8 - Meteorology and Weather Recognition Flashcards

Question

Moisture and Clouds Describe how clouds are formed

Answer 1

For cloud formation to be possible, the following properties must exist: 1. Moisture present in the air. 2. 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 3. 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.

Answer 2

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.

Answer 3

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.

Answer 4

There are four main groups of clouds: 1. Curriform, or fibrous 2. Cumuliform, or heaped 3. Stratiform, or layered 4. 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): 1. Cirro, or high-level cloud: (cloud base> 16,500-20,000 ft 2. Alto, or medium-level cloud: cloud base> 6500 ft 3. 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.)

Answer 5

Cumulonimbus clouds (CBs).

Answer 6

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.)

Answer 7

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.

Answer 8

The most common different types of fog are 1. Radiation fog 2. Advection fog, including sea fog 3. Frontal fog, including hill fog

Answer 9

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.

Answer 10

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.

Answer 11

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.

Answer 12

Dew is a water cover on the earth's surface that is formed when the following conditions exist: 1. 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. 2. Moist air. With a high relative humidity that only requires a slight cooling to reach its dewpoint temperature. 3. 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.

Answer 13

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.

Answer 14

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.

Answer 15

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: 1. A high moisture content in the air. 2. 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 3. Adiabatic cooling of the rising air. 4. 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. 1. 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. 2. 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 ``` 3. 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.

Answer 16

Thunderstorms can produce the following hazards to all aircraft types: 1. 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.) 2. Severe turbulence, which can cause a. Possible loss of control b. Possible structural damage (See Q: What is turbulence? page 253.) 3. 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.) 4. Airframe structural damage from hail 5. Reduced visibility 6. Lightning strikes, which can cause damage to the electrical system 7. 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.

Answer 17

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.

Answer 18

Wind is expressed in terms of direction and strength. 1. 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. 2. Wind strength. This is measured and expressed in nautical knots.

Answer 19

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.

Answer 20

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.)

Answer 21

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.

Answer 22

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.)

Answer 23

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.

Answer 24

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.

Answer 25

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.)

Answer 26

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.

Answer 27

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.

Answer 28

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.

Answer 29

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... 1. 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. 2. 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.

Answer 30

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.

Answer 31

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.)

Answer 32

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.

Answer 33

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).

Answer 34

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.

Answer 35

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.)

Answer 36

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.

Answer 37

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.

Answer 38

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.

Answer 39

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.

Answer 40

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.)

Answer 41

A low-pressure system, which is also known as a depression, has the following characteristics: 1. 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. 2. 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 3. Wind direction. (See Q: What is the wind direction around a low pressure system? page 244.) 4. 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. 5. 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. 6. Weather. (See Qs: Describe the weather associated with a depression, page 244; What are the different types of depressions? page 244.)

Answer 42

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.)

Answer 43

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: 1. Frontal depression 2. Thermal depression 3. Tropical storm depressions 4. Orographic depression

Answer 44

In general, the weather associated with a low-pressure system is as follows: 1. 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. 2. 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. 3. 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. 4. 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.)

Answer 45

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.

Answer 46

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).

Answer 47

A high-pressure system, which is also known as an anticyclone, has the following characteristics: 1. 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. 2. 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 3. Wind direction. (See Q: What is the wind direction around a high-pressure system? page 247.) 4. Airmass. A high-pressure system usually will be made up of only one air mass and therefore wiII have no defined fronts. 5. 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. 6. Weather. (See Q: Describe the weather associated with a high-pressure system, page 247.)

Answer 48

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.

Answer 49

In general, the weather associated with a high-pressure system is as follows: 1. 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. 2. 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. 3. 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.

Answer 50

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.)

Answer 51

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).

Answer 52

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).

Answer 53

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.)

Answer 54

A front is a boundary between two different air masses.

Answer 55

Frontal activity describes the interaction between at least two air masses as one air mass replaces another.

Answer 56

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.

Answer 57

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.

Answer 58

The passage of a warm front has the following general characteristics and associated weather: 1. 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. 2. 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.

Answer 59

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).

Answer 60

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.

Answer 61

The passage of a cold front has the following general characteristics and associated weather: 1. 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.

Answer 62

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.

Answer 63

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 1. Convection turbulence, caused by solar radiation heating the ground and producing rising thermal currents 2. Obstruction and orographic (terrain-generated) turbulence The other main forms of turbulence (considered to be horizontal) are.... 1. Jetstreams 2. 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.)

Answer 64

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.

Answer 65

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.

Answer 66

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.) 2. Frontal passage 3. 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

Answer 67

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: 1. 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. 2. 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). ```

Answer 68

Windshear is a change of wind speed and/or direction (including vertical updrafts/downdrafts) that affects the lift capability of an aircraft.

Answer 69

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.

Answer 70

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.

Answer 71

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.

Answer 72

The typical indications of a microburst are 1. Mature cumulonimbus clouds with thunderstorm activity, especially rain 2. 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. 3. 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. 4. Flight path and indicated airspeed (lAS) fluctuations, especially on the approach path (See Q: How does a microburst affect an aircraft? page 258.) 5. Wind direction and speed changes, which can be either reported or measured by aircraft systems, if fitted

Answer 73

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. 1. 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. 2. 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. 3. 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.)

Answer 74

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.

Answer 75

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.

Answer 76

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: 1. 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) 2. 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.) 3. Fronts. An active front produces a marked horizontal windshear, which can produce CAT. 4. 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.) 5. Wake turbulence. (See Q: What is wake turbulence? page 260.)

Answer 77

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.

Answer 78

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.

Answer 79

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: 1. Water vapor, by sublimation, causing hoar frost. (See Q: What is sublimation? page 261.) 2. 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.

Answer 80

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.

Answer 81

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.

Answer 82

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.

Answer 83

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: 1. Adverse aerodynamic performance effects. 2. Control surface effects. 3. Increase in aircraft weight effects. 4. Reduced engine power. 5. Vent blockage effects. 6. Degraded navigation and radio communication effects. 1. 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. 2. Control surface effects. Ice formation may freeze on control surfaces, leading to restricted movement and even loss of control in extreme circumstances. 3. 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. 4. 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.) 5. 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.) 6. Degraded navigation and radio communication effects. If ice builds up on navigation and radio aerials, then these systems will be degraded and unreliable.

Answer 84

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.

Answer 85

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.

Answer 86

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.

Answer 87

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.

Answer 88

Visibility is reported in the following ways: 1. General visibility, from overall meteorologic reports 2. 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.)

Answer 89

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.)

Answer 90

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.

Answer 91

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.

Answer 92

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.

Answer 93

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: 1. 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. 2. 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.)

Answer 94

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.

Answer 95

A meteorologic report is an observation of the actual weather at a specific time, i.e., either past or present.

Answer 96

The common aviation meteorologic reports are 1. METARS, SIGMETS, and SPECI (See Qs: What are METARS? page 268; What is a SlGMET? page 269; What are SPECls? page 269.) 2. Automatic Terminal Information Service (ATIS) 3. In-flight weather reports (i.e., Volmets, ATIS, and by radio communications with an air traffic service unit or flight information service)

Answer 97

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)

Answer 98

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)

Answer 99

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.

Answer 100

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 2. Tropical revolving storms 3. Severe line squalls 4. Heavy rain 5. Severe turbulence 6. Severe airframe icing 7. Marked mountain waves 8. Widespread dust or sandstorms

Answer 101

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.

Answer 102

In-flight weather reports that can be accessed include 1. Flight information service or air traffic (control) service 2. ATIS (See Q: What is ATIS? page 26.9.) 3. VOLMET (See Q: What is VOLMET? page 270.)

Answer 103

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.

Answer 104

VOLMET is a continuous broadcast on a VHF/HF frequency that includes 1. The actual weather report 2. The landing forecast 3. A forecast trend for the 2 hours following 4. A SIGMET (significant weather, if any) of several selected aerodromes that produce meteorologic reports within a given region.

Answer 105

A forecast is a prediction, or prognosis, of what the weather is likely to be for a given route, area, or aerodrome.

Answer 106

The common types of aviation forecasts are... 1. Area forecasts for preflight briefings (Note that cloud bases in area forecasts are given above mean sea level.) 2. 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.) 3. Special forecasts (See Q: What is a special forecast, and when is it requested? page 271.)

Answer 107

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.

Answer 108

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)

Answer 109

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.

Answer 110

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.

Answer 111

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.

Answer 112

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.

Answer 113

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.

Answer 114

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.

Answer 115

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.

Answer 116

When the following conditions occur simultaneously: 1. Visibility equal or greater than 10 km 2. 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 3. 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.