Inversion. As air rises and expands in the atmosphere, the temperature decreases. There is an atmospheric anomaly that can occur;.

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12-1Weather TheoryChapter 12IntroductionWeather is an important factor that influences aircraft performance and flying safety. It 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). The term fiweatherfl can also apply to adverse or destructive atmospheric conditions, such as high winds.This chapter explains basic weather theory and offers pilots background knowledge of weather principles. It is designed to help them gain a good understanding of how weather affects daily flying activities. Understanding the theories behind weather helps a pilot make sound weather decisions based on the reports and forecasts obtained from a Flight Service Station (FSS) weather specialist and other aviation weather services.Be it a local flight or a long cross-country flight, decisions based on weather can dramatically affect the safety of the flight.

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12-221%21%78%78%Nitrogen Nitrogen Oxygen Oxygen 1% Figure 12-1. Composition of the atmosphere. Troposphere Stratosphere Mesosphere Thermosphere 280,000 feet 160,000 feet 20,000 feetFigure 12-2. Layers of the atmosphere.AtmosphereThe atmosphere is a blanket of air made up of a mixture of gases that surrounds the Earth and reaches almost 350 miles from the surface of the Earth. This mixture is in constant motion. If the atmosphere were visible, it might look like an ocean with swirls and eddies, rising and falling air, and waves that travel for great distances.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.Composition of the AtmosphereIn any given volume of air, nitrogen accounts for 78 percent of the gases that comprise the atmosphere, while oxygen makes up 21 percent. Argon, carbon dioxide, and traces of other gases make up the remaining one percent. This volume of air also contains some water vapor, varying from zero to about five percent by volume. This small amount of water vapor is responsible for major changes in the weather. [Figure 12-1]The envelope of gases surrounding the Earth changes from the ground up. Four distinct layers or spheres of the atmosphere have been identified using thermal characteristics (temperature changes), chemical composition, movement, and density. [Figure 12-2]The first layer, known as the troposphere, extends from 6 to 20 kilometers (km) (4 to 12 miles) over the northern and southern poles and up to 48,000 feet (14.5 km) over the equatorial regions. The vast majority of weather, clouds, storms, and temperature variances occur within this first layer of the atmosphere. Inside the troposphere, the average temperature decreases at a rate of about 2 °Celsius (C) every

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12-3Figure 12-3. Circulation pattern in a static environment.1,000 feet of altitude gain, and the pressure decreases at a rate of about one inch per 1,000 feet of altitude gain. 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 varies with latitude and with the season of the year; therefore, it takes on an elliptical shape as opposed to round. Location of the tropopause is important because it is commonly associated with the location of the jet stream and possible clear air turbulence. Above the tropopause are three more atmospheric levels. The first is the stratosphere, which extends from the tropopause to a height of about 160,000 feet (50 km). Little weather exists in this layer and the air remains stable, although certain types of clouds occasionally extend in it. Above the stratosphere are the mesosphere and thermosphere, which have little influence over weather.Atmospheric CirculationAs noted earlier, the atmosphere is in constant motion. Certain factors combine to set the atmosphere in motion, but a major factor is the uneven heating of the Earth™s surface. This heating upsets the equilibrium of the atmosphere, creating changes in air movement and atmospheric pressure. The movement of air around the surface of the Earth is called atmospheric circulation. Heating of the Earth™s surface is accomplished by several processes, but in the simple convection-only model used for this discussion, the Earth is warmed by energy radiating from the sun. The process causes a circular motion that results when warm air rises and is replaced by cooler air.Warm air rises because heat causes air molecules to spread apart. As the air expands, it becomes less dense and lighter than the surrounding air. As air cools, the molecules pack together more closely, becoming denser and heavier than warm air. As a result, cool, heavy air tends to sink and replace warmer, rising air. Because the Earth has a curved surface that rotates on a tilted axis while orbiting the sun, the equatorial regions of the Earth receive a greater amount of heat from the sun than the polar regions. The amount of solar energy that heats the Earth depends on the time of year and the latitude of the specific region. All of these factors affect the length of time and the angle at which sunlight strikes the surface.Solar heating causes higher temperatures in equatorial areas, which causes the air to be less dense and rise. As the warm air flows toward the poles, it cools, becoming denser and sinks back toward the surface. [Figure 12-3] Atmospheric PressureThe unequal heating of the Earth™s surface not only modifies air density and creates circulation patterns; it also causes changes in air pressure or the force exerted by the weight of air molecules. Although air molecules are invisible, they still have weight and take up space. Imagine a sealed column of air that has a footprint of one square inch and is 350 miles high. It would take 14.7 pounds of effort to lift that column. This represents the air™s weight; if the column is shortened, the pressure exerted at the bottom (and its weight) would be less. The weight of the shortened column of air at 18,000 feet is approximately 7.4 pounds; almost 50 percent that at sea level. For instance, if a bathroom scale (calibrated for sea level) were raised to 18,000 feet, the column of air weighing 14.7 pounds at sea level would be 18,000 feet shorter and would weigh approximately 7.3 pounds (50 percent) less than at sea level. [Figure 12-4]The actual pressure at a given place and time differs with altitude, temperature, and density of the air. These conditions also affect aircraft performance, especially with regard to takeoff, rate of climb, and landings.Coriolis ForceIn general atmospheric circulation theory, areas of low pressure exist over the equatorial regions and areas of high pressure exist over the polar regions due to a difference in temperature. The resulting low pressure allows the high-pressure air at the poles to flow along the planet™s surface toward the equator. While this pattern of air circulation is

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12-414.7lb1 1 Square Inch 1 Square Inch7.4lb18,000 feet1 1 Square Inch 1 Square InchSea levelFigure 12-4. Atmosphere weights.Figure 12-5. Three-cell circulation pattern due to the rotation of the Earth.correct in theory, the circulation of air is modified by several forces, the most important of which is the rotation of the Earth. The force created by the rotation of the Earth is known as the Coriolis force. This force is not perceptible to humans as they walk around because humans move slowly and travel relatively short distances compared to the size and rotation rate of the Earth. However, the Coriolis force significantly affects motion over large distances, such as an air mass or body of water. The Coriolis force deflects air to the right in the Northern Hemisphere, causing it to follow a curved path instead of a straight line. The amount of deflection differs depending on the latitude. It is greatest at the poles and diminishes to zero at the equator. The magnitude of Coriolis force also differs with the speed of the moving bodyŠthe greater the speed, the greater the deviation. In the Northern Hemisphere, the rotation of the Earth deflects moving air to the right and changes the general circulation pattern of the air.The Coriolis force causes the general flow to break up into three distinct cells in each hemisphere. [Figure 12-5] In the Northern Hemisphere, the warm air at the equator rises upward from the surface, travels northward, and is deflected eastward by the rotation of the Earth. By the time it has traveled one-third of the distance from the equator to the North Pole, it is no longer moving northward, but eastward. This air cools and sinks in a belt-like area at about 30° latitude, creating an area of high pressure as it sinks toward the surface. Then, it flows southward along the surface back toward the equator. Coriolis force bends the flow to the right, thus creating the northeasterly trade winds that prevail from 30° latitude to the equator. Similar forces create circulation cells that encircle the Earth between 30° and 60° latitude and between 60° and the poles. This circulation pattern results in the prevailing upper level westerly winds in the conterminous United States. Circulation patterns are further complicated by seasonal changes, differences between the surfaces of continents and oceans, and other factors such as frictional forces caused by the topography of the Earth™s surface that modify the movement of the air in the atmosphere. For example, within 2,000 feet of the ground, the friction between the surface and the atmosphere slows the moving air. The wind is diverted from its path because of the frictional force. Thus, the wind direction at the surface varies somewhat from the wind direction just a few thousand feet above the Earth.Measurement of Atmosphere PressureAtmospheric pressure historically was measured in inches of mercury (“Hg) by a mercurial barometer. [Figure 12-6] The barometer measures the height of a column of mercury inside a glass tube. A section of the mercury is exposed to the pressure of the atmosphere, which exerts a force on the mercury. An increase in pressure forces the mercury to rise inside the tube. When the pressure drops, mercury drains out of the tube decreasing the height of the column. This type of barometer is typically used in a laboratory or weather observation station, is not easily transported, and difficult to read.

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12-529.92 “Hg = 1,013.2 mb (hPa) = 14.7 lb/in2Sea level29.92” (760 mm)Height of mercuryAtmospheric pressureAt sea level in a standard atmosphere, the weight of the atmosphere (14.7 lb/in2) supportsa column of mercury29.92 inches high.Figure 12-6. Although mercurial barometers are no longer used in the U. S., they are still a good historical reference for where the altimeter setting came from (inches of mercury).LowerHigher Atmospheric pressureSealed aneroid cellSealed aneroid cellSealed aneroid cellFigure 12-7. Aneroid barometer.An aneroid barometer is the standard instrument used to measure pressure; it is easier to read and transport. [Figure 12-7] The aneroid barometer contains a closed vessel called an aneroid cell that contracts or expands with changes in pressure. The aneroid cell attaches to a pressure indicator with a mechanical linkage to provide pressure readings. The pressure sensing part of an aircraft altimeter is essentially an aneroid barometer. It is important to note that due to the linkage mechanism of an aneroid barometer, it is not as accurate as a mercurial barometer.To provide a common reference, the International Standard Atmosphere (ISA) has been established. These standard conditions are the basis for certain flight instruments and most aircraft performance data. Standard sea level pressure is defined as 29.92 “Hg and a standard temperature of 59 °F (15 °C). Atmospheric pressure is also reported in millibars (mb), with 1 “Hg equal to approximately 34 mb. Standard sea level pressure is 1,013.2 mb. Typical mb pressure readings range from 950.0 to 1,040.0 mb. Surface charts, high and low pressure centers, and hurricane data are reported using mb.Since weather stations are located around the globe, all local barometric pressure readings are converted to a sea level pressure to provide a standard for records and reports. To achieve this, 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 “Hg, reports a sea level pressure reading of 29.92 “Hg. [Figure 12-8] Using common sea level pressure readings helps ensure aircraft altimeters are set correctly, based on the current pressure readings.By tracking barometric pressure trends across a large area, weather forecasters can more accurately predict movement of pressure systems and the associated weather. For example, tracking a pattern of rising pressure at a single weather station generally indicates the approach of fair weather. Conversely, decreasing or rapidly falling pressure usually indicates approaching bad weather and, possibly, severe storms.Altitude and Atmospheric PressureAs altitude increases, atmospheric pressure decreases. On average, with every 1,000 feet of increase in altitude, the atmospheric pressure decreases 1 “Hg. As pressure decreases, the air becomes less dense or thinner. This is the equivalent of being at a higher altitude and is referred to as density altitude. As pressure decreases, density altitude increases and has a pronounced effect on aircraft performance.Differences in air density caused by changes in temperature result in a change in pressure. This, in turn, creates motion in the atmosphere, both vertically and horizontally, in the form of currents and wind. The atmosphere is almost constantly in motion as it strives to reach equilibrium. These never-ending air movements set up chain reactions that cause a continuing variety in the weather.

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12-6Standard AtmosphereDenver 29.92 “Hg24.92 “HgStation PressureDenver29.92 “HgStation PressureNew OrleansNew Orleans 29.92 “HgFigure 12-8. Station pressure is converted to and reported in sea level pressure.1,590 feet745 feetPressure Altitude: Sea levelPressure Altitude: 8,000 feetTAKEOFF DISTANCEMAXIMUM WEIGHT 2,400 LBPressurealtitude(feet)S.L.1,0002,000 3,000 4,000 5,000 6,000 7,000 8,000Groundroll(feet) 745 815 895 980 1,075 1,185 1,305 1,440 1,590Total feetto clear50 foot obstacle1,320 1,445 1,585 1,740 1,920 2,125 2,360 2,635 2,9600 °CFigure 12-9. Takeoff distances increase with increased altitude.Altitude and FlightAltitude affects every aspect of flight from aircraft performance to human performance. At higher altitudes, with a decreased atmospheric pressure, takeoff and landing distances are increased, while climb rates decrease. When an aircraft takes off, lift is created by the flow of air around the wings. If the air is thin, more speed is required to obtain enough lift for takeoff; therefore, the ground run is longer. 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. [Figure 12-9] . It is also true that at higher altitudes, due to the decreased density of the air, aircraft engines and propellers are less efficient. This leads to reduced rates of climb and a greater ground run for obstacle clearance. Altitude and the Human BodyAs discussed earlier, nitrogen and other trace gases make up 79 percent of the atmosphere, while the remaining 21

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12-8Figure 12-11. Favorable winds near a high pressure system.Figure 12-12. Convective turbulence avoidance.cooler, denser air flowing in from over the water. This causes an onshore wind called a sea breeze. Conversely, at night land cools faster than water, as does the corresponding air. In this case, the warmer air over the water rises and is replaced by the cooler, denser air from the land, creating an offshore wind called a land breeze. This reverses the local wind circulation pattern. Convective currents can occur anywhere there is an uneven heating of the Earth™s surface. [Figure 12-13]Convective currents close to the ground can affect a pilot™s ability to control the aircraft. For example, on final approach, the rising air from terrain devoid of vegetation sometimes produces a ballooning effect that can cause a pilot to overshoot the intended landing spot. On the other hand, an approach over a large body of water or an area of thick vegetation tends to create a sinking effect that can cause an unwary pilot to land short of the intended landing spot. [Figure 12-14]Effect of Obstructions on WindAnother atmospheric hazard exists that can create problems for pilots. Obstructions on the ground affect the flow of wind and can be an unseen danger. Ground topography and large buildings can break up the flow of the wind and create wind gusts that change rapidly in direction and speed. These obstructions range from man-made structures, like hangars, to large natural obstructions, such as mountains, bluffs, or canyons. It is especially important to be vigilant when flying in or out of airports that have large buildings or natural obstructions located near the runway. [Figure 12-15] The intensity of the turbulence associated with ground obstructions depends on the size of the obstacle and the primary velocity of the wind. This can affect the takeoff and landing performance of any aircraft and can present a very serious hazard. During the landing phase of flight, an aircraft

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12-10 I N D W Figure 12-15. Turbulence caused by manmade obstructions.Figure 12-16. Turbulence in mountainous regions.a similar manner. As the air flows down the leeward side of the mountain, the air follows the contour of the terrain and is increasingly turbulent. This tends to push an aircraft into the side of a mountain. The stronger the wind, the greater the downward pressure and turbulence become.Due to the effect terrain has on the wind in valleys or canyons, downdrafts can be severe. Before conducting a flight in or may fidrop infl due to the turbulent air and be too low to clear obstacles during the approach.This same condition is even more noticeable when flying in mountainous regions. [Figure 12-16] While the wind flows smoothly up the windward side of the mountain and the upward currents help to carry an aircraft over the peak of the mountain, the wind on the leeward side does not act in

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12-11Intended Path1 2 3 4 Increasing tailwind Increasing headwindStrong downdraft OutflowOutflowFigure 12-17. Effects of a microburst wind.near mountainous terrain, it is helpful for a pilot unfamiliar with a mountainous area to get a checkout with a mountain qualified flight instructor.Low-Level Wind ShearWind shear is a sudden, drastic change in wind speed and/or direction over a very small area. Wind shear can subject an aircraft to violent updrafts and downdrafts, as well as abrupt changes to the horizontal movement of the aircraft. While wind shear can occur at any altitude, low-level wind shear is especially hazardous due to the proximity of an aircraft to the ground. Low-level wind shear is commonly associated with passing frontal systems, thunderstorms, temperature inversions, and strong upper level winds (greater than 25 knots).Wind shear is dangerous to an aircraft. It can rapidly change the performance of the aircraft and disrupt the normal flight attitude. For example, a tailwind quickly changing to a headwind causes an increase in airspeed and performance. Conversely, a headwind changing to a tailwind causes a decrease in airspeed and performance. In either case, a pilot must be prepared to react immediately to these changes to maintain control of the aircraft. The most severe type of low-level wind shear, a microburst, 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 and a ring of blowing dust is often the only visible clue. A typical microburst has a horizontal diameter of 1Œ2 miles and a nominal depth of 1,000 feet. The lifespan of a microburst is about 5Œ15 minutes during which time it can produce downdrafts of up to 6,000 feet per minute (fpm) and headwind losses of 30Œ90 knots, seriously degrading performance. It can also produce strong turbulence and hazardous wind direction changes. Consider Figure 12-17 : During an inadvertent takeoff into a microburst, the plane may first experience a performance-increasing headwind (1), followed by performance-decreasing downdrafts (2), followed by a rapidly increasing tailwind (3). This can result in terrain impact or flight dangerously close to the ground (4). An encounter during approach involves the same sequence of wind changes and could force the plane to the ground short of the runway.The FAA has made a substantial investment in microburst accident prevention. The totally redesigned LLWAS-NE, the TDWR, and the ASR-9 WSP are skillful microburst alerting systems installed at major airports. These three systems were extensively evaluated over a 3-year period. Each was seen to issue very few false alerts and to detect microbursts well above the 90 percent detection requirement established by Congress. Many flights involve airports that lack microburst alert equipment, so the FAA has also prepared wind shear training material: Advisory Circular (AC) 00-54, FAA Pilot Wind Shear Guide. Included is information on how to recognize the risk of a microburst encounter, how to avoid an encounter, and the best flight strategy for successful escape should an encounter occur.It is important to remember that wind shear can affect any flight and any pilot at any altitude. While wind shear may be reported, it often remains undetected and is a silent danger to aviation. Always be alert to the possibility of wind shear, especially when flying in and around thunderstorms and frontal systems.

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