The major source of all weather is the sun. Every physical process of weather is accompanied by, or is a result of, unequal heating (heat exchange) of the Earth’s surface. The heating of the Earth (and therefore the heating of the air in contact with the Earth) is unequal around the entire planet. Either north, south, east or west of a point directly under the sun, one square foot of sunrays is not concentrated over one square foot of the surface, but over a larger area. This lower concentration of sunrays produces less radiation (absorption) of heat over a given surface area and therefore, less atmospheric heating takes place in that area.
The unequal heating of the Earth’s atmosphere creates a large air cell circulation pattern (wind) because the warmer air has a tendency to rise (low pressure) and the colder air has a tendency to settle or descend (high pressure) and replace the rising warmer air. This unequal heating, which causes pressure variations, will also cause variations in altimeter settings between weather reporting points.
Because the Earth rotates, this large, simple air cell circulation pattern is greatly distorted by a phenomenon known as the Coriolis force. When wind—which is created by pressure differences, horizontal pressure gradient, and high pressure trying to flow into low pressure—first begins to move at higher altitudes, the Coriolis force deflects it to the right (in the Northern Hemisphere). This causes it to flow parallel to the isobars (lines of equal pressure). The Coriolis force prevents air from flowing directly from high-pressure areas to low-pressure areas because it tends to counterbalance the horizontal pressure gradient. These deflections of the large-cell circulation pattern create general wind patterns as depicted in Figure 6-1.

Figure 6-1. Prevailing wind systems
The jet stream is a river of high-speed winds (by definition, 50 knots or more) associated with a layer of atmosphere called the tropopause. The tropopause is actually the boundary layer between the troposphere and the stratosphere. Within the troposphere, temperature decreases with altitude, while the stratosphere is characterized by relatively small temperature changes. The tropopause itself is found between the two layers and is marked by an abrupt change in the temperature lapse rate.
The troposphere varies in height from around 65,000 feet at the equator to about 20,000 feet over the poles, averaging about 37,000 feet in the mid-latitudes. It also is higher in the summer than in the winter. The height of the tropopause does not change uniformly, but rather tends to change in “steps.” The jet stream is often found at or near these steps. Since the tropopause height also changes with the seasons, the location of the jet stream changes seasonally. In the winter, the jet stream moves south and increases in speed, and during the summer, the jet stream moves north and decreases in speed.
The strong winds of the jet stream create narrow zones of wind shear which often generate hazardous turbulence. The jet stream maximum is not constant; rather, it is broken into segments, shaped something like a boomerang. Jet stream segments move with pressure ridges and troughs in the upper atmosphere.
A common location of clear air turbulence (CAT) and strong wind shear exists with a curving jet stream. This curve is created by an upper or lower low-pressure trough. The wind speed, shown by isotachs (lines of constant wind speed), decreases outward from the jet core. The greatest rate of decrease of wind speed is on the polar side as compared to the equatorial side. Strong wind shear and CAT can be expected on the low-pressure side or polar side of a jet stream where the speed at the core is greater than 110 knots. Air travels in a corkscrew path around the jet core with upward motion on the equatorial side. When high-level moisture is present, cirriform (cirrus) clouds may be visible, identifying the jet stream along with its associated turbulence.
In aviation, temperature is measured in degrees Celsius (°C). The standard temperature at sea level is 15°C (59°F). The average decrease in temperature with altitude (standard lapse rate) is 2°C (3.5°F) per 1,000 feet. Since this is an average, the exact value seldom exists; in fact, temperature sometimes increases with altitude—this is known as an inversion. The most frequent type of ground- or surface-based temperature inversion is one that is produced on clear, cool nights, with calm or light wind. See Figure 6-2.

Figure 6-2. Temperature inversions
The circulation patterns for high- and low-pressure areas are caused by the Coriolis force.
The general circulation and wind rules in the Northern Hemisphere are:

Figure 6-3. Gradient and surface wind
For preflight planning, it is useful to know that air flows out and downward (or descends) from a high-pressure area in a clockwise direction and flows upward (rises) and into a low-pressure area in a counterclockwise direction. Assume a flight from point A to point B as shown in Figure 6-4. Going direct would involve fighting the wind flowing around the low. However, by traveling south of the low-pressure area, the circulation pattern could help instead of hinder. Generally speaking, in the Northern Hemisphere, when traveling west to east, the most favorable winds can be found by flying north of high-pressure areas and south of low-pressure areas. Conversely, when flying east to west, the most favorable winds can be found south of high-pressure areas and north of low-pressure areas. If flying directly into a low-pressure area in the Northern Hemisphere, the wind direction and speed will be from the left and increasing.

Figure 6-4. Circulation and wind
Air contains invisible water vapor. Water vapor content or 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 in the air. Temperature largely determines the maximum amount of water vapor air can hold. Warm air can hold more water vapor than cold air. See Figure 6-5. Air with 100% relative humidity is said to be saturated, and air with less than 100% is unsaturated.

Figure 6-5. Capacity of air to hold water
Dew point is the temperature to which air must be cooled to become saturated by the water already present in the air. See Figure 6-6.

Figure 6-6. Relative humidity and dew point
Water vapor can be added to the air by either evaporation or sublimation. Water vapor is removed from the air by either condensation or sublimation.
When water vapor condenses on large objects, such as leaves, windshields, or airplanes, it will form dew, and when it condenses on microscopic particles (condensation nuclei), such as salt, dust, or combustion by-products, 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, the weather most likely to develop will be fog or low clouds.
To summarize, relative humidity can be increased either by lowering the air temperature or by increasing the amount of water vapor in the air. This causes a decreased air temperature and temperature/dew point spread as the relative humidity increases.
Atmospheric stability is defined as the resistance of the atmosphere to vertical motion. A stable atmosphere resists an upward or downward movement. An unstable atmosphere allows an upward or downward disturbance to grow into a vertical (convective) current.
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 rate (a constant 3°C per 1,000 feet lapse rate).
A stable layer of air would be associated with a temperature inversion. Warming from below, on the other hand, would decrease the stability of an air mass.
The conditions shown in Figure 6-7 are characteristic of stable or unstable air masses.

Figure 6-7. Characteristics of air masses
Stability determines which of two types of clouds will be formed: cumuliform or stratiform.
Cumuliform clouds are the billowy-type clouds having considerable vertical development, which enhances the growth rate of precipitation. They are formed in unstable conditions, and they produce showery precipitation made up of large water droplets. See Figure 6-8.

Figure 6-8. Cumulus clouds
Stratiform clouds are the flat, more evenly based clouds formed in stable conditions. They produce steady, continuous light rain and drizzle made up of much smaller raindrops. See Figure 6-9.

Figure 6-9. Stratiform clouds
Steady precipitation (in contrast to showery) preceding a front is an indication of stratiform clouds with little or no turbulence.
Clouds are divided into four families according to their height range: low, middle, high, and clouds with extensive vertical development. See Figure 6-10.

Figure 6-10. Cloud families
The first three families—low, middle, and high—are further classified according to the way they are formed. Clouds formed by vertical currents (unstable) are cumulus (heap) and are billowy in appearance. Clouds formed by the cooling of a stable layer are stratus (layered) and are flat and sheet-like in appearance. A further classification is the prefix “nimbo-” or suffix “-nimbus,” which means raincloud. High clouds, called cirrus, are composed mainly of ice crystals; therefore, they are least likely to contribute to structural icing (since it requires water droplets).
The base of a cloud (AGL) that is formed by vertical currents (cumuliform clouds) can be roughly calculated by dividing the difference between the surface temperature and dew point by 4.4 and multiplying the remainder by 1,000. The convergence of the temperature and the dew point lapse rate is 4.4°F per 1,000 feet.
Problem:
What is the approximate base of the cumulus clouds if the temperature at 2,000 feet MSL is 10°C and the dew point is 1°C?
Solution:
In a convection current, the temperature and dew point converge at about 2.5°C per 1,000 feet. An estimate of convective cloud bases can be found by dividing the convergence into the temperature spread.
When a body of air comes to rest on, or moves slowly over, an extensive area having fairly uniform properties of temperature and moisture, the air takes on these properties. The area from which the air mass acquires its identifying distribution of temperature and moisture is its source region. As this air mass moves from its source region, it tends to take on the properties of the new underlying surface. The trend toward change is called air mass modification.
A ridge is an elongated area of high pressure. A trough is an elongated area of low pressure. All fronts lie in troughs. A cold front is the leading edge of an advancing cold air mass. A warm front is the leading edge of an advancing warm air mass. Warm fronts move about half as fast as cold fronts. Frontal waves and cyclones (areas of low pressure) usually form on slow-moving cold fronts or stationary fronts. Figure 6-11 shows the symbols that would appear on a weather map.

Figure 6-11. Weather map symbols
The physical manifestations of a warm or cold front can be different with each front. They vary with the speed of the air mass on the move and the degree of stability of the air mass being overtaken. A stable air mass forced aloft will continue to exhibit stable characteristics, while an unstable air mass forced to ascend will continue to be characterized by cumulus clouds, turbulence, showery precipitation, and good visibility.
Occlusions form because cold fronts move faster than warm fronts. In a cold front occlusion, the coldest air is under the cold front. When it overtakes the warm front, it lifts the warm front aloft and the cold air replaces cool air at the surface.
Frontal passage will be indicated by the following discontinuities:
Cumulus clouds are formed by convective currents (heating from below). Therefore, a pilot can expect turbulence below or inside cumulus clouds, especially towering cumulus clouds. The greatest turbulence could be expected inside cumulonimbus clouds. Strong winds (35+ knots) across ridges and mountain ranges can also cause severe turbulence and severe downdrafts on the lee side. The greatest potential danger from turbulent air currents exists when flying into the wind while on the leeward side of ridges and mountain ranges. See Figure 6-12.

Figure 6-12. Mountain turbulence
Winds blowing across a mountain may produce an almond- or lens-shaped cloud (lenticular cloud), which appears stationary, but which may contain winds of 50 knots or more. The presence of these clouds is an indication of very strong turbulence. The stationary crests of standing mountain waves downwind of a mountain also resemble the almond or lens shape and are referred to as standing lenticular clouds. Favorable conditions for a strong mountain wave consist of a stable layer of air being disturbed by the mountains with winds of at least 20 knots across the ridge. One of the most dangerous features of mountain waves is the turbulent areas in and below rotor clouds that form under lenticular clouds.
Structural icing occurs on an aircraft whenever supercooled droplets of water make contact with any part of the aircraft that is also at a temperature below freezing. An inflight condition necessary for structural icing to form is visible moisture (clouds or raindrops).
Icing in precipitation (rain) is of concern to the VFR pilot because it can occur outside of clouds. Aircraft structural ice will most likely have the highest accumulation in freezing rain, which indicates warmer temperature (more than 32°F) at a higher altitude. See Figures 6-13 and 6-14. But the air temperature at the point where freezing precipitation is encountered is 32°F or less, causing the supercooled droplet to freeze on impact with the aircraft’s surface.

Figure 6-13. Clear and rime ice

Figure 6-14. Effects of structural icing
If rain falling through colder air freezes during descent, ice pellets form. The presence of ice pellets at the surface is evidence that there is freezing rain at a higher altitude, while wet snow indicates that the temperature at your altitude is above freezing.
Chances for structural icing increase in the vicinity of fronts.
Frost is described as ice deposits formed by sublimation on a surface when the temperature of the collecting surface is at or below the dew point of the adjacent air, and the dew point is below freezing. Frost causes early airflow separation on an airfoil that results in a loss of lift, causing the airplane to stall at an angle of attack lower than normal. Therefore, all frost should be removed from the lifting surfaces of an airplane before flight, or it may prevent the airplane from becoming airborne.
Thunderstorms present many hazards to flying. Three conditions necessary to the formation of a thunderstorm are:
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: cumulus, mature, and dissipating. See Figure 6-15.

Figure 6-15. Stages of a thunderstorm
The cumulus stage consists of continuous updrafts, and these updrafts create low-pressure areas. Thunderstorms reach their greatest intensity during the mature stage, which is characterized by updrafts and downdrafts inside the cloud. Precipitation inside the cloud assists the development of these downdrafts, and the start of rain at the Earth’s surface signals the beginning of the mature stage. Precipitation that evaporates before it reaches the ground is called virga.
When lightning occurs, the cloud is classified as a thunderstorm. Very frequent lightning, cumulonimbus clouds, and roll clouds indicate extreme turbulence in a thunderstorm. The dissipating stage of a thunderstorm features mainly downdrafts. Lightning is always associated with a thunderstorm.
Hail is formed inside thunderstorms (or cumulonimbus clouds) by the constant freezing, melting, and refreezing of water as it is carried about by the up- and downdrafts. Hailstones may be thrown outward from a storm cloud for several miles.
A pilot should always expect the hazardous and invisible atmospheric phenomena called wind shear turbulence when operating anywhere near a thunderstorm (within 20 NM). Wind shear is thought to be the most hazardous condition associated with a thunderstorm.
Thunderstorms that generally produce the most intense hazard to aircraft are called squall-line thunderstorms. These non-frontal, narrow bands of thunderstorms often contain severe steady-state thunderstorms that develop ahead of a cold front. The intense hazards found in these storms include destructive winds, heavy hail, and tornadoes. Embedded thunderstorms are those that are obscured by massive cloud layers and cannot be seen visually.
Airborne weather avoidance radar detects only precipitation drops; it does not detect minute cloud droplets. Therefore, the radar scope provides no assurance of avoiding instrument weather in clouds and fog. Weather radar precisely measures rainfall density which can be related to turbulence associated with the radar echoes. The most intense echoes are severe thunderstorms, and should be avoided by at least 20 miles. You should avoid flying between these intense echoes unless they are separated by at least 40 miles.
Fog is a surface-based cloud which restricts visibility, composed of either water droplets or ice crystals. 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:
Radiation fog (ground fog) is formed when terrestrial radiation cools the ground (land areas only), 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 (no wind) nights.
Advection fog (sea fog) is formed when warm, moist air moves (wind is required) over colder ground or water; for example, an air mass moving inland from the coast in winter. Advection fog is usually more extensive and much more persistent than radiation fog. It can move in rapidly regardless of the time of day or night. This fog deepens as wind speed increases up to about 15 knots. Winds much stronger than 15 knots lift the fog into a layer of low stratus clouds.
Upslope fog is formed when moist, stable air is cooled to its dew point as it moves up sloping terrain (wind is required). Cooling will be at the dry adiabatic lapse rate of approximately 3°C per 1,000 feet.
Precipitation-induced fog (frontal fog) is formed when relatively warm rain or drizzle falls through cool air; evaporation from precipitation saturates cool air and forms fog. It is most commonly associated with warm fronts, but can occur with slow moving cold fronts and with stationary fronts.
Steam fog forms in winter when cold, dry air passes from land areas over comparatively warm ocean waters. Condensation takes place just above the surface of the water and appears as steam rising from the ocean.
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 exist in both horizontal and vertical direction. The amount of wind shear 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.
Potentially hazardous wind shear may be encountered during periods of a strong temperature inversion with calm or light surface winds, and strong winds above the inversion. Eddies (turbulence) in the shear zone cause airspeed fluctuation as an aircraft climbs or descends through the inversions. During an approach, the most easily recognized means of detecting possible windshear conditions includes monitoring the rate of descent (vertical velocity) and power required. The power needed to hold the glide slope will be different from a no-shear situation.
There are two potentially hazardous shear situations:

Figure 6-16. Tailwind shearing to headwind or calm

Figure 6-17. Headwind shearing to tailwind or calm
Some airports can report boundary winds as well as the wind at the tower. When a tower reports a boundary wind which is significantly different from the airport wind, there is a possibility of hazardous wind shear.
[10-2024]