Pilot's Handbook of Aeronautical Knowledge

Chapter 12: Weather Theory

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Weather is the state of the atmosphere at a given time and place with respect to variables, such as temperature (heat or cold), moisture (wetness or dryness), wind velocity (calm or storm), visibility (clearness or cloudiness), and barometric pressure (high or low).

Understanding the theories behind weather helps a pilot make sound weather decisions based on the reports and forecasts obtained from Flight Service and other sources.

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Atmosphere

Life on Earth is supported by the atmosphere, solar energy, and the planet's magnetic fields. The atmosphere absorbs energy from the sun, recycles water and other chemicals, and works with the electrical and magnetic forces to provide a moderate climate. The atmosphere also protects life on Earth from high-energy radiation and the frigid vacuum of space.

The atmosphere is a mixture of gases in constant motion, reaching nearly 350 miles high. If the atmosphere were visible, we would see waves, swirls, eddies, and rising and falling air.

Composition of the Atmosphere

Nitrogen accounts for 78 percent of the gases that constitute the atmosphere. Oxygen makes up 21 percent. Argon, carbon dioxide, and traces of other gases make up the remaining one percent. A small amount of water vapor in the air (0-5% by volume) is responsible for major changes in the weather.

There are four distinct layers, or "spheres," of the atmosphere.

The troposphere extends four (4) to twelve (12) miles over the north and south poles. It reaches and up to 48,000 feet (9.1 statute miles) over the equatorial regions. The vast majority of weather, clouds, storms, and temperature variances occur within the troposphere.

Within the troposphere, the average temperature lapse rate is 2° C (3.5° F) for every 1,000 feet of altitude. The average pressure lapse rate is about one (1) inch Hg for every 1,000 feet of altitude.

At the top of the troposphere is a boundary known as the tropopause, which traps moisture and the associated weather in the troposphere. The altitude of the tropopause is elliptical, varying with latitude and season. The tropopause is commonly associated with the jet stream and clear air turbulence.

The stratosphere extends from the tropopause to a height of about 160,000 feet (30.3 statue miles, 50 km). Little weather exists in stratosphere, and the air is stable. However, certain types of clouds occasionally extend in the stratosphere. Above the stratosphere are the mesosphere and thermosphere, which generally do not affect weather.

Atmospheric Circulation

The Earth is warmed by energy radiating from the sun. When warm air rises and is replaced by cooler air (which is heavier and tends to sink), a circular motion results. Uneven heating of the Earth's surface, creating these changes in air movement and atmospheric pressure, is the primary reason why the atmosphere in motion. This movement is called atmospheric circulation.

The equatorial regions of the Earth receive a greater amount of heat from the sun than the polar regions. As the warm air rises and flows toward the poles, it cools, becoming denser and sinking back toward the surface.

Atmospheric Pressure

The unequal heating of the Earth's surface also causes changes in air pressure, i.e. the force exerted by the weight of air molecules.

In a standard atmosphere (i.e., at sea level), it would take 14.7 pounds of effort to lift a one-square-inch column of air that's 350 miles high. At an elevation of 18,000 feet, it would take 7.4 lbs of effort to raise the same column of air. Actual pressure differs with altitude, temperature, and air density.

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Coriolis Force

Air circulation is modified by the rotation of the Earth, which is called Coriolis force.

The Coriolis force deflects air to the right in the Northern Hemisphere, causing it to follow a curved path instead of a straight line. This deflection is greatest at the poles and diminishes to zero at the equator.

Because of Coriolis force, atmospheric flow is broken up into three distinct cells in each hemisphere. The size of each is one-third of the total distance between each pole and the equator (found from the equator to 30°, from 30° to 60°, and from 60° to each pole). This circulation pattern results in the prevailing upper level westerly winds in the conterminous United States.

Seasonal changes, continents, and oceans also contribute to circulation patterns.

Because of ground friction, wind direction at the surface varies from the wind direction just a few thousand feet above the surface.

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Measurement of Atmosphere Pressure

Atmospheric pressure historically was measured in inches of mercury ("Hg) by a mercurial barometer, which measures the height of a column of mercury inside a glass tube. This type of barometer is typically used in a laboratory or weather observation station, is not easily transported, and difficult to read. Mercurial barometers are no longer used in the United States.

An aneroid barometer contains a closed vessel called an aneroid cell that contracts or expands with changes in pressure. A mechanical linkage provides pressure readings. This linkage makes it less accurate than a mercury barometer, but it is easily transported and easy to read. An aircraft altimeter is essentially an aneroid barometer.

The International Standard Atmosphere (ISA), also known as "standard conditions," is the basis for pitot-static flight instruments and aircraft performance.

Standard sea level pressure is defined as 29.92" Hg. Standard temperature is 59° F (15° C).

Air pressure also can be reported in millibars (mb), with 1" of pressure corresponding to 34 mb. Standard sea level pressure is 1,013.2 mb.

All local barometric pressure readings are converted to a sea level pressure to provide a standard for records and reports. Each station converts its barometric pressure by adding approximately 1 "Hg for every 1,000 feet of elevation. For example, a station at 5,000 feet above sea level with a reading of 24.92" reports a sea level pressure reading of 29.92". Using common sea level pressure readings helps ensure aircraft altimeters are set correctly.

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Altitude and Atmospheric Pressure

As altitude increases, atmospheric pressure decreases and the air becomes less dense. This is referred to as density altitude. A decrease in pressure, which corresponds to an increase in density altitude, has a pronounced effect on aircraft performance.

At higher altitudes, with a decreased atmospheric pressure, takeoff and landing distances are increased, while climb rates decrease. When the air is thin, more speed is required to obtain enough lift for takeoff. This increases the ground run. An aircraft that requires 745 feet of ground run at sea level requires more than double that at a pressure altitude of 8,000 feet. Engines and propellers also become less efficient at higher field elevations.

The reactions of the average person become impaired above altitudes of about 10,000 feet. Symptoms range from mild disorientation to total incapacitation, depending on body tolerance and altitude. Supplemental oxygen or cabin pressurization systems help overcome the effects of oxygen deprivation.

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Wind and Currents

Air always seeks out lower pressure. Because of this, air flows from areas of high pressure into areas of low pressure.

The combination of atmospheric pressure differences, Coriolis force, friction, and temperature differences of the air near the earth cause two kinds of atmospheric motion: convective currents (upward and downward motion) and wind (horizontal motion).

Wind Patterns

In the Northern Hemisphere, the flow of air from areas of high to low pressure is deflected to the right and produces a clockwise circulation around an area of high pressure. This is known as anticyclonic circulation. Air flow to low pressure is deflected to create a counterclockwise or cyclonic circulation. (The opposite is true in the Southern Hemisphere.)

High-pressure systems are generally areas of dry, descending air, resulting in "good" weather.

Low-pressure systems feature an inflow of descending air to replace rising air, resulting in "bad" weather with increased cloudiness and precipitation. At the area of lowest pressure, the air cannot flow out and must rise.

When planning a flight from west to east, favorable winds would be encountered along the northern side of a high-pressure system or the southern side of a low-pressure system. East to west, the most favorable winds would be along the southern side of the same high-pressure system or the northern side of a low-pressure system.

Convective Currents

Uneven heating of the air creates small areas of local circulation called convective currents. Plowed ground, rocks, sand, and barren land absorb solar energy quickly and can therefore give off a large amount of heat. Water, trees, and other areas of vegetation tend to more slowly absorb heat.

Convective currents can cause bumpy, turbulent air, often experienced in flight at lower altitudes during warmer weather. Updrafts are likely to occur over pavement or barren places. Downdrafts often occur over water or expansive areas of vegetation. Turbulent conditions can be avoided by flying at higher altitudes.

Convective currents are particularly noticeable in areas with a land mass directly adjacent to a large body of water. Land heats faster than water. Air rises and is replaced by cooler air flowing in from over the water. The resulting onshore wind is called a sea breeze. At night, as land cools faster than water, the conditions reverse, resulting in a land breeze.

On final approach, rising air from terrain devoid of vegetation sometimes produces a ballooning effect, causing an overshoot. Approach over a large body of water or thick vegetation tends to create a sinking effect, causing an undershoot.

Effect of Obstructions on Wind

Obstructions on the ground affect the flow of wind and can be an unseen danger. Topography and large buildings can break up the flow of the wind and create wind gusts that change rapidly in direction and speed. This can affect the takeoff and landing performance of any aircraft and can present a very serious hazard.

Wind flows smoothly up the windward side of a mountain. The upward currents help to carry an aircraft over the peak of the mountain. However, the wind on the leeward side follows the contour of the terrain and is increasingly turbulent. The stronger the wind, the greater the downward pressure and turbulence become. Collision with terrain is possible. Pilots unfamiliar with a mountainous area should get a checkout with a mountain-qualified flight instructor.

Low-Level Wind Shear

Wind shear is a sudden, drastic change in wind speed and/or direction over a very small area, subjecting an aircraft to violent updrafts and downdrafts. Low-level wind shear is especially hazardous due to the proximity of an aircraft to the ground. This type of wind shear is commonly associated with passing frontal systems, thunderstorms, temperature inversions, and strong upper level winds (greater than 25 knots).

In wind shear, a tailwind quickly changing to a headwind causes an increase in airspeed and performance. A headwind changing to a tailwind causes a decrease in airspeed and performance.

A microburst is the most severe type of low-level wind shear. It is associated with convective precipitation into dry air at cloud base. Microburst activity may be indicated by an intense rain shaft at the surface but virga at cloud base. A ring of blowing dust is often the only visible clue.

A typical microburst has a horizontal diameter of one (1) to two (2) miles and a nominal depth of 1,000 feet.

The lifespan of a microburst is about five (5) to fifteen (15) minutes.

A microburst can produce downdrafts of up to 6,000 feet per minute (FPM). Headwind losses can range from 30 to 90 knots. Strong turbulence and hazardous wind direction changes are possible.

During an inadvertent takeoff into a microburst, the plane may first experience a performance-increasing headwind, followed by performance-decreasing downdrafts, followed by a rapidly increasing tailwind. Terrain impact is possible.

Microburst alerting systems have been installed at major airports.

Always be alert to the possibility of wind shear, especially when flying in and around thunderstorms and frontal systems. It often remains undetected and presents serious risk to flight operations.

Wind and Pressure Representation on Surface Weather Maps

Surface weather maps provide information about fronts, areas of high and low pressure, and surface winds and pressures for each station. These map also depicts the wind and pressure at the surface for each location.

The station circle represents the head of the arrow, with the arrow pointing in the direction from which the wind is blowing. However, winds are described by the direction from which they blow.

The speed of the wind is depicted by barbs or pennants placed on the wind line. Each barb represents a speed of ten knots, while half a barb is equal to five knots, and a pennant is equal to 50 knots.

Pressure for each station is recorded on the weather chart and is shown in mb.

Isobars are lines drawn on the chart to depict lines of equal pressure. These lines result in a pattern that reveals the pressure gradient or change in pressure over distance. Isobars that are closely spaced indicate a steep pressure gradient where strong winds prevail.

A high is an area of high pressure surrounded by lower pressure. A low is an area of low pressure surrounded by higher pressure.

A ridge is an elongated area of high pressure. A trough is an elongated area of low pressure.

Close to the ground, wind direction is modified by the friction and wind speed decreases due to friction with the surface. At levels 2,000 to 3,000 feet above the surface, wind speed is greater and wind direction becomes more parallel to the isobars.

Generally, the wind 2,000 feet above ground level (AGL) is 20° to 40° to the right of surface winds.

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Atmospheric Stability

The stability of the atmosphere depends on its ability to resist vertical motion. In an unstable atmosphere, small vertical air movements tend to become larger, resulting in turbulent airflow and convective activity. This can lead to significant turbulence, extensive vertical clouds, and severe weather.

The combination of moisture and temperature determine the stability of the air and the resulting weather.

Cool, dry air is very stable and resists vertical movement, which leads to "good," clear weather.

Moist, warm air is unstable with vertical movement. This condition is common in tropical regions during summer, where thunderstorms appear on a daily basis.

Adiabatic cooling results from rising air that expands and cools, since air pressure decreases as altitude increases. Adiabatic heating is caused by the rising temperature of descending air as it is compressed at lower altitudes.

The rate at which temperature decreases with an increase in altitude is referred to as its lapse rate. As air ascends through the atmosphere, the average rate of temperature change is 2° C (3.5° F) per 1,000 feet, also known as the standard temperature lapse rate.

Water vapor is lighter than air. As moisture increases in air, air density decreases, causing the air to rise. As moisture decreases, air becomes denser and tends to sink.

Moist air cools at a slower rate than dry air. This means moist air is generally less stable than dry air, since the moist air must rise higher before its temperature cools to that of the surrounding air.

The dry adiabatic lapse rate (unsaturated air) is 3° C (5.4° F) per 1,000 feet.

The moist adiabatic lapse rate varies from 1.1° C to 2.8° C (2° F to 5° F) per 1,000 feet.

Inversion

When the temperature of the air rises with altitude, a temperature inversion exists. Inversion layers are commonly shallow layers of smooth, stable air close to the ground. The temperature of the air increases with altitude to a certain point, which is the top of the inversion. The air at the top of the layer acts as a lid, keeping weather and pollutants trapped below.

Surface-based temperature inversions occur on clear, cool nights when the air close to the ground is cooled by the lowering temperature of the ground. The air within a few hundred feet of the surface becomes cooler than the air above it.

Frontal inversions occur when warm air spreads over a layer of cooler air, or cooler air is forced under a layer of warmer air.

Moisture and Temperature

The amount of moisture present in the atmosphere is dependent upon the temperature of the air. Every 20° F increase in temperature doubles the amount of moisture the air can hold.

Water can be present in the atmosphere as liquid, solid, and/or gas. Water can change between these states through evaporation, sublimation, condensation, deposition, melting, or freezing. Sublimation is the process wherein water changes directly from solid to gas. Deposition is the process wherein water changes directly from gas to solid. Evaporation and sublimation add water directly to the atmosphere.

As water vapor forms, it absorbs heat from the nearest available source. This heat exchange is known as the latent heat of evaporation, which creates a cooling effect. (This is why you feel cold when you get out of the shower and the moisture on your skin evaporates.)

Relative Humidity

Humidity refers to the amount of water vapor present in the atmosphere at a given time.

Relative humidity is the actual amount of moisture in the air compared to the total amount of moisture the air could hold at that temperature.

If the current relative humidity is 50 percent, the air is holding 50 percent of the total amount of moisture that it is capable of holding at that temperature and pressure.

The western United States rarely experiences high humidity. High relative humidity readings (75% to 90%) are common in the southern United States during warm weather.

Temperature/Dew Point Relationship

The dew point, given in degrees, is the temperature at which the air can hold no more moisture. When the air is completely saturated, moisture begins to condense in the form of fog, dew, frost, clouds, rain, or snow.

As moist, unstable air rises, clouds often form at the altitude where temperature and dew point reach the same value.

A formula can be used to determine the base of clouds, based on the convergence of temperature and dew point at a rate of 4.4° F.

Temperature (T) = 85 °F
Dew point (DP) = 71 °F
Temperature (T) minus the dew point (DP) = the Temperature/Dew Point Spread (TDS), in this case 14.
Divide the TDS by the Convergence Rate (CR = 4.4°), which results in 3.18.
The height of the cloud base, in AGL, is the result factored in thousands.
The clouds are 3,180 feet above ground.

Methods by Which Air Reaches the Saturation Point

If air reaches the saturation point while temperature and dew point are close together, it is highly likely that fog, low clouds, and precipitation will form.

There are four methods by which air can reach the saturation point:

• When warm air moves over a cold surface.
• When cold air and warm air mix.
• When air cools at night through contact with the cooler ground.
• When air is lifted or is forced upward.

Dew and Frost

On cool, clear, calm nights, the temperature of the ground and objects on the surface can cause temperatures of the surrounding air to drop below the dew point. Moisture in the air condenses and deposits on surfaces. When above freezing, the moisture is called dew. When below freezing, the moisture is called frost.

Dew poses no threat to an aircraft. However, frost poses a flight safety hazard by reducing lift and increasing drag. Aircraft must be free of frost prior to flight.

Fog

Fog is a cloud that is on the surface, occurring when the temperature of air near the ground is cooled to the dew point, causing water vapor to condense.

Fog is classified according to the manner in which it forms.

Radiation fog may develop on clear nights with relatively little to no wind present. This is caused by rapid ground cooling due to terrestrial radiation. It's typically found in low-lying areas, such as valleys. Rising temperatures and wind will cause radiation fog to dissipate.

If radiation fog is less than 20 feet thick, it is known as ground fog.

Advection fog is likely to occur when a layer of warm, moist air moves over a cold surface. Wind below 15 knots is required to form advection fog. It is common in coastal areas where sea breezes can blow warm air over cooler landmasses.

Upslope fog occurs when moist, stable air is forced up sloping land features like a mountain range. As with advection fog, it requires wind. Unlike radiation fog, it may not burn of with rising temperatures and can persist for days.

Steam fog (or "sea smoke") forms when cold, dry air moves over warm water. This is common over bodies of water during cold weather.

Ice fog occurs in cold weather when the temperatures are far below freezing and water vapor forms directly into ice crystals. It is similar to radiation fog, but temperatures typically are 25° or colder. It's most common in arctic regions.

Clouds & Thunderstorms

For clouds to form, there must be adequate water vapor and condensation nuclei, as well as a method by which the air can be cooled. As moisture condenses or sublimates, it attaches to condensation nuclei, which are small particles of matter like dust, salt, and smoke. The nuclei provide a means for the moisture to change from one state to another.

Cloud type is determined by its height, shape, and characteristic.

Low clouds are those that form near the Earth's surface and extend up to about 6,500 feet AGL. Fog is also classified as a type of low cloud formation. Clouds in this family can change rapidly and influence flight planning. VFR flight may not be possible.

Middle clouds form around 6,500 feet AGL and extend up to 20,000 feet AGL. They are composed of water, ice crystals, and supercooled water droplets. Altostratus and altocumulus clouds may be encountered on cross-country flights at higher altitudes, and they may contain turbulence and icing.

High clouds form above 20,000 feet AGL, usually only in stable air. They are made up of ice crystals and pose no real threat of turbulence or aircraft icing. These include cirrus, cirrostratus, and cirrocumulus clouds.

Clouds with extensive vertical development are cumulus clouds that build vertically into towering cumulus or cumulonimbus clouds. The bases form in the low to middle region. They can extend into the high altitude cloud level. Towering cumulus clouds indicate areas of instability in the atmosphere. They often develop into cumulonimbus clouds, i.e. thunderstorms.

To pilots, the cumulonimbus cloud is perhaps the most dangerous cloud type. Flight hazards include lightning, hail, tornadoes, gusty winds, and wind shear. They can appear individually or in groups.

Aircraft entering thunderstorms can experience updrafts and downdrafts that exceed 3,000 fpm. Large hailstones, damaging lightning, tornadoes, and large quantities of water can be expected.

Heating of the air near the Earth's surface creates an air mass thunderstorm. The upslope motion of air in the mountainous regions causes orographic thunderstorms. Cumulonimbus clouds that form in a continuous line are nonfrontal bands of thunderstorms or squall lines.

When extensive vertical clouds are obscured by other cloud formations they are said to be embedded thunderstorms.

Cloud Types

• Cumulus: Heaped or piled clouds
• Stratus: Formed in layers
• Cirrus: Ringlets, fibrous clouds, also high level clouds above 20,000 feet
• Castellanus: Common base with separate vertical development, castle-like
• Lenticularus: Lens-shaped, formed over mountains in strong winds
• Nimbus: Rain-bearing clouds
• Fracto: Ragged or broken
• Alto: Middle level clouds existing at 5,000 to 20,000 feet

Ceiling, Visibility, and Precipitation

A ceiling is the lowest layer of clouds reported as being broken or overcast. It can be the vertical visibility into an obscuration like fog or haze.

Clouds are reported as "broken" when five-eighths to seven-eighths of the sky is covered with clouds.

Visibility refers to the greatest horizontal distance at which prominent objects can be viewed with the naked eye.

Precipitation refers to any type of water particles that form in the atmosphere and fall to the ground. Precipitation occurs because water or ice particles in clouds grow in size until the atmosphere can no longer support them. Precipitation in any form poses a threat to safety of flight. Often, precipitation is accompanied by low ceilings and reduced visibility.

Drizzle is classified as very small water droplets, smaller than 0.02 inches in diameter. Water droplets of larger size are referred to as rain.

Virga is rain that falls through the atmosphere but evaporates prior to striking the ground.

Freezing rain and freezing drizzle occur when the temperature of the surface is below freezing and precipitation freezes on contact.

If rain falls through a temperature inversion, it may freeze as it passes through the underlying cold air and fall to the ground in the form of ice pellets, which are an indication of a temperature inversion.

Hail is freezing water droplets that are carried up and down by drafts inside cumulonimbus clouds. They grow larger as they come in contact with more moisture and then escape the updrafts when they are too large. Hail can vary in size from very small to several inches in diameter.

Snow is precipitation in the form of ice crystals.

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Air Masses & Fronts

Air masses are large bodies of air that take on the characteristics of the surrounding area or source region. They are classified according to the regions where they originate. After an initial period of stagnation, the air mass takes on the temperature and moisture characteristics of the source region. Air masses are generally identified as polar or tropical.

As the air mass moves from its source region and passes over land or water, the air mass is subjected to the varying conditions of the land or water which modify the nature of the air mass.

A front is the boundary layer between two types of air masses. An approaching front always means that the weather will change.

There are four types of fronts: warm, cold, stationary, and occluded.

Warm Front

A warm front occurs when a warm mass of air advances and replaces a body of colder air. Warm fronts move slowly. The slope of the advancing front slides over the top of the cooler air and gradually pushes it out of the area. Cirriform or stratiform clouds, along with fog, can be expected to form along the frontal boundary. Light to moderate precipitation is probable. In hot weather, thunderstorms are likely to develop.

During the passage of a warm front, stratiform clouds are visible and drizzle may be falling. The visibility is generally poor, but improves with variable winds. The temperature rises steadily from the inflow of relatively warmer air. For the most part, the dew point remains steady and the pressure levels off.

After the passage of a warm front, visibility improves and temperatures rise. The wind is south-southwest. The dew point rises and then levels off. Barometric pressure rises and then decreases.

In a flight into a warm front, departure from a cold air mass will be VFR. As the flight progresses, clouds deepen, ceilings lower, and visibility is reduced. Eventually, conditions deteriorate to marginal VFR or IFR flight. Fog may develop, and conditions may not permit further flight.

Frontal passage may take up to two days.

Cold Front

A cold front occurs when a mass of cold, dense, and stable air advances and replaces a body of warmer air.

Cold fronts move more rapidly than warm fronts. They are dense, remaining close to the ground, with a frontal surface in the shape of a snowplow. As a cold front slides under warmer air, less dense air is forced aloft. As this rising air cools, clouds form.

Prior to a cold front passage, cirriform or towering cumulus clouds are present. Thunderstorms may develop. A high dew point and falling barometric pressure are indicative of imminent cold front passage. During passage, heavy rain showers form. Visibility is poor. Winds are variable and gusty. The barometer quickly falls and then begins a gradual increase. In severe weather, tornados can develop.

After a cold front passage, precipitation decreases and good visibility prevails. Temperatures remain cool.

Fast-moving cold fronts are pushed by intense pressure systems far behind the actual front. Friction between the ground and the cold front slows the front, which steepens the frontal surface. If the displaced warm air is unstable, a continuous line of thunderstorms, or squall line, may form along or ahead of the front.

In flight from a warm front to a cold front, departure typically will be VFR, with scattered clouds and limited visibility due to haze. As the flight progresses, the clouds show signs of vertical development. Barometric pressure falls. Weather deteriorates to low overcast, limited visibility, and heavy rain or thunderstorms. On the other side of the cold front, ceilings and visibility are VFR.

Flight into an approaching cold front is not generally recommended, due to the risk of encountering thunderstorms or other severe weather.

Comparison of Cold and Warm Fronts

Warm fronts and cold fronts vary in speed, composition, and weather phenomenon.

Violent weather is associated with cold fronts, directly along the frontal boundary. However, squall lines can form many miles ahead of an approaching cold front. Warm fronts bring low ceilings, poor visibility, and rain.

Cold fronts travel 20 to 35 mph, whereas warm fronts travel 10 to 25 mph.

Cold fronts are fast approaching with little or no warning, and they bring about a complete weather change in just a few hours. Warm fronts provide advance warning of their approach and can take days to pass through a region.

Wind Shifts

Wind around a high-pressure system rotates clockwise, while low-pressure winds rotate counter-clockwise.

When two high pressure systems are adjacent, the winds are almost in direct opposition to each other at the point of contact.

Fronts are the boundaries between two areas of pressure, and therefore, wind shifts are continually occurring within a front. Shifting wind direction is most pronounced in conjunction with cold fronts.

Stationary Front

When the forces of two air masses are relatively equal, the boundary or front that separates them remains stationary and influences the local weather for days. This front is called a stationary front. The weather is typically a mixture that can be found in both warm and cold fronts.

Occluded Front

An occluded front occurs when a fast-moving cold front catches up with a slow-moving warm front. Prior to passage, warm front weather prevails, but this is immediately followed by cold front weather.

A cold front occlusion occurs when a fast-moving cold front is colder than the air ahead of the slow moving warm front. When this occurs, the cold air replaces the cool air and forces the warm front aloft into the atmosphere. This creates a mixture of weather found in both warm and cold fronts.

A warm front occlusion occurs when the air ahead of the warm front is colder than the air of the cold front. In this case, the cold front rides up and over the warm front. If the air forced aloft is unstable, embedded thunderstorms, rain, and fog are likely to occur.

Prior to the passage of the typical occluded front, cirriform and stratiform clouds prevail, light to heavy precipitation falls, visibility is poor, dew point is steady, and barometric pressure drops.

During passage, nimbostratus and cumulonimbus clouds predominate. Towering cumulus clouds may form. Light to heavy precipitation falls. Visibility is poor. Winds are variable. Barometric pressure levels off.

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Thunderstorms

For a thunderstorm to form, the air must have sufficient water vapor, an unstable lapse rate, and an initial lifting action to start the storm process.

An air mass thunderstorm generally results from surface heating. They occur at random in unstable air, last for only an hour or two, and produce only moderate wind gusts and rainfall.

Steady-state thunderstorms are associated with weather systems. Fronts, converging winds, and troughs aloft force upward motion. These storms often form into squall lines. Updrafts become stronger and last much longer than in air mass storms.

A thunderstorm has three distinct stages. In the cumulus stage, lifting action of the air begins. With sufficient moisture and instability, the vertical height increases. Continuous, strong updrafts prohibit moisture from falling

The mature stage is reached in about 15 minutes. Drops of moisture (rain or ice) are too heavy for the cloud to support and begin falling in the form of rain or hail, creating a downward motion of the air.

This the most violent time period of the thunderstorm's life cycle. Warm air rises. Cool, precipitation-induced air descends. Severe turbulence occurs. Down-rushing air increases surface winds and decreases the temperature.

In the dissipating stage, vertical motion near the top of the cloud slows down, and the top of the cloud spreads out, taking an anvil-like shape. Downdrafts spread out, replacing the updrafts needed to sustain the storm.

Thunderstorm Hazards

Severe thunderstorms can cross the tropopause, reaching 50,000 to 60,000 feet at some latitudes. Flying over the top of a thunderstorm in a small aircraft is impossible. Flying under thunderstorms can subject aircraft to rain, hail, damaging lightning, and violent turbulence.

All thunderstorms have conditions that are a hazard to aviation. Thunderstorms should be circumnavigated by at least 20 NM, in order to avoid far-flung hail. Waiting for thunderstorms to pass also is a good option.

A squall line is the single most intense weather hazard to aircraft. This is a narrow band of active thunderstorms, often ahead of a cold front. It often contains steady-state thunderstorms. It usually forms rapidly, generally reaching maximum intensity during the late afternoon and the first few hours of darkness. The line may be too long to detour easily, forcing a diversion to land until frontal passage.

If a thunderstorm's incoming air has any initial rotating motion, it often forms an extremely concentrated vortex. Extremely low barometric pressure and winds exceeding 200 knots are possible. The low pressure generates a funnel-shaped cloud extending downward from the cumulonimbus base. If the cloud does not reach the surface, it is a funnel cloud. If the cloud touches a land surface, it is a tornado. If the cloud touches water, it is a waterspout.

Extreme turbulence within a thunderstorm exists with shear between updrafts and downdrafts. Shear turbulence also can exist far above and 20 miles laterally from a severe storm. A gust front, as much as 15 miles ahead of a thunderstorm, causes a rapid, and sometimes drastic, change in surface wind. The shear zone is a low-level turbulent area associated with the gust front. Often, a roll cloud on the leading edge of a storm marks this zone.

Updrafts with abundant moisture, carried above the freezing level, creates supercooled water, as water vapor sublimates as ice crystals. Supercooled water freezes on impact with an aircraft, which causing aircraft icing.

Hail develops as supercooled drops above the freezing level begin to freeze. Hailstones accumulate moisture, becoming large chunks of ice. Hailstones larger than one-half inch in diameter can significantly damage an aircraft in a few seconds.

Barometric pressure falls rapidly as a thunderstorm approaches, rises sharply, and then falls back to normal as the storm moves on. This cycle can occur in 15 minutes. Without a corrected altimeter setting, the altimeter may be more than 100 feet in error.

A lightning strike can puncture the skin of an aircraft and damage communications and electronic navigational equipment. While serious accidents due to lightning strikes are rare, lightning can temporarily blind the pilot.

If the updraft velocity in a thunderstorm approaches or exceeds the terminal velocity of the falling raindrops, very high concentrations of water can be in excess of the quantity of water turbine engines are designed to ingest. Severe thunderstorms could result in flameout and/or structural failure of one or more turbine engines.

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