Chapter 1
Weather

The Earth’s Atmosphere

We classify the atmosphere into layers, or spheres, by characteristics exhibited in these layers. The troposphere is the layer from the surface to an average altitude of about 7 miles (37,000 feet). It is characterized by an overall decrease of temperature with increasing altitude. The height of the troposphere varies with latitude and season. It slopes from about 20,000 feet over the poles to about 65,000 feet over the Equator; and it is higher in summer than in winter.

At the top of the troposphere is the tropopause, a very thin layer marking the boundary between the troposphere and the layer above. It is characterized by an abrupt change in temperature lapse rate.

Above the tropopause is the stratosphere. This layer is typified by relatively small changes in temperature with height except for a warming trend near the top. See Figure 1-1.

Figure 1-1. Layers of the atmosphere

High Altitude Weather

The jet stream is a river of high speed winds (50 knots or more) associated with the tropopause. The location of the jet stream changes seasonally. In the winter, the jet stream moves south and increases in velocity. During the summer, the jet stream moves north and slows.

Temperature

The major source of all weather is the sun. Changes or variations of weather patterns are caused by the unequal heating of the Earth’s surface. In aviation, surface and aloft temperature is measured in degrees Celsius (°C).

Standard temperature is 15°C at sea level. To calculate International Standard Atmosphere (ISA), use the average lapse rate of 2°C per 1,000 feet.

Wind

The rules in the Northern Hemisphere are:

1. Air circulates in a clockwise direction around a high pressure system.

2. Air circulates in a counterclockwise direction around a low pressure system.

3. The closer the isobars are together, the stronger the wind speed.

4. Due to surface friction (up to about 2,000 feet AGL), surface winds do not exactly parallel the isobars, but move outward from the center of the high toward lower pressure.

5. Coriolis force is at a right angle to wind direction and directly proportional to wind speed. The force deflects air to the right in the Northern Hemisphere.

Figure 1-2. Gradient and surface wind

Moisture and Precipitation

Air contains moisture (water vapor). The water vapor content of air can be expressed in two different ways: relative humidity and dew point.

Relative humidity relates the actual water vapor present in the air to that which could be present. Temperature largely determines the maximum amount of water vapor the air can hold. Warm air can hold more water vapor than cold air can. Air with 100 percent relative humidity is said to be saturated, and air with less than 100 percent is unsaturated.

Dew point is the temperature to which air must be cooled to become saturated by the water already present in the air.

When water vapor condenses on large objects, such as leaves, windshields, or airplanes, it will form dew. When it condenses on microscopic particles, such as salt, dust, or combustion by-products (condensation nuclei), it will form clouds or fog.

If the temperature and dew point spread is small and decreasing, condensation is about to occur. If the temperature is above freezing, fog or low clouds will be most likely to develop.

The growth rate of precipitation is enhanced by upward currents. Cloud particles collide and merge into a larger drop in the more rapid growth process. This process produces larger precipitation particles and does so more rapidly than the simple condensation growth process. Upward currents also support larger drops.

If wet snow is encountered at your flight altitude, then the temperature is above freezing at your altitude. Since melting snow has been encountered, the freezing level must be at a higher altitude.

The presence of ice pellets at the surface is evidence that there is freezing rain at a higher altitude. Rain falling through colder air may become supercooled, freezing on impact as freezing rain; or it may freeze during its descent, falling as ice pellets.

Air Masses, Fronts, Stability

Atmospheric stability is defined as the resistance of the atmosphere to vertical motion. A stable atmosphere resists any upward or downward movement. An unstable atmosphere allows an upward or downward disturbance to grow into a vertical (convective) current.

Characteristics of unstable air are: cumuliform clouds, showery precipitation, rough air (turbulence), and good visibility, except in blowing obstructions. Characteristics of stable air are: stratiform clouds and fog, continuous precipitation, smooth air, and fair to poor visibility in haze and smoke.

Determining the stability of the atmosphere requires measuring the difference between the actual existing (ambient) temperature lapse rate of a given parcel of air and the dry adiabatic (3°C per 1,000 feet) lapse rate.

When a body of air (or air mass) comes to rest, or moves slowly over a large geographical area that has fairly uniform properties of temperature and moisture, the air mass acquires the temperature/moisture properties of that area it covers.

As air masses move out of their source regions, they come in contact with other air masses of different properties. The zone between two different air masses is a frontal zone or front. Across this zone, temperature, humidity and wind often change rapidly over short distances. The three principal types of fronts are the cold front, the warm front, and the stationary front. Frontal waves and cyclones (areas of low pressure) usually form on slow-moving cold fronts or stationary fronts.

Figure 1-3. Properties of unstable and stable air

Figure 1-4. Temperature effects on ambient lapse rate

Clouds

Cloud types are divided into four families: high clouds, middle clouds, low clouds, and clouds with extensive vertical development. The first three families are further classified according to the way they are formed. The high cloud family is cirriform and includes cirrus, cirrocumulus, and cirrostratus. They are composed almost entirely of ice crystals. Clouds formed by vertical currents in unstable air are cumulus, meaning “accumulation or heap.” They are characterized by their lumpy, billowy appearance. Clouds formed by the cooling of a stable layer are stratus, meaning “stratified or layered.” They are characterized by their uniform, sheet-like appearance.

In addition to the above, the prefix nimbo or the suffix nimbus means “raincloud.” A heavy, swelling cumulus type cloud which produces precipitation is a cumulonimbus. The high cloud family is cirriform and composed almost entirely of ice crystals.

Lenticular clouds are associated with a mountain wave. Crests of standing waves may be marked by stationary, lens-shaped clouds known as standing lenticular clouds. Mountain waves and lenticular clouds indicate strong turbulence.

Fog 

Fog is a surface-based cloud composed of either water droplets or ice crystals. Fog is the most frequent cause of surface visibility below 3 miles, and is one of the most common and persistent weather hazards encountered in aviation. Fog may form by cooling the air to its dew point or by adding moisture to the air near the ground. A small temperature/dew point spread is essential to the formation of fog. An abundance of condensation nuclei from combustion products makes fog prevalent in industrial areas. Fog is classified by the way it is formed:

1. Radiation fog (ground fog) is formed when terrestrial radiation cools the ground, which in turn cools the air in contact with it. When the air is cooled to its dew point (or within a few degrees), fog will form. This fog will form most readily in warm, moist air over low, flatland areas on clear, calm nights.

2. Advection fog (sea fog) is formed when warm, moist air moves (wind is required) over colder ground or water. An example is an air mass moving inland from the coast in winter.

3. Upslope fog is formed when moist, stable air is cooled to its dew point as it moves (wind is required) up sloping terrain. Cooling will be at the dry adiabatic lapse rate of approximately 3°C per 1,000 feet.

Turbulence

If severe turbulence is encountered either inside or outside of clouds, the airplane’s airspeed should be reduced to maneuvering speed and the pilot should attempt to maintain a level flight attitude because the amount of excess load that can be imposed on the wing will be decreased. An attempt to maintain a constant altitude will greatly increase the stresses that are applied to the aircraft. See Figure 1-5.

Figure 1-5. Turbulence Reporting Criteria

Thunderstorms

A thunderstorm is a local storm produced by a cumulonimbus cloud. It is always accompanied by lightning and thunder, usually with strong gusts of wind, heavy rain, and sometimes, with hail. Three conditions necessary for the formation of a thunderstorm are: sufficient water vapor, an unstable lapse rate, and an initial upward boost (lifting). The initial upward boost can be caused by heating from below, frontal lifting, or by mechanical lifting (wind blowing air upslope on a mountain).

There are three stages of a thunderstorm:

1. The cumulus stage is characterized by continuous updrafts, and these updrafts create low-pressure areas.

2. The mature stage is characterized by updrafts and downdrafts inside the cloud. Precipitation inside the cloud aids in the development of these downdrafts, and the start of rain from the base of the cloud signals the beginning of the mature stage. Thunderstorms reach their greatest intensity during the mature stage.

3. The dissipating stage is characterized predominantly by downdrafts.

Thunderstorms that generally produce the most intense hazard to aircraft are called squall-line thunderstorms. These nonfrontal, narrow bands of thunderstorms often develop ahead of a cold front.

Embedded thunderstorms are those that are obscured by massive cloud layers and cannot be seen.

Microbursts

Microbursts are small-scale intense downdrafts which, as they get near the ground, spread outward from the center in all directions. Maximum downdrafts at the center of a microburst may be as strong as 6,000 feet per minute. As your aircraft approaches, encounters, and exits a microburst it will experience a sudden increase in performance due to a headwind followed by a strong downdraft at the microburst center and then a severe loss of performance as the downdraft becomes a sudden tailwind. Since the initial headwind may be 45 knots and the tailwind also 45 knots, the total wind shear passing across the microburst would be 90 knots.

An individual microburst will seldom last longer than 15 minutes from the time it strikes the ground until dissipation. The horizontal winds continue to increase during the first 5 minutes with the maximum intensity winds lasting approximately 2–4 minutes.

Icing

One of the hazards to flight is aircraft icing. Pilots should be aware of the conditions conducive to icing, the types of icing, the effects of icing on aircraft control and performance, and the use and limitations of aircraft deice and anti-ice equipment. For ice to form, there must be moisture present in the air and the air must be cooled to a temperature of 0°C (32°F) or less. Aerodynamic cooling can lower the temperature of an airfoil to 0°C even though the ambient temperature is a few degrees warmer.

Ice is identified as clear, rime, or mixed. Rime ice forms if the droplets are small and freeze immediately when contacting the aircraft surface. This type of ice usually forms on areas such as the leading edges of wings or struts. It has a somewhat rough looking appearance and is a milky white color. Clear ice is usually formed from larger water droplets or freezing rain that can spread over a surface. This is the most dangerous type of ice since it is clear, hard to see, and can change the shape of the airfoil. Mixed ice is a mixture of clear ice and rime ice. It has the bad characteristics of both types and can form rapidly.

Wind Shear

Wind shear is defined as a change in wind direction and/or speed in a very short distance in the atmosphere. This can occur at any level of the atmosphere and can be detected by the pilot as a sudden change in airspeed.

Low-level (low altitude) wind shear can be expected during strong temperature inversions, on all sides of a thunderstorm and directly below the cell. Low-level wind shear can also be found near frontal activity because winds can be significantly different in the two air masses which meet to form the front.

In warm front conditions, the most critical period is before the front passes. Warm front shear may exist below 5,000 feet for about 6 hours before surface passage of the front. The wind shear associated with a warm front is usually more extreme than that found in cold fronts.

The shear associated with cold fronts is usually found behind the front. If the front is moving at 30 knots or more, the shear zone will be 5,000 feet above the surface 3 hours after frontal passage.

There are two potentially hazardous shear situations:

1. Loss of tailwind—A tailwind may shear to either a calm or headwind component. In this instance, initially the airspeed increases, the aircraft pitches up, and altitude increases. Lower than normal power would be required initially, followed by a further decrease as the shear is encountered, and then an increase as glide slope is regained.

2. Loss of headwind—A headwind may shear to a calm or tailwind component. Initially, the airspeed decreases, the aircraft pitches down, and altitude decreases.