Chapter 11: Weather Theory - FAA

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At the time of departure from Pittsburgh, the weather is VFRwith three miles visibility in smoke and a scattered layer ofclouds at 3,500 feet. As the flight progresses westward toColumbus and closer to the oncoming cold front, the cloudsshow signs of vertical development with a broken layer at2,500 feet. The visibility is six miles in haze with a fallingbarometric pressure. Approaching Indianapolis, the weatherhas deteriorated to overcast clouds at 1,000 feet, and threemiles visibility with thunderstorms and heavy rain showers.At St. Louis, the weather gets better with scattered clouds at1,000 feet and a ten mile visibility.A pilot using sound judgment based on the knowledge offrontal conditions would most likely remain in Indianapolisuntil the front had passed. Trying to fly below a line ofthunderstorms or a squall line is hazardous, and flight overthe top of or around the storm is not an option. Thunderstormscan extend up to well over the capability of small airplanesand can extend in a line for 300 to 500 miles.Comparison of Cold and Warm FrontsWarm fronts and cold fronts are very different in nature asare the hazards associated with each front. They vary inspeed, composition, weather phenomenon, and prediction.Cold fronts, which move at 20 to 35 mph, move very quicklyin comparison to warm fronts, which move at only 10 to25 mph. Cold fronts also possess a steeper frontal slope.Violent weather activity is associated with cold fronts, andthe weather usually occurs along the frontal boundary, not inadvance. However, squall lines can form during the summermonths as far as 200 miles in advance of a severe cold front.Whereas warm fronts bring low ceilings, poor visibility,and rain, cold fronts bring sudden storms, gusty winds,turbulence, and sometimes hail or tornadoes.Cold fronts are fast approaching with little or no warning, andthey make a complete weather change in just a few hours.The weather clears rapidly after passage and drier air withunlimited visibilities prevail. Warm fronts, on the other hand,provide advance warning of their approach and can take daysto pass through a region.Wind ShiftsWind around a high pressure system rotates in a clockwisefashion, while low pressure winds rotate in a counterclockwisemanner. When two pressure systems are adjacent,the winds are almost in direct opposition to each other atthe point of contact. Fronts are the boundaries between twoareas of pressure, and therefore, wind shifts are continuallyoccurring within a front. Shifting wind direction is mostpronounced in conjunction with cold fronts.Stationary FrontWhen the forces of two air masses are relatively equal, theboundary or front that separates them remains stationary andinfluences the local weather for days. This front is called astationary front. The weather associated with a stationaryfront is typically a mixture that can be found in both warmand cold fronts.Occluded FrontAn occluded front occurs when a fast-moving cold frontcatches up with a slow-moving warm front. As the occludedfront approaches, warm front weather prevails, but isimmediately followed by cold front weather. There are twotypes of occluded fronts that can occur, and the temperaturesof the colliding frontal systems play a large part in definingthe type of front and the resulting weather. A cold frontocclusion occurs when a fast moving cold front is colderthan the air ahead of the slow moving warm front. Whenthis occurs, the cold air replaces the cool air and forces thewarm front aloft into the atmosphere. Typically, the coldfront occlusion creates a mixture of weather found in bothwarm and cold fronts, providing the air is relatively stable.A warm front occlusion occurs when the air ahead of thewarm front is colder than the air of the cold front. When thisis the case, the cold front rides up and over the warm front. Ifthe air forced aloft by the warm front occlusion is unstable,the weather is more severe than the weather found in a coldfront occlusion. Embedded thunderstorms, rain, and fog arelikely to occur.Figure 11-28 depicts a cross-section of a typical cold frontocclusion. The warm front slopes over the prevailing coolerair and produces the warm front type weather. Prior to thepassage of the typical occluded front, cirriform and stratiformclouds prevail, light to heavy precipitation is falling, visibilityis poor, dew point is steady, and barometric pressure isfalling. During the passage of the front, nimbostratus andcumulonimbus clouds predominate, and towering cumulusmay also be possible. Light to heavy precipitation is falling,visibility is poor, winds are variable, and the barometricpressure is leveling off. After the passage of the front,nimbostratus and altostratus clouds are visible, precipitationis decreasing and clearing, and visibility is improving.ThunderstormsFor a thunderstorm to form, the air must have sufficient watervapor, an unstable lapse rate, and an initial lifting action tostart the storm process. Some storms occur at random inunstable air, last for only an hour or two, and produce onlymoderate wind gusts and rainfall. These are known as air11-22
CUMULONIMBUSWARM AIRCIRROSTRATUSCIRRUSALTOSTRATUSCOLD AIRNIMBOSTRATUSCOLD AIRSt. LouisIndianapolis200 milesColumbus400 milesPittsburgh600 miles1006 1005 1002999999 1002 1005 10061011101410171020METAR KSTL 1950Z 31023G40KT 8SMSCT035 05/M03 A297642 0768 3226St. Louis661 26220588Indianapolis5225114234Columbus4774020020Pittsburgh1023METAR KIND 1950Z 29028G45KT 1/2SM TSRAGRVV005 18/16 A2970METAR KCMH 1950Z 16017KT 2SM BROVC080 11/10 A29701011 1014 1017 1020 1023METAR KPIT 1950Z 13012KT 75SMBKN130 08/04 A3012Figure 11-28. Occluded front cross-section with a weather chart depiction and associated METAR.mass thunderstorms and are generally a result of surfaceheating. Steady-state thunderstorms are associated withweather systems. Fronts, converging winds, and troughs aloftforce upward motion spawning these storms which oftenform into squall lines. In the mature stage, updrafts becomestronger and last much longer than in air mass storms, hencethe name steady state. [Figure 11-29]Knowledge of thunderstorms and the hazards associated withthem is critical to the safety of flight.HazardsWeather can pose serious hazards to flight and a thunderstormpacks just about every weather hazard known to aviation intoone vicious bundle. These hazards occur individually or incombinations and most can be found in a squall line.Squall LineA squall line is a narrow band of active thunderstorms. Oftenit develops on or ahead of a cold front in moist, unstableair, but it may develop in unstable air far removed fromany front. The line may be too long to detour easily and toowide and severe to penetrate. It often contains steady-statethunderstorms and presents the single most intense weatherhazard to aircraft. It usually forms rapidly, generally reachingmaximum intensity during the late afternoon and the first fewhours of darkness.TornadoesThe most violent thunderstorms draw air into their cloud baseswith great vigor. If the incoming air has any initial rotatingmotion, it often forms an extremely concentrated vortex fromthe surface well into the cloud. Meteorologists have estimatedthat wind in such a vortex can exceed 200 knots with pressureinside the vortex quite low. The strong winds gather dustand debris and the low pressure generates a funnel-shapedcloud extending downward from the cumulonimbus base. Ifthe cloud does not reach the surface, it is a funnel cloud; ifit touches a land surface, it is a tornado.Tornadoes occur with both isolated and squall linethunderstorms. Reports for forecasts of tornadoes indicate thatatmospheric conditions are favorable for violent turbulence.An aircraft entering a tornado vortex is almost certain tosuffer structural damage. Since the vortex extends well intothe cloud, any pilot inadvertently caught on instruments in asevere thunderstorm could encounter a hidden vortex.Families of tornadoes have been observed as appendages ofthe main cloud extending several miles outward from the area11-23
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Page 11 and 12: Low-Level Wind ShearWind shear is a
Page 13 and 14: The adiabatic process takes place i
Page 15 and 16: Dew and FrostOn cool, calm nights,
Page 17 and 18: which lifting action of the air beg
Page 19 and 20: AcPNote standard air mass abbreviat
Page 21: Prior to the passage of a typical c
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