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.
