention) capacity of the surface.
(a) Absorption. The type of surface detennines how much heat energy is absorbed, i.e., its reflective quality.(b) Specific heat capacity of the surface. (See Q: What is specific heat capacity? page 222.)
Heat and Temperature
What is the difference between Celsius and Fahrenheit?
Celsius and Fahrenheit are both scales used for measuring temperature.
The Celsius scale (*C) divides the temperature between the boiling point and the freezing point of water into 100 degrees.
That is, the boiling point of water is lOO°C, and the freezing point of water is O°C.
The Fahrenheit scale (OF), is based on water boiling
at 212°F and freezing at 32°F.
Heat and Temperature
What is the formula to convert Celsius and Fahrenheit?
- F= 1.8(*C+32)
* C=0.55(*F-32)
Heat and Temperature
Describe OAT.
OAT is the ambient outside air temperature.
Heat and Temperature
Describe SAT.
SAT is the ambient static air temperature.
This is commonly used as a different name for outside air temperature (OAT).
Heat and Temperature
Describe TAT.
TAT is the total air temperature indicated on the air temperature instrument it is a product of the static air temperature (SAT) and the adiabatic compression (ram) rise in temperature experienced on the temperature probe.
Note: Therefore, TAT is a higher temperature than outside air temperature
(OAT) whenever there is an airflow into the temperature
probe, which is sometimes referred to as a heating error when you
need to calculate the actual OAT.
Heat and Temperature
How does a change in air temperature affect an aircraft’s flight level?
An air temperature that differs from the international standard atmosphere (ISA) temperature will result in a different actual flight level (i.e., height above the ground) than the pressure level read by the altimeter.
A higher than ISA air temperature makes the air less dense and lighter in weight, causing the density altitude to differ from the pressure
altitude.
This results in the actual flight level being higher than
the pressure level read by the altimeter.
However, a lower than ISA air temperature makes the air denser and heavier in weight, causing the density altitude to differ from the pressure
altitude.
This results in the actual flight level being lower than
the pressure level read by the altimeter.
(See Q: What density errors
are commonly experienced? page 119.)
Therefore, when flying from a high to low (temperature), beware
below, because your actual flight level (and therefore ground clearance)
is lower than indicated by your altimeter. In other words, your
altimeter overreads.
This high-to-Iow mnemonic applies equally to pressure values as it does to temperature.
(See Q: How does a change in pressure affect the
aircraft’s flight level? page 249.)
Heat and Temperature
What is a temperature inversion/layer?
A temperature inversion occurs when the air closest to the ground,
or even the ground itself, is cooler than the air above it.
In other
words, the air temperature increases with height (rather than the
usual decrease).
A temperature inversion layer is the heigh/altitude where the air
temperature changes from the temperature increasing with height
(i.e., a temperature inversion) to a normal decrease in temperature
with height state.
When a temperature inversion occurs, it acts like a blanket, stopping
vertical movement/currents; i.e., air that starts to rise meets an
inversion layer and so stops rising.
Heat and Temperature
What is an isothermal layer?
An isothermal layer is one where the air remains at the same temperature through a vertical section of the atmosphere (i.e., a constant
10*C between 5000 and 10,000 ft).
Note: Remember that temperature usually decreases with altitude.
Moisture and Clouds
What is the adiabatic process?
The adiabatic process is one in which heat is neither added nor removed from a system, but any expansion or compression of its gases
changes the temperature of the system with no overall loss or gain of
energy.
That is, compressing air increases its temperature, and decompressing it (expansion) reduces its temperature.
A common adiabatic process for pilots is the expansion of a cooling parcel of air when it rises in the atmosphere.
(See Qs: What is
ELR/DALR/SALR? page 226 and 227; Explain relative humidity, page
227; What is dew point? page 227.)
Moisture and Clouds
What is ELR?
Environmental lapse rate (ELR) is the rate of temperature change with height of the general surrounding atmosphere.
The international standard atmosphere (lSA) assumes an ELR of 2°C per 1000 ft of heigh/altitude gained. The actual ELR in a real atmosphere, however, may differ greatly from this; in fact, it can be zero (isothermal layer) or even a negative value (inversion).
Moisture and Clouds
What is DALR?
Dry adiabatic lapse rate (DALR) is the adiabatic temperature change for unsaturated air as it rises.
Unsaturated air is known as dry air,
and its change in temperature is a rather regular drop of 3°C per 1000 ft of height /altitude gained.
Moisture and Clouds
What is SALR?
Saturated adiabatic lapse rate (SALR) is the adiabatic change in temperature
for saturated air as it rises.
SALR commences at a height where a parcel of air’s temperature is
reduced to its dew point temperature, 100 percent relative humidity,
and its water starts to condense out to form a cloud.
(See Qs: Explain
humidity/relative humidity, page 227; What is dewpoint? page 227.)
Above this height, the now saturated air will continue to cool as it
rises, but because it releases latent heat as the water vapor condenses
into a liquid form, i.e., clouds, its cooling rate is reduced to a rather
regular drop of 1.5°C per 1000 ft of height/altitude gained.
Moisture and Clouds
Explain humidity/relative humidity.
Humidity is the water vapor in the air, and relative humidity is a measure of the amount of water vapor present in a parcel of air compared with the maximum amount it can support (Le., when the air is saturated)
at the same temperature.
Relative humidity is usually expressed as a percentage, i.e., relative humidity is 100 percent when
the air is saturated.
Moisture and Clouds
How does air temperature affect relative humidity?
The amount of water vapor a parcel of air can hold depends on its temperature.
That is, warm air is able to hold more water (in a vapor or
liquid state) than colder air. In other words, cooler air supports less
water vapor.
Moisture and Clouds
What is dewpoint?
The dewpoint is the temperature at which a parcel of air becomes saturated.
That is, its capacity to hold water vapor is equal to that which it is actually holding, or in other words, its relative humidity is 100 percent.
Note: Dewpoint is also sometimes called the saturated temperature.
The higher the moisture content in the air, the higher is its dewpoint
temperature. (See Q: Describe how clouds are formed, page 228.)
Moisture and Clouds
Describe how clouds are formed
For cloud formation to be possible, the following properties must exist:
- Moisture present in the air.
- A lifting action to cause a parcel of air to rise. The four main lifting actions are
a. Convection
b. Turbulence
c. Frontal
d. Orographic - Adiabatic cooling of the rising air.
If a parcel of air containing water vapor is lifted sufficiently, it will
cool adiabatically, and its capacity to hold water vapor will decrease (i.e., cooler air supports less water).
Therefore, its relative humidity
increases until the parcel of air cools to its dewpoint temperature,
where its capacity to hold water vapor is equal to that which it is
actually holding, and the parcel of air is said to be saturated (i.e., its
relative humidity is 100 percent).
Any further cooling will cause some of the water vapor to condense out of its vapor state as water
droplets and form clouds.
Further, if the air is unable to support these water droplets, then they will fall as precipitation in the form
of rain, hail, or snow.
Moisture and Clouds
How is the height of a cloud base determined?
Provided that the properties for a cloud to form are present (i.e., moisture,
lifting action, adiabatic cooling, etc.), the height of a cloud base is
determined by the difference between the dewpoint temperature and
the ground temperature.
Cloud Base (ft) = (Surface Temp - Dewpoint ) / 3 (DALR)
It is worth noting that the relationship of the temperatures in the cloud and the surrounding air (i.e., the ELR/DALR/SALR) determines
not only the type of cloud formation but also the height of the cloud top.
That is, when the surrounding temperature is the same as or warmer than the temperature inside the cloud, then the air in the cloud will become stable and stop rising.
This limits the height of the cloud top.
Moisture and Clouds
Why do cumulus clouds have flat bases and round tops?
Cumulus clouds have flat bases due to the uniform decrease in temperature of the dry adiabatic lapse rate (DALR).
(See Q: How is the height of a cloud base determined? page 228.)
They have round and uneven tops because of the uneven decrease in the environmental lapse rate (ELR) temperature and different magnitudes in movement of the rising currents inside the cloud.
Note: lf the ELR is high (i.e., the temperature of the surrounding air reduces more quickly with height gained than the rising DALR/SALR parcel of air), then this parcel of air will always be warmer than the
surrounding (ELR) air.
Therefore, it will be lighter and thus will keep
rising.
This air is said to be unstable and will produce cumiliform (i.e.,heaped) clouds.
Moisture and Clouds
How are cloud types classified?
There are four main groups of clouds:
- Curriform, or fibrous
- Cumuliform, or heaped
- Stratiform, or layered
- Nimbus, or rain-bearing
These groups are further subdivided with the following prefixed
names according to the level of their base above mean sea level (MSL):
- Cirro, or high-level cloud: (cloud base> 16,500-20,000 ft
- Alto, or medium-level cloud: cloud base> 6500 ft
- No prefix, for low-level clouds: cloud base < 6500 ft
Note: The state or extent of any cloud formation depends on the
stability ofthe air.
(See Q: Describe how clouds are formed, page 228.)
Moisture and Clouds
If cumulus clouds were present in the morning, what would you expect later?
Cumulonimbus clouds (CBs).
Moisture and Clouds
Describe the formation of mountain (lenticular) clouds.
Airflow rises over mountains due to orographic uplift and cools adiabatically.
If it cools below its dewpoint temperature, then the water vapor will condense out and form clouds, either as lenticular clouds, often on the hillside when there is a stable layer of air above the mountain, or
as cumulus or even cumulonimbus clouds when there is unstable air above the mountain.
Rotor, or roll, clouds, particularly common with lenticular cloud formation, also may form at a low level downstream of the mountain as a result of surface turbulence.
(See Q: What do lenticular clouds
indicate? page 259.)
Moisture and Clouds
What is mist and fog?
Mist and fog are simply parcels of low-level air in contact with the ground that have small suspended water droplets that have the effect of reducing visibility.
It is usual for mist to precede and to follow fog unless an already formed fog patch is blown in across an area, e.g., sea fog.
Moisture and Clouds
What are the different types of fog?
The most common different types of fog are
- Radiation fog
- Advection fog, including sea fog
- Frontal fog, including hill fog
Moisture and Clouds
How is Radiation fog formed?
This requires the following conditions:
a. Cloudless night. This allows the earth’s surface to lose heat by radiation.
This causes its water vapor to condense out in liquid
form.
b. Moist air. With a high relative humidity, which only requires a slight cooling to reach its dewpoint temperature. c. Light winds. Between 2 and 8 knots. Radiation fog occurs inland, especially in valleys and low-lying areas.
Moisture and Clouds
How is Advection fog formed?
The term advection means heat transfer by the horizontal flow of air.
Fog formed in this manner is called advection
fog and can occur quite suddenly, day or night, land or sea, if the following conditions exist:
a. A warm, moist air mass flowing across a significantly colder
surface, which is cooled from below.
b. Light to moderate winds that encourages the mixing of the lower
levels to give a layer of fog.
c. Sea fog. Sea smoke occurs in the reverse conditions; i.e., very cold air, often in an inversion, passes over a warmer sea that causes evaporating moisture to condense out into whispers of vapor.
Moisture and Clouds
How is Frontal fog formed?
Usually forms in the cold air ahead of a warm occluded
front as a pre-frontal widespread fog. It forms due to the interaction of two air masses.
Moisture and Clouds
What is dew, and how is it formed?
Dew is a water cover on the earth’s surface that is formed when the following conditions exist:
- Cloudless night. This allows the earth’s surface to lose heat by radiation and causes its water vapor to condense out as a water liquid.
- Moist air. With a high relative humidity that only requires a slight cooling to reach its dewpoint temperature.
- Light winds. Less than 2 knots.
Note: The conditions for dew to form are the same as for radiation fog except for the lower or nil wind.
Moisture and Clouds
What is frost, and how is it formed?
Frost is a frozen water cover on the earth’s surface that is formed in the same manner as dew except that the earth’s surface has a subzero temperature that causes the water droplets that have condensed out of the air to freeze on the ground.
(See Q: What is dew, and how is it
formed? page 232.)
For smog questions, see sub-chapter, “Visibility,” page 265.
Storms and Precipitation
What is virga?
Virga is rain that falls from the base of a cloud but evaporates at a lower level in drier warmer air before it reaches the ground.
This is a sign of a temperature inversion, which in turn is an indication of possible windshear.
(See Q: Where do you find windshear? page 254.)
Virga is not really a form of precipitation because it does not reach the ground; however, it is generally considered to be precipitation, and it is important for pilots to recognize it because it can affect an aircraft’s
flight path.
Storms and Precipitation
Describe the formation of a thunderstorm.
Thunderstorms are associated with cumulonimbus clouds, and there may be several thunderstorm cells within a single cloud.
Therefore, we first have to examine how a cumulonimbus cloud starts to form.
Four conditions are required for a cumulonimbus cloud to develop:
- A high moisture content in the air.
- A trigger lifting action (or catalyst) to cause a parcel of air to start rising.
The four main lifting actions are
a. Convection b. Turbulence c. Frontal d. Orographic
- Adiabatic cooling of the rising air.
- A highly unstable atmosphere so that once the air starts to rise, it will continue rising.
Effectively, the environmental lapse rate (ELR)
must be greater than the saturated adiabatic lapse rate (SALR) for over 10,000 ft.
(See Q: Describe how clouds are formed? page 228.)
Next, we have to examine the life cycle of the cumulonimbus cloud and its associated thunderstorm.
This life cycle can be divided into three
phases:
Developing stage
Mature stage
Decaying stage.
- Developing stage:
During the development of the cumulonimbus
cloud, updrafts move air aloft, allowing condensation to take place throughout the ascent of the convective currents. - Mature stage:
During this stage, water drops start to fall through
the cloud, drawing air down with them. Although it is dependent on the shape of the storm and the prevailing wind gradient, this downdraft
is often in the middle of the cloud/storm, surrounded on all sides by strong continuing updrafts, which are providing further fuel for the storm.
During this stage, downdrafts can reach 3000 ft/min,
and updrafts can reach 5000 to 6000 ft/min.
The mature phase of a cumulonimbus cloud is also the most hazardous stage of its thunderstorms.
In short, the dangers include
a. Torrential rain
b. Hail
c. Severe turbulence
d. Severe icing
e. Windshear and micro bursts
f Lightning
- Decaying stage:
This is the final stage of the cumulonimbus cloud.
It starts with the end of the thunderstorm, which is marked by the end of continuous rain and the start of sporadic showers, sometimes as virga due to a temperature inversion beneath the cloud base,
which can still cause a marked windshear.
At the higher levels it may take on the familiar anvil shape as upper winds spread out under the tropopause.
An anvil can have marked downward vertical
currents beneath it, which cause a strong windshear that also should be avoided.
Storms and Precipitation
How are thunderstorms a hazard to aviation?
Thunderstorms can produce the following hazards to all aircraft types:
- Severe windshear, which can cause
a. Handling problems
b. Flight path deviations, especially vertically
c. Loss of airspeed
d. Possible structural damage
(See Q: What is winds hear? page 254.)
- Severe turbulence, which can cause
a. Possible loss of control
b. Possible structural damage
(See Q: What is turbulence? page 253.) - Severe icing, especially clear ice formed from super-cooled water droplets (SWDs) striking a surface with a subzero temperature.
(See Qs: What are SWDs? page 262; What hazards to aviation does icing cause? page 262.)
- Airframe structural damage from hail
- Reduced visibility
- Lightning strikes, which can cause damage to the electrical system
- Radio communication and navigation interference from static electricity present in the thunderstorm
These hazards exist inside, under, and for some distance around a thunderstorm’s associated cumulonimbus cloud.
Therefore, cumulonimbus clouds (and thunderstorms) should be avoided by a minimum of 10 nautical miles and in severe conditions (i.e., the mature stage of
a cumulonimbus cloud) by at least 20 nautical miles.
Storms and Precipitation
When is lightning most likely to occur?
Lightning is most likely to occur when the outside air temperature
(OAT) is + 10°C to -10°C and within or close to a thunderstorm associated with a mature-stage cumulonimbus cloud.
Winds
How is wind described or expressed?
Wind is expressed in terms of direction and strength.
- Wind direction. This is the direction from which the wind blows and is expressed in degrees measured clockwise from true north, with two main exceptions when the wind direction is measured from
magnetic north:a. The reported wind given from the tower, either air traffic control (ATC) or Automatic Terminal Information Service (ATIS), is expressed in degrees magnetic so that the runway direction and reported wind are relative to each other. This is extremely important when taking off or landing.
b. The upper winds used for airways planning are expressed in
degrees magnetic so that the airway direction and reported wind
are relative to each other. - Wind strength. This is measured and expressed in nautical knots.
Winds
What is wind velocity?
Wind velocity relates to the wind’s direction and strength.
It is usually written in the following form: 180/25; i.e., a wind blowing from 1800 at a strength of 25 knots.
Winds
Describe a veering and backing wind.
A wind is said to be veering, or is said to have veered, when it changes its direction in a clockwise direction, e.g., 100/10 to 160/10.
A wind is said to be backing, or is said to have backed, when it changes its direction in an counterclockwise direction, e.g., 160/10 to 100/10.
(See Q: Describe the characteristics of a surface wind, page 238.)
Winds
What is buys ballot’s law?
Buys ballot’s law is if you stand with your back to the wind in the northern hemisphere, the low pressure (temperature) will be on your left.
Note: Conversely, in the southern hemisphere, the low pressure (temperature) will be on your right.
Winds
What is the pressure gradient force?
A pressure gradient force is a natural force generated by a difference in pressure across a horizontal distance; i.e., gradient between two
places.
It acts at right angles to the isobars and is usually responsible for starting the movement of a parcel of air from an area of high pressure to an area of low pressure.
(See Q: What is an isobar? page 221.)
Winds
What is the Coriolis force (or geostrophic force)?
The coriolis (geostrophic) force is an apparent force that acts on a parcel of air that is moving over the rotating earth’s surface.
This means that the air does not flow simply from a high- to low-pressure system but is deflected to the left or right according to which hemisphere you are in.
This is known as the coriolis effect. The coriolis force is a product of the earth’s rotational properties.
Note: In the northern hemisphere, the coriolis force deflects the airflow to the right (i.e., as a westerly wind), and in the southern hemisphere, the airflow is deflected to the left.
Winds
What is the geostrophic wind?
The geostrophic wind is the balanced flow of air from a westerly direction that is parallel to straight isobars with a low-pressure system to its left, i.e., buys ballot’s law, and at a strength directly proportional to the
spacing of the isobars (i.e., pressure gradient).
It is usually found at low to medium heights from approximately 2000 ft and above.
The geostrophic wind is created when the two forces of pressure gradient force and coriolis (geostrophic) force are balanced.
Winds
What is a gradient wind?
The gradient wind is the resulting wind that blows around the curved isobars common to circular low- or high-pressure patterns.
It is usually found at low to medium heights from approximately 2000 ft and above.
Note: The geostrophic wind assumes straight west to east isobars.
(See Q: What is the geostrophic wind? page 236.)
Winds
Describe upper winds.
Upper winds are determined by the thermal gradient.
A difference in temperature between two columns of air will cause a pressure difference at height even if both columns of air have the same sea level
pressure.
This pressure difference creates a wind (parallel to the isobars) at altitude that is different from the wind experienced at sea level, even if no wind was present at sea level.
The vector sum of the isotherm thermal wind component and the surface and upper isobar pressure-driven geostrophic (or gradient)
wind produces the direction and speed of the upper wind.
In the northern hemisphere, the thermal gradient is generally north-south (north cold and south warm), and therefore, the upper winds generally are westerly in direction (i.e., from the west), with the highest
wind speed where the thermal gradient is greatest, e.g., jetstreams.
Note: However, there is a light easterly wind over the thermal equator that can extend all around the globe at certain times of the year and can reach speeds of up to 70 knots.
Winds
What is a thermal wind, and how is it generated?
Thermal winds (of quite different direction and strength to the low- and medium-level geostrophic and gradient winds) are generated by a difference in temperature (thermal gradient) between two columns of air over large areas and at great upper heights.
The direction of a thermal wind is parallel to the isotherms. (See Q: What is an isotherm? page 221.)
That is, the thermal wind direction is such that if you stand with your back to the thermal component
in the northern hemisphere, the low temperature will be to your left.
The strength or speed of the thermal wind is directly proportional to the temperature gradient (i.e., the spacing of the isotherms) between
the two columns of air.
Winds
What is a jetstream?
Jetstreams are simply narrow bands of high-speed upper thermal winds at very high altitudes.
(See Q: What is a thermal wind, and how
is it generated? page 237.)
The official definition of a jetstream is a strong, narrow current on a quasi-horizontal axis in the upper tropopause or stratosphere characterized
by strong vertical and/or lateral windshear (CAT).
The wind speed must be greater than 60 knots for a wind to be classified as a jetstream.
Jetstreams typically are 1500 nautical miles long, 200 nautical miles wide, and 12,000 ft deep, and their speed is directly proportional to the thermal gradient; i.e., the greater the thermal gradient, the greater is the speed of the jetstream.
Winds
Where do you find jetstreams?
Jetstreams are driven by thermal gradients and therefore are found wherever the thermal gradient is high enough.
There are two bands of rapid temperature changes (i.e., high / maximum thermal gradient) in each hemisphere that are marked enough to produce a jetstream.
They are…
- At the polar front around 60 degrees oflatitude, where the polar air
meets the subtropical air.
This is a polar front jetstream, and is the
most marked thermal gradient to be found, especially when it is
over land in the winter.
- At the inter-tropical front, where the subtropical air meets the tropical
air.
This is known as the inter-tropical front jetstream.
The jetstream exists just below the tropopause in the warm air of a pressure system; i.e., in the subtropical air at the polar front and in the tropical air at the inter-tropical front, but appears on the surface
chart to be in the cold sector.
This is so because of the slope of the front
with height.
The jet moves with the front (i.e., south in the winter), and its direction
is not always westerly because the jet follows the pressure system.
In fact, the polar front jetstream can blow from 190 to 350 degrees
around the polar front. However, its overall direction generally is regarded as being from west to east.
The maximum windshearl / clear air turbulence (CAT) associated with a jetstream can be found level with or just above the jet core in the warm air but on the cold polar air side of the jet.
Winds
Describe the characteristics of a surface wind.
At the surface, the wind weakens in strength (speed) and backs in
direction in the northern hemisphere (veers in direction in the southern
hemisphere).
The wind speed reduces near the surface compared with the free air
geostrophidgradient wind at 2000 ft due to the friction forces between
the moving air and the ground.
Winds
Describe the diurnal variation of the surface wind.
The diurnal variation (time of day) affects the degree to which the underlying trend of the surface wind is altered, i.e., to weaken in strength and back in direction (northern hemisphere) compared with the free air gradient wind (at approximately 2000 ft).
(See Q: Describe the characteristics of a surface wind, page 238.)
By day;
The day surface wind loses less of its strength/speed and therefore backs only slightly compared with the free air gradient wind or is a stronger wind that has veered compared with the night
surface wind.
By night:
The night surface wind will drop in strength (speed) and
back in direction significantly compared with the free air gradient
wind and to a slightly lesser extent compared with the daytime surface
wind.
Therefore, the surface wind by day will resemble the gradient wind more closely than the surface wind at night.
(See Q: What are the approximate changes in the surface wind direction and speed compared with the free air 2000-ft gradient wind? page 239.)
Winds
How does the wind (direction and speed) change with height?
In general, the wind in the northern hemisphere increases in speed and veers in direction with an increase of height.
And in the southern hemisphere, the wind increases in speed and backs in direction with an increase of height.
Winds
What are the approximate changes in the surface wind direction and speed
compared with the free air 2000-ft gradient wind?
The surface wind reduces in speed and in the northern hemisphere
backs in direction compared with the free air gradient wind at 2000 ft.
(See Q: Describe the characteristics of a surface wind, page 238.)
However, the degree to which it changes direction and speed is a function of
1. Diurnal variation (i.e., night or daytime). (See Q: Describe the diurnal
variation of the surface wind, page 239. )
2. Surface variation (i.e., land or sea).
Winds
Why is the surface wind important to pilots?
The surface wind is important to pilots because it relates directly, in terms of both its speed and its direction, to the effect it has on their aircraft during takeoffs and landings.
Winds
What are trade winds?
Trade winds are steady and predictable surface winds that rarely exceed 15 knots at the surface but can extend up to 10,000 ft.
They blow from the subtropical highs into the equatorial low (ITCZ),
i.e., blowing from the northeast in the northern hemisphere and from the southeast
in the southern hemisphere.
(See Q: What is the ITCZ? page 266.)
Winds
Explain land/sea breezes, especially in connection with coastal airports.
Land/sea breezes are a product of surface heating and atmospheric convection currents that produce small local airflow circulation cells.
Land/sea breezes are particularly important to aviators in connection with coastal airports because they can determine the wind direction at a local level, which may be considerably different from the general
wind direction.
In addition, they also may cause some windshear
and / or turbulence as an airplane passes from one body of air to another.
Sea breezes:
These occur during the daytime, usually between midday
and late afternoon on hot sunny days, when the land heats up quicker
than the sea.
The air above the land becomes warmer and rises,
causing the pressure in the warmer air column over the land to be
greater than that at a similar height over the sea. Therefore, a pressure
difference is born at height, i.e., 2000 ft, that produces an airflow
at height from the land to the sea.
This induces an opposite pressure
difference at the surface; namely, the outflow of air at height cools,
gets denser and heavier, and therefore descends over the sea, creating
a high surface pressure.
This induces a flow of air at the surface
from the sea to the land (which has a low surface pressure due to the rising convective current air), which is the sea breeze.
Land breezes:
These occur at night when the land cools quicker than the sea, causing the air above it to cool and subside. The air over the sea, however, retains its heat better and becomes the warmer of the two columns of air, and therefore, its air rises.
The pressure in the warmer air column above the sea is greater than that over the land at height, so the air flows at height from the sea to the land.
This induces an opposite pressure difference at the surface between the land and the sea; namely, the outflow of air at height cools, gets
denser and heavier, and therefore descends over the land, creating a high surface pressure.
This induces a flow of air at the surface from
the land to the sea (which has a low surface pressure due to the rising
warmer air), which is the offshore land breeze.
Winds
What is the Fohn wind effect?
The fohn wind effect is when air is cooled as it is forced to rise over
high ground, first at the dry adiabatic lapse rate (DALR) and then,
after the dewpoint / condensation level is reached, at the saturated
adiabatic lapse rate (SALR).
After the condensation level, clouds will
form, usually on the hillside, and moisture will be lost, falling as
either rain or dew.
Then, as the air descends over the far side of the hill, it has a lower
water content, and so the condensation level is higher.
A longer period
of warming at the greater DALR means that the air on the far (downwind)
side of the hills is warmer and dryer than it was on the upwind
side of the hill.
Note: As a rough guide, the air temperature can rise by 1.5°C per
1000 ft of hill, which of course is the difference between the DALR and
the SALR.
Therefore, the fohn effect (i.e., descending air) can influence the wind direction and strength and in some cases even generate a wind
that blows down the farllee side (downwind) of the hill.
This is known as the fohn wind. The fohn wind effect can be seen in various places, including the Rocky Mountains (Chinook winds) and the Alps.
Winds
What is a katabatic/anabatic wind?
A katabatic wind is a local valley wind that flows down the side of a hill.
An anabatic wind is a local valley wind that flows up the side of
a hill.
Air Masses/Pressure Systems/Fronts
What is an airmass?
An airmass is a large parcel of air with fairly similar temperature and humidity (moisture content) properties throughout.
For this definition it follows that the environmental lapse rate (ELR) (see Q: What is
ELR? page 226) will be fairly constant throughout the airmass, and all the air will behave in the same way when lifted, heated, or cooled.
Air Masses/Pressure Systems/Fronts
What is a pressure system?
A pressure system is a circulating airmass that is classified as either a low or high, which relates to the direction of pressure change toward the center of the airmass at the surface, i.e., gets lower or higher.
A low-pressure system typically will have more than one airmass, e.g., a warm airmass incorporated into a cold unstable airmass, with fronts in between.
(See Q: What is a front? page 249.)
On the other hand, a high-pressure system is typically made up of a single stable airmass.
(See Qs: Describe the characteristics of a low-pressure system, page 243;
Describe the characteristics of a high-pressure system, page 246.)
Air Masses/Pressure Systems/Fronts
Describe the characteristics of a low-pressure system.
A low-pressure system, which is also known as a depression, has the following characteristics:
- Pressure gradient. In a low-pressure system, the surface barometric
air pressure rises as you move away from its center; in other words,
its pressure drops as you move toward its center. - Airflow pattern. The airflow pattern of a depression can be categorized as follows:
a. Convergence (inflow) at the lower layers
b. Rising air at the center
c. Divergence (outflow) in the upper layers - Wind direction. (See Q: What is the wind direction around a low pressure
system? page 244.) - Airmass. A low-pressure system usually will be made up of at least
two different air masses, i.e., a cold and a warm air mass, with cold
and warm fronts. - Movement. Low-pressure systems generally are more intense than highs because they are more concentrated in terms of area and have
a stronger pressure gradient (pressure change with distance).
Therefore, lows move faster across the surface ofthe earth and tend
to have a shorter life span than highs.
- Weather. (See Qs: Describe the weather associated with a depression,
page 244; What are the different types of depressions? page 244.)
Air Masses/Pressure Systems/Fronts
What is the wind direction around a low-pressure system?
The wind circulates counterclockwise around a low-pressure system in
the northern hemisphere and clockwise in the southern hemisphere.
Note: Flying toward a low in the northern hemisphere, an aircraft
will experience right (starboard) drift.
(See Q: What drift would you
experience when flying from a high- to a low-pressure system? page 245.)
Air Masses/Pressure Systems/Fronts
What are the different types of depressions?
What are the different types of depressions?
The different types of depressions (low-pressure systems) are developed
and classified according to their trigger phenomenon.
Depressions are
classified as follows:
- Frontal depression
- Thermal depression
- Tropical storm depressions
- Orographic depression
Air Masses/Pressure Systems/Fronts
Describe the weather associated with a depression.
In general, the weather associated with a low-pressure system is as
follows:
- Cloud formation and related weather are present, i.e., precipitation, etc.
This is due to the adiabatic cooling experienced by the ascending air in a depression.
Any instability in the rising air may
lead to the vertical development of cumuliform clouds accompanied
by rain showers.
- Visibility may be good (except in the rain showers). This is so because
the vertical motion will tend to carry away most of the particles
suspended in the air. - Moderate to strong winds are present. This is so because a low pressure
system typically has a marked pressure gradient, which
represents the strength of the wind. - Frontal weather is present because fronts are associated with low pressure
systems; thus frontal weather is nonnally the most prominent
weather associated with a depression.
(See Q: Describe the
characteristics and weather common to the passage of a warm/cold
front, pages 250 and 252.)
Air Masses/Pressure Systems/Fronts
What is a trough?
A trough is a V-shaped extension of a low-pressure system.
Air flows into a trough (convergence) and rises. If the air is unstable, weather
similar to that found in a depression or a cold front will occur, e.g.,
cumuli form clouds, cumulonimbus clouds, and thunderstorm activity.
Air Masses/Pressure Systems/Fronts
What drift would you experience when flying from a high- to a low-pressure
system?
In the northern hemisphere, you would experience a drift to the right
(starboard) when flying from a high- to a low-pressure system.
This is so because the wind circulates counter-clockwise around a low in the northern hemisphere, and therefore, flying toward the center of a low-pressure system, you have a wind from the left (port) causing you to drift to the right (starboard).
Air Masses/Pressure Systems/Fronts
Describe the characteristics of a high-pressure system.
A high-pressure system, which is also known as an anticyclone, has the
following characteristics:
- Pressure gradient. In a high-pressure system, the surface barometric air pressure rises as you move toward its center; in other words, its
pressure drops as you move away from its center.
The pressure gradient (which is depicted by the closeness of the isobars) relates
to the wind speed; i.e., a weak pressure gradient = wide isobar
spacing = a weak wind.
- Airflow pattern. The airflow pattern of an anticyclone can be categorized
as follows:
a. Convergence (inflow) at the upper layers
b. Subsiding air at the center
c. Divergence (outflow) at the lower layers - Wind direction. (See Q: What is the wind direction around a high-pressure
system? page 247.) - Airmass. A high-pressure system usually will be made up of only
one air mass and therefore wiII have no defined fronts. - Movement. High-pressure systems generally are greater in extent but
with a weaker pressure gradient (change of pressure with distance)
and therefore are slower moving, more persistent, and last longer
compared with a low-pressure system. - Weather. (See Q: Describe the weather associated with a high-pressure
system, page 247.)
Air Masses/Pressure Systems/Fronts
What is the wind direction around a high-pressure system?
The wind circulates clockwise around a high-pressure system in the northern hemisphere and counterclockwise in the southern hemisphere.
Note: Flying toward a high in the northern hemisphere, an aircraft will experience left (port) drift.
(See Q: What drift would you experience
when flying from a high- to a low-pressure system? page 245.)
However, it should be remembered that above a surface high there is an upper low system.
(See Q: Describe the characteristics of a high-pressure
system, page 246.)
Therefore, the wind direction will reverse
at height in a high-pressure system.
Air Masses/Pressure Systems/Fronts
Describe the weather associated with a high-pressure system.
In general, the weather associated with a high-pressure system is as
follows:
- Clear upper skies with little or no low-level cloud formation and precipitation.
This is so because the descending stable air (subsiding
air is very stable) in a high-pressure system is warmed as it
descends.
Therefore, the dewpoint temperature is increased, and
the relative humidity is reduced. Thus cloud formation and any
associated precipitation will tend to disperse.
- Light winds.
This is so because a high-pressure system has a weak
pressure gradient, and its wind strength is represented by its pressure
gradient. - Possible poor visibility at low levels.
This is so because the descending
air may bring airborne particles from the upper levels down to the
lower levels that we as pilots fly in or may warm sufficiently to create
an inversion layer as a result of the upper descending air warming to
a greater temperature than the lower surface turbulent layer.
This phenomenon may cause strati-form clouds to form and/or trap airborne
particles, dust, smoke, and moisture (fog) beneath the inversion layer.
Air Masses/Pressure Systems/Fronts
What is a ridge?
A ridge is a U-shaped extension to a high-pressure system.
Stable air subsides into a ridge, similar to a high-pressure system,
and therefore, weather similar to that found in an anticyclone will
occur in a ridge.
(See Q: Describe the weather associated with a high-pressure
system, page 247.)
Air Masses/Pressure Systems/Fronts
What drift would you experience when flying from a low- to a high-pressure
system?
In the northern hemisphere - drift to the left
(port) when flying from a low- to a high-pressure system.
In the southern hemisphere - drift to the right (starboard).
Air Masses/Pressure Systems/Fronts
Describe the airflow between a low- and a high-pressure system.
The airflow between a low- and a high-pressure systems is pressure driven.
As a random starting point, at the surface of a high-pressure system the air flows outward (divergence) at low levels into the center of a surface low-pressure system (convergence).
Air Masses/Pressure Systems/Fronts
How does a change in pressure affect an aircraft’s flight level?
A change in atmospheric pressure causes a barometric pressure error in the altimeter.
altimeter. (See Q: What pressure altitude error is commonly experienced?
page 117.)
Therefore, when flying from high to low (pressure),
beware below because your actual flight level is lower than your altimeter indicated level. In other words, the altimeter over-reads.
Conversely, the opposite is true when flying from low- to high-pressure areas.
This high- to low-mnemonic applies equally to temperature values as it does pressure.
(See Q: How does a change in air temperature
affect an aircraft’s flight level? page 225.)
Air Masses/Pressure Systems/Fronts
What is a front?
A front is a boundary between two different air masses.
Air Masses/Pressure Systems/Fronts
What is frontal activity?
Frontal activity describes the interaction between at least two air masses as one air mass replaces another.
Air Masses/Pressure Systems/Fronts
How are frontal depressions developed?
Frontal depressions (low-pressure frontal system) develop when two different air masses meet (touch) but do not mix together because one of them is warmer and less dense than the other.
Air Masses/Pressure Systems/Fronts
What is a warm front?
A warm front is the boundary produced between two air masses (i.e.,
warm air behind cold air), where the warmer, less dense air mass rises
up and replaces at altitude (slides over) the colder air mass at the surface.
In a warm front, the frontal air at altitude is actually well ahead of its depicted position on a weather chart.
Note: A front is shown as a line on a weather chart that represents
the front’s surface position.
A warm front is represented by half circles
along the front line.
Air Masses/Pressure Systems/Fronts
Describe the characteristics and weather common to the passage of a warm front.
The passage of a warm front has the following general characteristics
and associated weather:
- As the warm front approaches:a. A lowering of the cloud base is experienced. This is represented
by cirrus clouds giving way to cirrostratus clouds giving way to
altostratus clouds with possible virga rain giving way to nimbostratus
clouds with increased rainfall.b. Poor visibility is experienced. This is so because with the passage
of a warm front, visibility is reduced due to the increase
in low-level douds and the more consistent rainfall and possibly
raIn Ice.c. The atmospheric pressure usually will fall as a warm front
approaches. - As the wann front passes:a. A rise in air temperature. This is due to the arrival of the warm
sector air mass.b. Low-level cloud base or fog.c. Wind veers in direction in the northern hemisphere and backs in
the southern hemisphere.d. The atmospheric pressure usually will stop falling and may even
rise.e. Good visibility. In general, the visibility will be good, especially above the low-level clouds, although obviously the visibility will be poor in the cloud or fog.
Air Masses/Pressure Systems/Fronts
What is a cold front?
A cold front is the boundary between two air masses (i.e., cold air behind
wann air), where the colder, denser air mass undercuts and replaces the
wanner preceding air mass from the surface upward (slides under).
Air Masses/Pressure Systems/Fronts
Why does a warm front move slower than a cold front?
A warm front moves slower than a cold front because of the tendency of the warm air (in the warm front) to rise up and slide over the cold,
more resistant, denser air (mass) in front of it.
Air Masses/Pressure Systems/Fronts
Describe the characteristics and weather common to the passage of a
cold front.
The passage of a cold front has the following general characteristics
and associated weather:
- As the cold front approaches:
a. Cumulus and even cumulonimbus cloud cover is experienced.
This results in the following severe weather changes:
(1) Heavy rain(2) Thunderstorms
(3) Turbulence (possibly severe)
(4) Windshear
b. Poor visibility is experienced.
This is so because with the passage
of a cold front, visibility is reduced due to the increase in heavy clouds through a large vertical band and the heavy, consistent rainfall.
c. The atmospheric pressure usually will fall as the cold front approaches.
2. As the cold front passes:
a. A sudden drop in air temperature is seen. This is due to the
arrival of the cold sector airmass.
b. Clear skies with isolated cumulus clouds are seen.
c. There is good visibility, except within the isolated cumulus clouds.
d. Wind veers in direction in the northern hemisphere and backs in
the southern hemisphere.
e. The atmospheric pressure will stop falling and may even rise.
Air Masses/Pressure Systems/Fronts
What is an occluded front?
An occluded front is a combination of both a cold and a warm front.
An occlusion occurs because the cold front moves faster than the warm front, and therefore, the cold front inevitably will catch up with the warm front.
Turbulence
What is turbulence?
Turbulence is the eddy motions in the atmosphere, which vary with time, from place to place, and in magnitude.
Some form of turbulence is always present in the atmosphere, with severe or even moderate turbulence at best being uncomfortable and at worst capable of overstressing some airframe types.
Turbulence is mainly considered to be vertical gusts. The two main
forms of vertical turbulence are
- Convection turbulence, caused by solar radiation heating the
ground and producing rising thermal currents - Obstruction and orographic (terrain-generated) turbulence
The other main forms of turbulence (considered to be horizontal) are….- Jetstreams
- Wake turbulence (See Q: What is wake turbulence? page 260.)
In addition, other (horizontal) forms of less dynamic turbulence are frontal, temperature, and any general wind direction and/or speed
changes.
(See Q: What is CAT, and give examples? page 259.)
Air Masses/Pressure Systems/Fronts
What causes surface turbulence?
Surface turbulence is caused by the surface wind being blown over and around surface obstacles, such as hills, trees, and buildings.
This causes the surface wind to form turbulent eddies, the size of which depends on the size of both the obstruction and the wind speed.
Air Masses/Pressure Systems/Fronts
What is windshear?
Windshear is any variation of wind speed and/or direction from place to place, including updrafts and downdrafts.
The stronger the change
and/or the shorter the distance within which it occurs, the greater is the windshear.
However, in practical terms, only a wind change of a
magnitude that causes turbulence or a loss of energy disturbance to an
aircraft’s flight path is generally considered to be windshear.
It should be remembered that windshear is a complex subject and is still not fully
understood. (See Q: Where do you find windshear? page 254.
Air Masses/Pressure Systems/Fronts
Where do you find windshear?
Most forms of windshear are found at low levels, i.e., below 3000 ft.
Therefore, the term low-level windshear is used to specify the windshear
along a final approach path prior to landing, along a runway,
and along a takeoff and initial climb-out flight path.
Low-level windshear, i.e., near the ground, generally is present to
some extent as an aircraft approaches the ground because of the difference
of speed and/or direction of the surface wind compared with the
wind at altitude.
This is critical in terms of aircraft safety because the
loss of energy caused by these changes can easily lead to the aircraft
losing altitude and/or stalling.
(See Q: How would you fly an approach
if you suspect windshear? page 319.)
Low-level windshear includes
1. Clear air turbulence (CAT). (See Q: What is CAT, and give examples?
page 259.)
- Frontal passage
- Microburst and thunderstorm gusts. (See Q: What is a microburst?
page 256.)
Medium- and high-level windshear also can be experienced at high
or medium altitudes and include
1. CAT in the form of jet streams. (See Qs: What is a jetstream? page 237; Where do you find jetstreams? page 237.)
2. Frontal passage (e.g., a low-level frontal passage
Air Masses/Pressure Systems/Fronts
How is windshear detected?
Windshear is detected by identifying a difference in wind (direction and/or speed) and / or temperature between two places or identifying certain weather phenomenon, e.g., cumulonimbus clouds, that give rise
to possible windshear conditions from weather reports.
This can be done in the following ways:
- Pilot appreciation of differences in reported wind and/or temperature between two places and the location of certain weather and terrain
phenomenon.
For example, the following phenomena should alert
the pilot instinctively of the possibility of windshear along his or her’s flight path:
a. Cumulonimbus clouds in the general area (microbursts and / or thunderstorm gusts)
b. Heavy rain or even thunderstorms in the vicinity c. Fronts (change in wind direction) d. Virga rain (temperature inversion), e.g., rain that evaporates before it reaches the ground, absorbs latent heat out of the surrounding air that creates denser /heavier air with an associated higher pressure causing a downdraft windshear. e. Land/sea breezes at coastal airports, especially at dawn and dusk periods f. Terrain, i.e., trees, mountains, or even surface obstructions
Note: Pilots are required to report windshear whenever they
encounter it.
- Aerodrome equipmentireporting. Many of the major aerodromes
around the world have windshear measuring equipment.
3. Aircraft warning equipment. Modern aircraft have windshear warning systems, which usually are incorporated into the ground proximity warning system (GPWS).
Air Masses/Pressure Systems/Fronts
How does windshear affect an aircraft?
Windshear is a change of wind speed and/or direction (including vertical
updrafts/downdrafts) that affects the lift capability of an aircraft.
Air Masses/Pressure Systems/Fronts
What is a microburst?
A microburst is a severe downdraft, i.e., vertical wind, emanating from
the base of a cumulonimbus cloud during a thunderstorm.
Note: A microburst is a severe form of windshear.
Air Masses/Pressure Systems/Fronts
Where do you find microbursts?
Microbursts are found close to, normally underneath, mature cumulonimbus clouds and are associated with thunderstorms.
The microburst downdraft is highly concentrated, typically only 5 km across, and often centered in the middle of a thunderstorm, surrounded
on all sides by strong updrafts.
Air Masses/Pressure Systems/Fronts
What do you know about microbursts?
Microbursts are severe vertical downdrafts associated with mature cumulonimbus clouds.
(1) They are usually highly concentrated,
only about 5 km across, and are found in the middle of a thunderstorm
under a mature cumulonimbus cloud.
(2) The mechanics that produce a microburst are found at the height of
a mature cumulonimbus cloud’s cycle; e.g., they only last for a few
minutes (up to 10 minutes) because they are centered in the mature
stage of a cumulonimbus cloud (where updrafts are produced that fuel
the downdrafts), which itself only last for about 40 minutes.
(3) A mature cumulonimbus cloud produces strong continuous updrafts around its outer edges (and sometimes outside the cloud itself) that build up to fuel
the downdrafts in the center of the cloud.
(4) The microburst is a result of the downdrafts breaking out of the base of the cloud being colder than the surrounding air because it has only been warmed at the saturated adiabatic lapse rate (SALR) during the descent within the cloud.
Air Masses/Pressure Systems/Fronts
What identifies (are the indications of) a microburst?
The typical indications of a microburst are
- Mature cumulonimbus clouds with thunderstorm activity, especially
rain - Roll cloud formation around a cumulonimbus cloud
Note: Roll clouds are formed by the turbulent outflow, associated with microburst downdrafts, from underneath a cumulonimbus cloud. - Virga rain, especially beneath or near to a cumulonimbus cloud
Note: Rain that evaporates before it reaches the ground absorbs latent heat out of the surrounding air and creates denser air with an associated higher pressure, causing the downdrafts to fall to the
ground at an even greater rate.
Therefore, virga rain indicates a possibly
severe microburst.
- Flight path and indicated airspeed (lAS) fluctuations, especially on
the approach path (See Q: How does a microburst affect an aircraft?
page 258.) - Wind direction and speed changes, which can be either reported or
measured by aircraft systems, if fitted
Air Masses/Pressure Systems/Fronts
How does a microburst affect an aircraft?
1 - aircraft will experience an updraft
2- aircraft will experience an strong downdraft in a severe change in short distance.
3- aircraft will experience severe downdraft and a tailwind directly from the cloud base.
- Initially, when entering a zone underneath a cumulonimbus cloud with a micro burst present, an aircraft will experience an updraft
(as a result of the downburst spreading outward and rebounding off the ground as an updraft). This has the following effects on an aircraft’s
flight path:
a. Energy gain from an increasing headwind will cause the aircraft’s
nose to rise.
Note: In an extreme case, the updraft may be severe enough to
cause the aircraft to exceed its stalling angle.
b. airrspeed (lAS) will rise. c. Rate of descent will be reduced.
The overall effect in this portion of flight is to cause an overshoot
effect as the aircraft tends to fly above the desired flight path.
Note: When such an updraft is encountered at the start of an
approach underneath a cumulonimbus cloud, a pilot should be
aware of the potential for the initial overshoot effect of the aircraft
to be reversed as the aircraft proceeds along the flight path.
This is
known as windshear reversal effect, i.e., an overshoot followed by undershoot.
- Approaching the central area underneath a cumulonimbus cloud
with a microburst present, an aircraft would start to experience a
strong downdraft, which would be a severe change, over a short
distance, having previously experienced an updraft.
This has the
following effects on an aircraft’s flight path:
a. There will be an energy loss from a reducing headwind, which
will cause the aircraft’s nose to fall.
b. airspeed (lAS) starts to fall (due to the reducing headwind).
c. Rate of descent starts to increase, with a tendency to go below
the glide path.
- As the aircraft’s flight path progresses to the far side of the cloud with
a microburst, the aircraft will then experience a severe downdraft
and a tailwind directly from the cloud base. This has the following
effects on the aircraft’s flight path: ‘
a. There will be energy loss from an increasing tailwind.
b. Airspeed (lAS) continues to fall sharply.
c. Rate of descent continues to increase with a tendency to go further
below the glide path, with even the possibility of ground contact
if not checked by initiating a missed approach, and even then,
success depends on the power height and speed reserves available being sufficient enough to overcome the downdraft.
The overall effect in this portion of flight is to cause an undershoot
effect as the aircraft tends to fly below the desired flight path.
It should be understood clearly that microbursts can be of such a
strength that they can down an aircraft no matter how much power,
time, and height the aircraft has available.
Therefore, always, always
avoid possible microburst areas. (See Q: How would you fly an approach
if you suspect a microburst? page 320.)
Air Masses/Pressure Systems/Fronts
What is the definition of wind gust factors?
Wind gust factors are an indication of how much wind speed change
you can expect in varying wind conditions and are calculated by taking
the difference between the maximum and minimum wind speeds in
the gusts and dividing by the mean wind speed.
Thus a wind gusting
from 15 to 25 knots with a mean of 20 knots will have a gust factor of
10/20, or 0.5.
Air Masses/Pressure Systems/Fronts
What do lenticular clouds indicate?
Lenticular (lens-shaped) clouds indicate standing (mountain) wave
clear air turbulence (CAT).
They are found at height in rising air above
the downwind side of a range of hills, often extending for up to 100 nautical miles downward of a line of hills and at a height of up to 25,000 ft.
[See Qs: What is CAT, and give examples? page 259; Describe
the formation of mountain {lenticular} clouds, page 230.
Air Masses/Pressure Systems/Fronts
What is CAT, and give examples?
CAT is an acronym for clear air turbulence, i.e., no signs of visible moisture content in turbulent air.
Examples of clear air turbulence are
as follows:
- Low-level CAT. This is caused by
a. Temperature inversions
b. A difference between the surface and gradient wind due to the
turbulent mixing of the boundary layer near the surface
c. Local surface winds, i.e., land/sea breezes
d. Terrain-generated winds (e.g., mountain/standing waves on the
downwind side of a range of hills) - Jetstreams. The most severe CAT can be found level or just above the
jet core in the warm air but on the cold air/polar side of the jet. (See Qs:
What is ajetstream? page 237; Where do you findjetstreams? page 237.) - Fronts. An active front produces a marked horizontal windshear,
which can produce CAT. - Thunderstorms. Serious CAT windshear can be found at low levels
under and near thunderstorms, e.g., microbursts. (See Q: What is a
microburst? page 256.) - Wake turbulence. (See Q: What is wake turbulence? page 260.)
Air Masses/Pressure Systems/Fronts
Can you detect CAT?
Clear air turbulence (CAT) is one of the hardest forms of turbulence to detect.
In fact, no specific equipment has been developed to detect
CAT.
Therefore, CAT can only be detected by using meteorologic appraisal of the prevailing conditions, typically shown on met charts,
and identifying the locations of conditions that give rise to CAT.
Because CAT is a form of windshear, any reported wind and/or temperature
change between two places and the presence of certain
weather conditions, such as cumulonimbus clouds, lenticular clouds,
fronts, or jetstreams, are good indications of possible CAT.
Note: In particular, a large, rapid fluctuation in the total air temperature
(approximately ± lOOC) in a few seconds is a very good indication
of CAT, as are broken engine trails of a preceding aircraft.
Air Masses/Pressure Systems/Fronts
What is wake turbulence?
Wake turbulence is the phenomenon of disturbed airflow; i.e., wing-tip
vortices, created behind an aircraft’s wing as the aircraft moves forward.
(See Q: What causes/are wing-tip vortices? page 10.)
Note: Wake turbulence can be a serious hazard to lighter aircraft following heavier aircraft.
Wake vortex turbulence is present behind every aircraft, including
helicopters, when in forward flight. It is extremely hazardous to a lighter
aircraft with a smaller wingspan during takeoff, initial climb, final
approach (including the circuit), and landing phases of flight following
a heavier aircraft with a larger wingspan.
Note: Wake turbulence also can be encountered in the cruise but is
not really a hazard, just an uncomfortable disturbance.
Icing
What is icing?
The formation of ice (icing) is the change of the state of water to a solid form when the temperature is less than the freezing point of water,
i.e.,O°C.
Note: There are three states of water: liquid (water drops), gas
(water vapor), and solid (ice). Ice can form from either of the other two
states of water:
- Water vapor, by sublimation, causing hoar frost.
(See Q: What is sublimation? page 261.) - Water droplets, by freezing rain or by freezing super-cooled water
drops (SWDs).
Note: Airframe icing occurs in freezing clouds when SWDs are present,
causing rime and/or clear ice.
[See Q: What are supercooled water
droplets (SWDs)? page 262.
Icing
What is sublimation?
Sublimation is the process of turning water vapor immediately into ice when the dewpoint/actual temperature is less than O°C.
The usual result of sublimation is hoar frost, which is in fact regarded as a type of icing.
Note: The dewpoint is now called the frost point.
Icing
What are super-cooled water droplets (SWDs)?
Supercooled water droplets (SWDs) are liquid water drops sized between 40 and 2000 nano meters that exist in a nonfrozen liquid form in the atmosphere
at temperatures down to as low as -45°C, well below the normal freezing
point of water (O°C). This is known as being supercooled.
Such supercooled water drops will freeze only partially on contact
with a colder, subzero surface, i.e., the skin of an aircraft or a propeller
blade. Usually, a SWD will turn progressively into ice as it is washed
back along the colder aircraft surface.
This is so because the SWDs
release latent heat (into the remaining water) as it starts to freeze (see
Q: What is latent heat? page 222), which reduces the rate offreezing of
the remaining waterlliquid, allowing it tD stay in a liquid state for longer,
which in turn gets spread backwards by the airflow along the aircraft’s
surface.
As the remaining liquid/water comes into direct contact with
another part of the subzero surface, it in turn freezes progressively.
This can cause long trails of ice to build up on and from the leading
edges, which the deicing systems of the aircraft must be capable of
removing.
For everyone degree of super cooling, %0 of the water drop
will change to ice on impact. Obviously, the larger the supercooled
water drops, the more severe is the icing.
Note: SWDs do not exist below approximately -45°C, where any
moisture is already an ice crystal in the atmosphere, and do not stick
to the aircraft’s airframe.
Therefore, airframe icing is only possible
between 0 and -45°C.
Icing
What conditions present an icing risk?
Icing from water drops that are present in clouds and as rain pose a
risk to aircraft when the tDtal air temperature (in flight) or outside air
temperature (on the ground) is between + lO*C and -40/45°C.
Icing
What hazards to aviation does icing cause?
Ice buildup on an aircraft or within its engine induction system can be
a serious hazard to flight safety, particularly at slow flight phases of
flight, e.g., takeoffs and landings, because of the following effects:
- Adverse aerodynamic performance effects.
- Control surface effects.
- Increase in aircraft weight effects.
- Reduced engine power.
- Vent blockage effects.
- Degraded navigation and radio communication effects.
- Adverse aerodynamic performance effects.
Ice build up on an aircraft’s
wings seliously disrupts the airflow pattern, causing it to
separate at a significantly lower angle of attack than it would from a clean wing.
This results in
a. Reduced stalling angle b. Reduced maximum lift capability c. Increased stall speed d. Increased drag e. Reduced lift (loss of lift) at a given angle attack, especially at higher angles of attack.
- Control surface effects. Ice formation may freeze on control surfaces,
leading to restricted movement and even loss of control in extreme
circumstances. - Increase in aircraft weight effects.
The buildup of ice on an aircraft will increase the overall weight of the aircraft, with all the associated
effects of higher weights, Le., higher stall speed, extra lift required,
etc. In addition, this uncontrolled increase in weight may change…
a. The position of the aircraft’s center of gravity
b. The balance of the various control surfaces and propellers, causing
severe vibration and/or control difficulties.
- Reduced engine power. Ice buildup in the jet engine intake or piston
engine carburetor can restrict the airflow into the engine, causing
a power loss and even engine failure in extreme circumstances.
(See Q: What is carburetor icing? page 264.)
- Vent blockage effects. Ice buildup on pitot and static probes will
produce errors in the aircraft’s pressure-driven flight instruments,
airspeed indicator (ASI), altimeter, and vertical speed indicator
(VSI).
(See Qs: What are the ASI, altimeter and VSI indications and actions for a blocked pitot and/or static probe? pages 129, 131, and 133.)
- Degraded navigation and radio communication effects. If ice builds
up on navigation and radio aerials, then these systems will be
degraded and unreliable.
Icing
What is carburetor icing?
A buildup of ice in a carburetor can disturb that can prevent the flow of air and fuel into the engine manifold, causing
it to lose power, run roughly, and even to stop the engine in extreme circumstances.
This effect is called carburetor icing.
Ice formation can occur in the engine induction system and in the carburetor
of piston engines, particularly in the venturi and around the
throttle valve, where acceleration of the air can produce a temperature
fall by as much as 25°C.
This, combined with the heat absorbed as the
fuel evaporates, can cause serious icing, even when there is no visible
moisture present.
Throttle icing (i.e., around the throttle valve) is more likely to occur at
low power settings (e.g., descents), when the partially closed butterfly
creates its own venturi cooling effect.
Visibility
What is atmospheric/meteorologic visibility?
Meteorological visibility is defined as the greatest horizontal distance at
which a specified object can be seen in daylight conditions. It is therefore
a measure of how transparent the atmosphere is to the human eye.
Visibility
What conditions reduce visibility?
Visibility is reduced whenever particles are present in the atmosphere
that absorb the light, e.g., water, ice, pollution, sand, dust, volcanic
ash, etc.
Poor visibility is usually associated with stable air, where the
moisture and/or atmospheric contamination is kept in situ, especially
at low levels.
Unstable air produces a convection current that carries
moisture and contamination aloft and thus produces good visibility
at low levels.
Visibility may be further reduced by the position of the Sun.
Visibility may be much greater flying “downsun,” where the pilot can
see the sunlit side of objects, than when flying into the sun.
As well as
reducing the visibility, flying into the sun may cause a glare.
If you are
landing into the sun, consideration should be given to altering the time
of arrival.
Visibility
What is the most important visibility to the pilot?
The most important visibility to the pilot is from the aircraft to the ground, i.e., the slant or oblique visibility, especially during and in the direction of takeoff and landing.
This may be very different from
horizontal visibility distance (normal meteorologic measured visibility direction).
For example, low-level stratus, smog, or fog may severely reduce the slant visibility (especially on an approach) when the vertical visibility might be unlimited.
Visibility
How is visibility reported?
Visibility is reported in the following ways:
- General visibility, from overall meteorologic reports
- Runway visual range (RVR) for instrument landings.
General visibility - In general meteorologic reports, the visibility
passed is always the least distance visible from the point of the
observation in all directions.
Visibility is reported in kilometers or,
in very poor conditions, in meters.
Runway visual range - (RVR) (See Q: What is RVR? page 278.)
Visibility
What is smog, and how is it formed?
Smog is a combination of smoke (and/or other airborne particles) and
fog.
(See Q: What is fog? page 231.)
Smog is usually found under an inversion layer, which acts like a blanket,
stopping vertical convective currents. (See Q: What is a temperature
inversion/layer? page 225.)
Climatology
Climatology questions can be about either
(1) General weather
mechanics (which are covered throughout Chapter 8, “Meteorology
and Weather Recognition,” or
2) weather conditions in specific parts
of the world.
Obviously, it is impossible to list the weather phenomena throughout the year for every airport location around the world in this
section. Therefore, it is left to the good sense of the reader to identify
the locations and their weather conditions that he or she is likely to be
asked about.
Climatology
What is the ITCZ?
The intertropical convergence zone (ITCZ) is where converging air
masses meet near the thermal equator.
Like the thermal equator,
ITCZ movement is a function of seasonal heating that is much greater
over the land than over the sea.
Over South America and southern
Africa, ITCZ movement is large, especially in the summer season,
whereas over the Atlantic Ocean, its movement is small.
In other
words, it is stable.
(See Q: What is the thermal equator? page 266.)
The
effect of the ITCZ determines the weather pattern over a significant
portion of the globe.
Climatology
What are tropical revolving storms?
Tropical revolving storms (TRSs) are deep, intense depressions, i.e.,
lows, found in equatorial regions around the intertropical convergence
zone (ITCZ).
(See Qs: What is a pressure system? page 243;
Describe the characteristics of a Low-pressure system, page 248.)
They are known as cyclones in the Indian and Pacific Oceans, hurricanes in the Caribbean
and Americas, and typhoons in the China Sea area.
Tropical revolving storms start at the edge of the ITCZ (see Q: What
is the ITCZ? page 266) as it retreats in late summer or early autumn.
This is so because they are enormous heat engines, and they take their
energy from water vapor off the warm sea, which is condensed aloft,
releasing latent heat.
Sea temperatures of at least 27°C are needed to
form a TRS, which is a temperature that the sea achieves only after
prolonged heating, i.e., after a long, hot summer. For this reason, TRSs
do not form over cold seas and die out when they pass over land or
move to colder sea region.
However, TRSs do not form at the equator
because the coriolis effect is zero at the equator, so they form at
approximately 5 to 20 degrees of latitude, where the sea temperature
is high enough and the coriolis effect is present. [See Q: What is the
coriolis force (or geostrophic force)? page 236.] On crossing land, TRSs
can cause immense damage.
Weather Forecasts and Reports
What is the primary method of preflight meteorologic briefings for aircrew?
The primary method of preflight meteorologic briefing for aircrew in most parts of the aviation world is self-briefing.
This is done in one of
two ways:
- Using facilities, information, and documentation routinely available or displayed in aerodrome briefing areas.
Note: Computerized systems providing specific route meteorologic
conditions are commonplace in most of the major airports around
the world.
- Using a telephone service to call an aviation authority met office;
e.g., area forecasts (TAFS) or reports (METARS) are available
directly from a meteorologic officer, and/or area forecasts are available
from an airmet telephone recording system, especially in the
United Kingdom.
(See Q: What is an AlRMET? page 267.)
Weather Forecasts and Reports
What is an AIRMET?
An AIRMET is a recorded telephone message that gives the meteorologic
forecast for a particular area. It can be accessed by area, and
telephone numbers for different areas are listed on an AIRMET
Areas Chart.
Weather Forecasts and Reports
What is a meteorologic report?
A meteorologic report is an observation of the actual weather at a specific
time, i.e., either past or present.
Weather Forecasts and Reports
What are the common aviation meteorologic reports?
The common aviation meteorologic reports are
- METARS, SIGMETS, and SPECI
(See Qs: What are METARS? page
268; What is a SlGMET? page 269; What are SPECls? page 269.)
- Automatic Terminal Information Service (ATIS)
- In-flight weather reports
(i.e., Volmets, ATIS, and by radio communications
with an air traffic service unit or flight information service)
Weather Forecasts and Reports
What are METARs?
A metar is a written, coded routine aviation weather report for an
aerodrome.
It is an observation of the actual weather given by a meteorologic
observer at the aerodrome.
Note: Cloud base in a metar is given above ground level (AGL)
Weather Forecasts and Reports
Decode the following METAR.
METAR: EGCC 0920Z 210/15 G27 1000SW R24/P1500M SHRA BKN 025 CB 08/06 Q1013 RE TS
WS TK OF RWY 24/NOSIG
EGCC - Location identifier (Manchester, U.K.)
0920Z - Time the report was taken (09.20 hours UTC)
210/15 G27 - Wind direction and speed (210 degrees true at 15 knots, gusting to 27)
1000SW - Horizontal visibility (1000 m to the southwest)
R24/P1500M - Runway visual range (runway 24 plus 1500 m of visibility)
SHRA - Weather (rain showers)
BKN 025 CB - Clouds (broken at 2500 ft with cumulonimbus clouds)
08/06 - Temperature/dewpoint (8°C temperature, 6°C dewpoint)
Q1013 - QNH (1013 millibars or 29.92 in)
RE TS - Recent weather (recent thunderstorms)
WS TK OF RWY 24 - Windshear (windshear report on takeoff runway 24)
NOSIG - Trend (no significant change)
Weather Forecasts and Reports
What does trend mean in a meteorologic report?
A weather trend is usually attached to an aerodrome weather report,
i.e., METAR (see Q: What are METARs? page 268), and is commonly
referred to as a landing forecast.
The trend is a forecast of any significant weather changes expected
in the next 2 hours after the time of the report and is described using the normal coded weather format and abbreviations.
If no significant
change is expected, the observation (report) will be followed by NOSIG
(no significant change) as the trend.
Because a trend forecast period is much shorter than a normal aerodrome
forecast; i.e., a TAF, it should be much more accurate.
Note: Cloud bases in a trend are given above aerodrome level (AAL).
A trend can only be given by a qualified forecaster, whereas a report can be given by just an observer.
Weather Forecasts and Reports
What is a SIGMET?
A SIGMET is a meteorologic report that advises of significant meteorologic
(SIG/MET) conditions that may affect the safety of flight operations
in a general geographic area, i.e., en route or at an aerodrome.
The criteria for raising a SIGMET include
1. Active thunderstorms
- Tropical revolving storms
- Severe line squalls
- Heavy rain
- Severe turbulence
- Severe airframe icing
- Marked mountain waves
- Widespread dust or sandstorms
Weather Forecasts and Reports
What are SPECIs?
A SPECI is an Aviation Selected Special Weather Report for an aerodrome.
It is generated whenever a critical meteorologic condition
exists, e.g., windshear, microbursts, etc. It is similar in presentation to a METAR.
Weather Forecasts and Reports
What in-flight weather reports can you access?
In-flight weather reports that can be accessed include
- Flight information service or air traffic (control) service
- ATIS (See Q: What is ATIS? page 26.9.)
- VOLMET (See Q: What is VOLMET? page 270.)
Weather Forecasts and Reports
What is ATIS?
The Automatic Terminal Information Service (ATIS) is a prerecorded tape broadcast on an appropriate VOR or VHF channel to reduce the workload on air traffic control (ATC) communications frequencies that
give current information on aerodrome operations and weather.
Note: Some of the larger aerodromes have both an arrival and a departure ATIS.
The ATIS message is changed with any significant change in the reported conditions, and each new message has a new alphabetical designator
prefix, e.g., alpha, beta, etc., to distinguish the current from the old.
Weather Forecasts and Reports
What is VOLMET?
VOLMET is a continuous broadcast on a VHF/HF frequency that includes
- The actual weather report
- The landing forecast
- A forecast trend for the 2 hours following
- A SIGMET (significant weather, if any) of several selected aerodromes that produce meteorologic reports within a given region.
Weather Forecasts and Reports
What is a meteorologic forecast?
A forecast is a prediction, or prognosis, of what the weather is likely to be for a given route, area, or aerodrome.
Weather Forecasts and Reports
What are the common types of aviation forecasts?
The common types of aviation forecasts are…
- Area forecasts for preflight briefings (Note that cloud bases in area
forecasts are given above mean sea level.) - Aerodrome forecasts (e.g., TAFS and trends)
(See Qs: What is a TAF? page 270; What does trend mean in a meteorologic report? page 268.)
- Special forecasts (See Q: What is a special forecast, and when is it
requested? page 271.)
Weather Forecasts and Reports
What is a TAF?
A Terminal Aerodrome Forecast (TAF) is a coded routine weather forecast
for an aerodrome.
It is a weather forecast given by a qualified
meteorologic forecaster based at the aerodrome.
Note: Cloud base in a TAF is given above aerodrome level (AAL).
TAFs are usually issued for a 9-hour period and updated every 3
hours.
However, they may be issued for up to 24 hours 1iII-ith updates
every 6 hours, but their accuracy is not as high.
Weather Forecasts and Reports
Decode the following TAF:
TAF: EGCC 150600 0716 210/15 5000 SH RA BKN025 TEMPO 1116 3000
SH RA PROB30 1416 TS RA BKN 006CB.
EGCC - Location identifier (Manchester, u.K.)
150600 - Time the report was taken (15th day, 0600 hours UTC)
0716 - Validity time of forecast (0700 to 1600 hours UTC)
210/15 - Wind direction and speed (210 degrees true at 15 knots)
5000 - Horizontal visibility (5000 m)
SH RA - Weather (rain showers)
BKN025 - Clouds (broken at 2500 ft)
TEMPO - Variation (See Q: What is the definition of Tempo? page 271.)
1116 - Validity time of TEMPO (1100 to 1600 hours UTC)
SH RA - Weather (rain showers)
PROB30 - Probability (See Q: Wha.t is the definition of PROB? pa.ge 272.)
1416 - Validity time of PROB (1400 to 1600 hours UTC)
TS RA - Weather (thunderstorm, rain showers)
BKN 006CB Clouds (broken at 600 ft with cumulonimbus clouds)
Weather Forecasts and Reports
What is a special forecast, and when is it requested?
Special forecasts are meteorologic forecasts for flights over long routes outside the coverage of the local countries’ area forecast.
Note: Special forecasts also can include forecasts (TAFS) for the
departure, destination, and up to three alternative aerodromes.
Weather Forecasts and Reports
What is the definition of BECMG in a forecast?
BECMG means “Becoming” and is followed by a four-figure time group, which is two different whole UTC hours.
It indicates a permanent change in the forecasted conditions occurring at some time during the specified period.
Weather Forecasts and Reports
What is the definition of TEMPO in a forecast?
TEMPO relates to a temporary variation in the general forecasted weather lasting less than 1 hour or, if recurring, lasting in total less
than half the trend or TAF period it is included within.
Note: A TEMPO can relate to improvements as well as deterioration in wind, visibility, weather, or clouds.
Once the TEMPO weather events have finished, the original prevailing weather reasserts itself.
Weather Forecasts and Reports
What is the definition of INTER in a forecast?
INTER relates to intermittent variations in the general forecasted weather that are more frequent than TEMPOs, i.e., conditions fluctuating
almost constantly.
Note: An INTER can relate to improvements as well as deteriorations in wind, visibility, weather, or clouds.
Once the INTER weather events are finished, the original prevailing weather reasserts itself.
Weather Forecasts and Reports
What is the definition of GRADU in a forecast?
GRADU relates to gradual change in the original weather at approximately a constant rate throughout the period or during a specified part
thereof to a different and new weather condition.
Weather Forecasts and Reports
What is the definition of RAPID in a forecast?
RAPID relates to a rapid change, in less than half an hour (30 minutes),
of the original weather to a new and different prevailing
weather condition.
Weather Forecasts and Reports
What is the definition of PROB in a forecast?
PROB is used in weather forecasts when the forecaster is uncertain if the weather conditions will occur or not, and therefore, he or she assesses the probability of them occurring as less than 50 percent. (If greater than
50 percent, then it would be listed as a TEMPO.)
For example, PROB30
means a 30 percent chance of the weather conditions occurring.
Weather Forecasts and Reports
What is the definition of CAVOK?
When the following conditions occur simultaneously:
- Visibility equal or greater than 10 km
- No clouds below 5000 ft or below the highest minimum sector safe altitude (MSA), whichever is the greater, and no cumulonimbus clouds at any altitude
- No precipitation, thunderstorms, shallow fog, or low drifting snow then the term CAVOK is used to replace visibility, RVR, weather, and clouds in meteorologic reports and forecasts.
Note: CAVOK does not mean clear blue skies.
