Chapter 11: Weather Theory - FAA

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as broken when five-eighths to seven-eighths of the sky iscovered with clouds. Overcast means the entire sky is coveredwith clouds. Current ceiling information is reported by theaviation routine weather report (METAR) and automatedweather stations of various types.VisibilityClosely related to cloud cover and reported ceilings isvisibility information. Visibility refers to the greatesthorizontal distance at which prominent objects can beviewed with the naked eye. Current visibility is also reportedin METAR and other aviation weather reports, as well asby automated weather systems. Visibility information, aspredicted by meteorologists, is available for a pilot during apreflight weather briefing.PrecipitationPrecipitation refers to any type of water particles that form inthe atmosphere and fall to the ground. It has a profound impacton flight safety. Depending on the form of precipitation, it canreduce visibility, create icing situations, and affect landingand takeoff performance of an aircraft.Precipitation occurs because water or ice particles in cloudsgrow in size until the atmosphere can no longer support them.It can occur in several forms as it falls toward the Earth,including drizzle, rain, ice pellets, hail, snow, and ice.Drizzle is classified as very small water droplets, smallerthan 0.02 inches in diameter. Drizzle usually accompaniesfog or low stratus clouds. Water droplets of larger size arereferred to as rain. Rain that falls through the atmosphere butevaporates prior to striking the ground is known as virga.Freezing rain and freezing drizzle occur when the temperatureof the surface is below freezing; the rain freezes on contactwith the cooler surface.If rain falls through a temperature inversion, it may freeze asit passes through the underlying cold air and fall to the groundin the form of ice pellets. Ice pellets are an indication of atemperature inversion and that freezing rain exists at a higheraltitude. In the case of hail, freezing water droplets are carriedup and down by drafts inside clouds, growing larger in size asthey come in contact with more moisture. Once the updraftscan no longer hold the freezing water, it falls to the Earth inthe form of hail. Hail can be pea sized, or it can grow as largeas five inches in diameter, larger than a softball.Snow is precipitation in the form of ice crystals that fallsat a steady rate or in snow showers that begin, change inintensity, and end rapidly. Falling snow also varies in size,being very small grains or large flakes. Snow grains are theequivalent of drizzle in size.Precipitation in any form poses a threat to safety of flight.Often, precipitation is accompanied by low ceilings andreduced visibility. Aircraft that have ice, snow, or frost ontheir surfaces must be carefully cleaned prior to beginninga flight because of the possible airflow disruption andloss of lift. Rain can contribute to water in the fuel tanks.Precipitation can create hazards on the runway surface itself,making takeoffs and landings difficult, if not impossible, dueto snow, ice, or pooling water and very slick surfaces.Air MassesAir masses are classified according to the regions wherethey originate. They are large bodies of air that take on thecharacteristics of the surrounding area, or source region. Asource region is typically an area in which the air remainsrelatively stagnant for a period of days or longer. Duringthis time of stagnation, the air mass takes on the temperatureand moisture characteristics of the source region. Areas ofstagnation can be found in polar regions, tropical oceans, anddry deserts. Air masses are generally identified as polar ortropical based on temperature characteristics and maritimeor continental based on moisture content.A continental polar air mass forms over a polar region andbrings cool, dry air with it. Maritime tropical air masses formover warm tropical waters like the Caribbean Sea and bringwarm, moist air. As the air mass moves from its source regionand passes over land or water, the air mass is subjected to thevarying conditions of the land or water, and these modify thenature of the air mass. [Figure 11-24]An air mass passing over a warmer surface is warmed frombelow, and convective currents form, causing the air to rise.This creates an unstable air mass with good surface visibility.Moist, unstable air causes cumulus clouds, showers, andturbulence to form.Conversely, an air mass passing over a colder surface does notform convective currents, but instead creates a stable air masswith poor surface visibility. The poor surface visibility is dueto the fact that smoke, dust, and other particles cannot riseout of the air mass and are instead trapped near the surface.A stable air mass can produce low stratus clouds and fog.FrontsAs an air mass moves across bodies of water and land, iteventually comes in contact with another air mass withdifferent characteristics. The boundary layer between two typesof air masses is known as a front. An approaching front of anytype always means changes to the weather are imminent.11-18
AcPNote standard air mass abbreviations: arctic (A), continental polar(cP), maritime polar (mP), continental tropical (cT), and maritimetropical (mT).mPmPmTcTmTmTFigure 11-24. North American air mass source regions.There are four types of fronts, which are named accordingto the temperature of the advancing air relative to thetemperature of the air it is replacing: [Figure 11-25]• Warm• Cold• Stationary• OccludedAny discussion of frontal systems must be tempered withthe knowledge that no two fronts are the same. However,generalized weather conditions are associated with a specifictype of front that helps identify the front.Table ASymbols for surface fronts and other significant linesshown on the surface analysis chartWarm front (red)*Cold front (blue)*Stationary front (red/blue)*Occluded front (purple)** Note: Fronts may be black and white or color, depending on theirsource. Also, fronts shown in color code will not necessarily showfrontal symbols.Figure 11-25. Common chart symbology to depict weather frontlocation.Warm FrontA warm front occurs when a warm mass of air advancesand replaces a body of colder air. Warm fronts move slowly,typically 10 to 25 miles per hour (mph). The slope of theadvancing front slides over the top of the cooler air andgradually pushes it out of the area. Warm fronts contain warmair that often have very high humidity. As the warm air islifted, the temperature drops and condensation occurs.Generally, prior to the passage of a warm front, cirriformor stratiform clouds, along with fog, can be expected toform along the frontal boundary. In the summer months,cumulonimbus clouds (thunderstorms) are likely to develop.Light to moderate precipitation is probable, usually in theform of rain, sleet, snow, or drizzle, accentuated by poorvisibility. The wind blows from the south-southeast, and theoutside temperature is cool or cold, with an increasing dewpoint. Finally, as the warm front approaches, the barometricpressure continues to fall until the front passes completely.During the passage of a warm front, stratiform clouds arevisible and drizzle may be falling. The visibility is generallypoor, but improves with variable winds. The temperaturerises steadily from the inflow of relatively warmer air. Forthe most part, the dew point remains steady and the pressurelevels off.11-19
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Page 15 and 16: Dew and FrostOn cool, calm nights,
Page 17: which lifting action of the air beg
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Page 25 and 26: As hailstones fall through air whos

Chapter 11: Weather Theory - FAA

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