OVER AND BEYOND

the hazard. Since turbulence in the forecast vol.ume is patchy, you can expect to encounter it onlyintermittently and possibly not at all. A flightthrough forecast turbulence, on the average, en·counters only light and annoying turbulence 10to 15 percent of the time; about 2 to 3 percent ofthe time there is a need to have all objects secured;the pilot experiences control problems only abouttwo·tenths of 1 percent of the time-Ddds of thisgenuinely hazardous turbulence are about 1 in 500.Look again at figure 126. Where are the mostprobable areas of CAT? Turbulence would begreatest near the windspeed maxima, usually onthe polar sides where there is a combination ofstrong wind shear, curvature in the flow, and coldair advection. These areas would be to the northwest of Vancouver Island, from north of the GreatLakes to east of James Bay and over the Atlanticeast of Newfoundland. Also, turbulence in the formof mountain waves is probable in the vicinity ofthe jet stream from southern California across theRockies into the Central Plains.In flight planning, use upper air charts and forecasts to locate the jet stream, wind shears, and areasof most probable turbulence. AVIATION WEATHERSERVICES (AC 00-45) explains in detail how toobtain these parameters. If impractical to avoidcompletely an area of forecast turbulence, proceedwith caution. You will do well to avoid areas wherevertical shear exceeds 6 knots per 1,000 feet or horizontal shear exceeds 40 knots per 150 miles.What can you do if you get into CAT rougherthan you care to fly? If near the jet core, you couldclimb or descend a few thousand feet or youcould move farther from the jet core. If caught inCAT not associated with the jet stream, your bestbet is to change altitude since you have no positiveway of knowing in which direction the strongestshear lies. Pilot reports from other flights, whenavailable, are helpful.Flight maneuvers increase stresses on the aircraftas does turbulence. The increased stresses are cumulative when the aircraft maneuvers in turbulence. Maneuver gently when in turbulence tominimize stress. The patchy nature of CAT makescurrent pilot reports extremely helpful to observers,briefers, forecasters, air traffic controllers, and,most important, to your fellow pilots. Always, if atall possible, make inflight weather reports of CATor other turbulence encounters; negative reportsalso help when no CAT is experienced where itnormally might be expected.CONDENSATION TRAILSA condensation trail, popularly contracted to"contrail," is generally defined as a cloud-likestreamer which frequently is generated in the wakeof aircraft flying in clear, cold, humid air, figure130. Two distinct types are observed-exhaust trailsand aerodynamic trails. "Distrails," contracted fromdissipation trails, are produced differently from exhaust and aerodynamic trails.EXHAUST CONTRAILSThe exhaust contrail is formed by the additionto the atmosphere of sufficient water vapor fromaircraft exhaust gases to cause saturation or supersaturation of the air. Since heat is also added to theatmosphere in the wake of an aircraft, the additionof water vapor must be of such magnitude that itsaturates or supersaturates the atmosphere in spiteof the added heat. There is evidence to support theidea that the nuclei which are necessary for con·densation or sublimation may also be donated tothe atmosphere in the exhaust gases of aircraftengines, further aiding contrail formation. Thesenuclei are relatively large. Recent experiments,however, have revealed that visible exhaust contrails may be prevented by adding very minutenuclei material (dust, for example) to the exhaust.Condensation and sublimation on these smaller nuclei result in contrail particles too small to be visible.AERODYNAMIC CONTRAILSIn air that is almost saturated, aerodynamicpressure reduction around airfoils, engine nacelles,and propellers cools the air to saturation leavingcondensation trails from these components. Thistype of trail usually is neither as dense nor as persistent as exhaust trails. However, under criticalatmospheric conditions, an aerodynamic contrailmay trigger the formation and spreading of a deckof cirrus clouds.Contrails create one problem unique to militaryoperations in that they reveal the location of anaircraft attempting to fly undetected. A more general operational problem is a cirrus layer sometimes induced by the contrail. The induced layer143

FIGURE130.Contrails. The thin contrail is freshly formed by an aircraft (not visible) in the lower rightcenter of the photograph.may make necessary the strict use of instrumentsby a subsequent flight at that altitude.DISSIPATION TRAILS (DISTRAILS)The term dissipation trail applies to a rift inclouds caused by the heat of exhaust gases from anaircraft flying in a thin cloud layer. The exhaustgases sometimes warm the air to the extent that itis no longer saturated, and the affected part of thecloud evaporates. The cloud must be both thin andrelatively warm for a distrail to exist; therefore,they are not common.HAZE LAYERSHaze layers not visible from the ground are, attimes, of concern at high altitude. These layers arereally cirrus clouds with a very low density of icecrystals. Tops of these layers generally are verydefinite and are at the tropopause. High level hazeoccurs in stagnant air; it is rare in fresh outbreaksof cold polar air. Cirrus haze is common in Arctic144winter. Sometimes ice crystals restrict visibilityfrom the surface to the tropopause.Visibility in the haze sometimes may be nearzero, especially when one is facing the sun. Toavoid the poor visibility, climb into the lower stratosphere or descend below the haze. This change maybe several thousand feet.

r jCANOPY STATICCanopy static, similar to the precipitation staticsometimes encountered at lower levels, is producedby particles brushing against plastic-covered aircraft surfaces. The discharge of static electricityresults in a noisy disturbance that interferes withradio reception. Discharges can occur in such rapidsuccession that interference seems to be continuous.Since dust and ice crystals in cirrus clouds are theprimary producers of canopy static, usually youmay eliminate it by changing altitude.ICINGAlthough icing at high altitude is not as commonor extreme as at low altitudes, it can occur. It canform quickly on airfoils and exposed parts of jetengines. Structural icing at high altitudes usuallyis rime, although clear ice is possible.High altitude icing generally forms in tops oftall cumulus buildups, anvils, and even in detachedcirrus. Clouds over mountains are more likely tocontain liquid water than those over more gentlysloping terrain because of the added lift of themountains. Therefore, icing is more likely to occurand to be more hazardous over mountainous areas.Because ice generally accumulates slowly at highaltitudes, anti-icing equipment usually eliminatesany serious problems. However, anti-icing systemscurrently in use are not always adequate. If such isthe case, avoid the icing problem by changing altitude or by varying course to remain clear of theclouds. Chapter 10 discusses aircraft icing in moredetail.THUNDERSTORMSA well-developed thunderstorm may extend upward through the troposphere and penetrate thelower stratosphere. Sometimes the main updraft ina thunderstorm may toss hail out the top or theupper portions of the storm. An aircraft may encounter hail in clear air at a considerable distancefrom the thunderstorm, especially under the anvilcloud. Turbulence may be encountered in clearair for a considerable distance both above andaround a growing thunderstorm.Thunderstorm avoidance rules given in chapter11 apply equally at high altitude. When flying inthe clear, visually avoid all thunderstorm tops. In asevere thunderstorm situation, avoid tops by atleast 20 miles. When you are on instruments,weather avoidance radar assures you of avoidingthunderstorm hazards. If in an area of severe thunderstorms, avoid the most intense echoes by at least20 miles. Most air carriers now use this distance asthe minimum for thunderstorm avoidance.145

Chapter 14ARCTIC WEATHERThe Arctic, strictly speaking, is the region shownin figure 131 which lies north of the Arctic Circle(66 0 latitude). However, this chapter includesAlaskan weather even though much of Alaska liessouth of the Arctic Circle.Because of the lack of roads over most Arcticareas, aviation is the backbone of transportationbetween communities. As the economy expands,- sowill air transportation.Your most valuable source of information concerning flying the Arctic is the experienced Arcticflyer. To introduce you to Arctic flying weather,this chapter surveys climate, air masses, and frontsof the Arctic; introduces you to some Arctic weatherpeculiarities; discusses weather hazards in the Arctic; and comments on Arctic flying.147

FIGURE131. The Arctic. The Arctic Circle is at66 oN latitude.CLIMATE, AIR MASSES, AND FRONTSClimate of any region is largely determined bythe amount of energy received from the sun; butlocal characteristics of the area also influenceclimate.LONG DAYS AND NIGHTSA profound seasonal change in length of day andnight occurs in the Arctic because of the Earth'stilt and its revolution around the sun. Figure 132shows that any point north of the Arctic Circle has148autumn and winter days when the sun stays all daybelow the horizon and days in spring and summerwith 24 hours of sunshine. The number of thesedays increases toward the North Pole; there the sunstays below the horizon for 6 months and shinescontinuously during the other 6 months.Twilight in the Arctic is prolonged because ofthe shallow angle of the sun below the horizon. Inmore northern latitudes, it persists for days whenthe sun remains just below the horiwn. This

SUMMER2418SUNSHINE12I680 NORTH LAT.02418SUNSHINEHOURS12I6BARTER ISLAND 70 8' NORTH LAT.02418SUNSHINE12I60ANCHORAGE 61 10' NORTH LAT.JANFEBMARAPRMAYJUNAUGSEPTOCTNOVDEC132. Sunshine in the Northern Hemisphere. The sun shines a full 24 hours on the entire area north of the ArcticCircle (top) on June 21; the amount of sunshine decreases until none falls anywhere in the area on December 22. Graphs(below) show duration of sunshine and nautical twilight per day at two points north of the Arctic Circle and for Anchorage.Alaska, at a latitude about 5 Yz0 south of the circle.FIGURE149

abundance of twilight often makes visual referencepossible at night.LAND AND WATERFigure 131 shows the water and land distributionin the Arctic. Arctic mountain ranges are effectivebarriers to air movement. Large masses of air stagnate over the inland continental areas. Thus, theArctic continental areas are air mass source regions.A large portion of the Arctic Ocean is coveredthroughout the year by a deep layer of ice-thepermanent ice pack as shown in figure 133. Eventhough the ocean is ice-covered through much ofthe year, the ice and the water below contain moreheat than the surrounding cold land, thus moderating the climate to some extent. Oceanic and coastalareas have a milder climate during winter thanwould be expected and a cool climate in summer.As opposed to large water bodies, large land areasshow a more significant seasonal temperaturevariation.TEMPERATUREAs one would expect, the Arctic is very cold inwinter; but due to local terrain and the movementof pressure systems, occasionally some areas are sur-IOELAND. .fr,/1/ ! . JFIGURE150133.The pennanent Arctic ice pack.

prisingly warm. During winter, coastal areas average about 20 degrees warmer than the interior.During summer, interior areas are pleasantly warmwith many hours of sunshine. Coastal areas haverelatively cool short summers due to their proximityto water.CLOUDS AND PRECIPITATIONCloudiness over the Arctic is at a minimum during winter reaching a maximum in summer andfall, figure 134. Spring also brings many cloudy\.days. During summer afternoons, scattered cumulusclouds forming over the interior occasionally growinto thundershowers. These thundershowers, usuallycircumnavigable, move generally from northeast tosouthwest in the polar easterlies which is oppositethe general movement in midlatitudes.Precipitation in the Arctic is generally light. Annual amounts over the ice pack and along thecoastal areas are only 3 to 7 inches. The interior issomewhat wetter, with annual amounts of 5 to 15inches. Precipitation falls mostly in the form ofCAPE ,CHELIUSKIN"'' , \.)'I1!Jt./.jI"1/"""'jiIISEASON.MAYrHROUGH OCTOBERto'II'"l:,COLD SEASON·NOVEMBER'1/'/ THROUGH APRILt (FIGURE4134.zI",1//W,ARMto'I/1.' '.;!Iy.l./'i,Average number of cloudy days per month. Note that most stations show the greatest number of cloudy daysin the warmer season.151

snow over ice caps and oceanic areas and mostly assummer rain over interior areas.infrequent wintertime cloudiness and precipitationin the Arctic.AIR MASSES-SUMMERWINDStrong winds occur more often along the coaststhan elsewhere. The frequency of high winds incoastal areas is greatest in fall and winter. Windspeeds are generally light in the continental interiorduring the entire year, but are normally at theirstrongest during summer and fall.AIR MASSES-WINTERIn winter, air masses form over the expanded icepack and adjoining snow-covered land areas. Theseair masses are characterized by very cold surfaceair, very low humidity, and strong low-level temperature inversions. Occasionally, air from unfrozenocean areas flows northward over the Arctic. Theseintrusions of moist, cold air account for most of theDuring the summer, the top layer of the Arcticpermafrost layer melts leaving very moist ground,and the open water areas of the Polar Basin increase markedly. Thus, the entire area becomesmore humid, relatively mild, and semimaritime incharacter. The largest amount of cloudiness andprecipitation occurs inland during the summermonths.FRONTSOccluded fronts are the rule. Weather conditionswith occluded fronts are much the same in theArctic as elsewhere-low clouds, precipitation, poorvisibility, and sudden fog formation. Fronts aremuch more frequent over coastal areas than overthe interior.ARCTIC PECULIARITIESSeveral Arctic phenomena are peculiar to thatregion. At times, they have a direct bearing onArctic flying.EFFECTS OF TEMPERATURE INVERSIONThe intense low-level inversion over the Arcticduring much of the winter causes sound-includingpeople's voices-to carryover extremely long distances. Light rays are bent as they pass at low angles through the inversion. This bending creates aneffect known as looming-a form of mirage thatcauses objects beyond the horizon to appear abovethe horizon. Mirages distorting the shape of thesun, moon, and other objects are common withthese low level inversions.AURORA BOREALISIn theory, certain energy particles from the sunstrike the Earth's magnetic field and are carriedalong the lines of force where they tend to lowerand converge near the geomagnetic poles. The energy particles then pass through rarefied gases ofthe outer atmosphere, illuminating them in muchthe same way as an electrical charge illuminatesneon gas in neon signs.The Aurora Borealis takes place at high altitudesabove the Earth's surface and thus has been observed as far south as Florida. However, the highest152frequency of observations is over the northernUnited States and northward. Displays of auroravary from a faint glow to an illumination of theEarth's surface equal to a full moon. They frequently change shape and form and are also calleddancing lights or northern lights.LIGHT REFLECTION BYSNOW-COVERED SURFACESMuch more light is reflected by snow-coveredsurfaces than by darker surfaces. Snow often reflects Arctic sunlight sufficiently to blot out shadows, thus markedly decreasing the contrast betweenobjects. Dark distant mountains may be easily recognized, but a crevasse normally directly in viewmay be undetected due to lack of contrasts.LIGHT FROM CELESTIAL BODIESIllumination from the moon and stars is muchmore intense in the Arctic than in lower latitudes.Pilots have found that light from a half-moon overa snow-covered field may be sufficient for landing.Even illumination from the stars creates visibilityfar beyond that found elsewhere. Only under heavyovercast skies does the night darkness in the Arcticbegin to approach the degree of darkness in lowerlatitudes.

WEATHER HAZARDSWeather hazards include visibility restricting phenomena, blowing snow, icing, frost, and lack ofcontrast-whiteout.heating as the air descends downslope. Icing in advection fog is in the form of rime and may becomequite severe.FOGBLOWING SNOWFog limits landing and takeoff in the Arctic morethan any other visibility restriction. Water-dropletfog is the main hazard to aircraft operations incoastal areas during the summer. Ice fog is themajor restriction in winter.Over the frozen Arctic Ocean and along thecoastal areas, blowing snow and strong winds arecommon hazards during autumn and winter. Blowing snow is a greater hazard to flying operations inthe Arctic than in mid latitudes because the snow is«dry" and fine and can be picked up easily by lightwinds. Winds in excess of 8 knots may raise thesnow several feet off the ground obliterating objects such as runway markers as illustrated in figure135. A sudden increase in surface wind may causean unlimited visibility to drop to near zero in a fewminutes. This sudden loss of visibility occurs frequently without warning in the Arctic. Strongerwinds sometimes lift blowing snow to heights above1,000 feet and produce drifts over 30 feet deep.Ice FogIce fog is common in the Arctic. It forms inmoist air during extremely cold, calm conditions inwinter, occurring often and tending to persist. Effective visibility is reduced much more in ice fogwhen one is looking toward the sun. Ice fog maybe produced both naturally and artificially. Ice fogaffecting aviation operations most frequently is produced by the combustion of aircraft fuel in coldair. When the wind is very light and the temperature is about -30 0 F or colder, ice fog oftenforms instantaneously in the exhaust gases of automobiles and aircraft. It lasts from as little as a fewminutes to days.Steam FogSteam fog, often called "sea smoke," forms inwinter when cold, dry air passes from land areasover comparatively warm ocean waters. Moistureevaporates rapidly from the water surface; butsince the cold air can hold only a small amount ofwater vapor, condensation takes place just abovethe surface of the water and appears as "steam"rising from the ocean. This fog is composed entirelyof water droplets that often freeze quickly and fallback into the water as ice particles. Low level turbulence can occur and icing can become hazardous.Advection FogAdvection fog, which may be composed either ofwater droplets or of ice crystals, is most common inwinter and is often persistent. Advection fog formsalong coastal areas when comparatively warm,moist, oceanic air moves over cold land. If the landareas are hilly or mountainous, lifting of the airresults in a combination of low stratus and fog. Thestratus and fog quickly diminish inland. Lee sidesof islands and mountains usually are free of advection fog because of drying due to compressionalICINGIcing is most likely in spring and fall, but is alsoencountered in winter. During spring and fall, icingmay extend to upper levels along frontal zones.While icing is mostly a problem over water andcoastal areas, it does exist inland. It occurs typicallyas rime, but a combination of clear and rime is notunusual in coastal mountains.FROSTIn coastal areas during spring, fall, and winter,heavy frost and rime may form on aircraft parkedoutside, especially when fog or ice fog is present.This frost should be removed; it reduces lift and isespecially hazardous if surrounding terrain requiresa rapid rate of climb.WHITEOUT"Whiteout" is a visibility restrict

Artist's concept of the jet stream. Broad arrow shows direction of wind. stream. The jet maximum is not constant; rather, it is broken into segments, shaped something like a boomerang as diagrammed in figure 125. Jet stream segments move with pressure ridges and troughs in the upper atmosphere. In general