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Home , Convergence zone, Jet stream, Polar high, Stratosphere, Tropopause, Troposphere

Part TWO OVER AND BEYOND

1

Chapter 13 HIGH

II Many general aviation as well as air carrier and densation trails, high altitude "haze" layers, and military aircraft routinely fly the upper canopy static. This chapter explains these phenom­ j and lower . Weather phenomena of ena along with the high altitude aspects of the these higher include the ., the more common icing and hazards. , cirrus , clear air turbulence, con-

135 THE TROPOPAUSE

Why is the high altitude pilot interested in the tropopause is not continuous but generally descends tropopause? and vary greatly in step-wise from the to the poles. These the vicinity of the tropopause affecting efficiency, steps occur as "breaks." Figure 123 is a cross comfort, and safety of flight. Maximum gen­ section of the troposphere and lower stratosphere erally occur at levels near the tropopause. These showing the tropopause and associated features. strong winds create narrow zones of Note the break between the tropical and the which often generate hazardous turbulence. Pre­ tropopauses. flight knowledge of temperatu~e, . wind, and wind An abrupt change in temperature shear is important to . characterizes the tropopause. Note in figure 123 In chapter 1, we learned that the tropopause is how temperature above the tropical tropopause in­ a thin layer forming the boundary between the creases with height and how over the polar tropo­ troposphere and stratosphere. Height of the tropo­ pause, temperature remains almost constant with pause varies from about 65,000 feet over the Equa­ height. tor to 20,000 feet or lower over the poles. The

_ 045 0 _ 50· _____ ._600 I / --- , / .- 60,000' , / .... ,.- .:: I / 0 - 70 I I I I \ \ _ 70 0 I \ \ , _ 60·· 40,0000 , I _ 50° ,.- / 0 _ 45. - - .- O,AUSI _ 40° 'OLAR 110' _ 30 ° _ 40°. ------20° 20,000' 0 - _ 10· _ 30 ------.- - - ..- -- ..-

FIGURE 123. A cross section of the upper troposphere and lower stratosphere showing the tropopause and associated fea­ tures. Note the "break" between the high tropical and the lower polar tropopause. Maximum winds occur in the vicinity of this break.

THE JET STREAM

Diagrammed in figure 124, the jet stream is a the . The jet stream typically occurs in a narrow, shallow, meandering river of maximum break in the tropopause as shown in figure 123. winds extending around the globe in a wavelike Therefore, a jet stream occurs in an area of inten­ pattern. A second jet stream is not uncommon, and sified temperature gradients characteristic of the three at one time are not unknown. A jet may be break. as far south as the northern Tropics. A jet in. mid­ The concentrated winds, by arbitralY definition, latitudes generally is stronger than one in or near must be 50 knots or greater to classify as a jet

136 FIGURE 124. Artist's concept of the jet stream. Broad arrow shows direction of wind. stream. The jet maximum is not constant; rather, imum varies as the segments progress it is broken into segments, shaped something like through the systems. In midlatitude, wind speed in a boomerang as diagrammed in figure 125. the jet stream averages considerably stronger in Jet stream segments move with ridges than in . Also the jet shifts farther and troughs in the upper . In general south in winter than in summer. they travel faster than pressure systems, and max- I~~4------1000-3000mi ------~~ I

I-- ,00 - 400 mi

FIGURE 125. A jet stream segment.

137 In figure 123 note how wind speed decreases out­ zontal wind shear is evident on both sides of the jet ward from the jet core. Note also that the rate of and is greatest near the maximum wind segments. decrease of wind speed is considerably greater on Strong, long-trajectory jet streams usually are the polar side than on the equatorial side; hence, associated with well-developed surface lows and the magnitude of wind shear is greater on the polar frontal systems beneath deep upper troughs or lows. side than on the equatorial side. is usually south of the jet stream and Figure 126 shows a map with two jet streams. moves nearer as the low deepens. The occluding The paths of the jets approximately conform to the low moves north of the jet, and the jet crosses the shape of the contours. The northerly jet has three frontal system near the point of occlusion. Figure segments of maximum wind, and the southerly one 127 diagrams mean jet positions relative to surface has two. Note how spacing of the height contours systems. These long jets mark high level boundaries is closer and wind speeds higher in the vicinity of between warm and cold air and are favored places the jets than outward on either side. Thus hori- for cirriform cloudiness.

50-99

• 100--149

• 150 PLUS

FIGURE 126. Multiple jet streams. Note the "segments" of maximum winds embedded in the general pattern. Turbulence usually is greatest on the polar sides of these maxima.

138 FIGURE 127. M ean jet positions relative to surface systems. Cyclogenesis (development) of a surface low usually is south of the jet as shown on the left. The deepening low moves nearer the jet, center. As it occludes, the low moves north of the jet, right; the jet crosses the frontal system near the point of occlusion.

CIRRU S CLOUDS

Air travels in a "corkscrew" path around the jet clouds. Figure 128b is an infrared photo of the core with upward motion on the equatorial side. same system; the light shade of the cirrus band in­ Therefore, when high level moisture is available, dicates cold while warmer low clouds cirriform clouds form on the equatorial side of the are the darker shades. jet. Jet stream cloudiness can form independently The upper limit of dense, banded cirrus is near of well-defined pressure systems. Such cloudiness the tropopause; a band may be either a single layer ranges primarily from scattered to broken coverage or multiple layers 10,000 to 12,000 feet thick. in shallow layers or streaks. Their sometimes fish Dense, jet stream cirriform cloudiness is most prev­ hook and streamlined, wind-swept appearance al­ alent along midlatitude and polar jets. However, a ways indicates very strong upper wind usually quite cirrus band usually forms along the subtropical jet far from developing or intense weather systems. in winter when a deep upper plunges south­ The most dense cirriform clouds occur with well­ ward into the Tropics. defined systems. They appear in broad bands. Cirrus clouds, in themselves, have little effect on Cloudiness is rather dense in an upper trough, aircraft. However, dense, continuous coverage re­ thickens downstream, and becomes most dense at quires a pilot's constant reference to instruments; the crest of the downwind ridge. The clouds taper most pilots find this more tiring than flying with a off after passing the ridge crest into the area of de­ visual horizon even though IFR. scending air. The poleward boundary of the cirrus A more important aspect of the jet stream cirrus band often is quite abrupt and frequently casts a shield is its association with turbulence. Extensive shadow on lower clouds, especially in an occluded cirrus cloudiness often occurs with deepening sur­ frontal system. Figure 128a is a satellite photograph face and upper lows; and these deepening systems showing a cirrus band casting a shadow on lower produce the greatest turbulence.

139 FIGURE 128a. Satellite photograph of an occluded system centered at about 44° Nand 13T W. Here, the jet extends south-southwest to north-northeast along the polar (more westerly) boundary of the cirrus band from 35° N, 141° W through 43° N, 135° W to 51 ° N, 130° W. Shadow of the cirrus band is clearly evident as a narrow dark line from 45° N, 134.5° W to 49° N, 132° W.

140 FIGURE 128b. Infrared photograph of the system shown in figure 128a. The warmer the radiating surface, the darker the shade; the cold cirrus appears nearly white. Infrared clearly distinguishes the banded jet stream cirrus from other cirrus and lower clouds.

141 CLEAR AIR TURBULENCE

Clear air turbulence (CAT) implies turbulence CAT often is experienced in wind shears associated devoid of clouds. However, we commonly reserve with sharply curved contours of strong lows, troughs, the term for high level wind shear turbulence, even and ridges aloft, and in areas of strong, cold or when in cirrus clouds. warm air advection. Also mountain waves can cre­ Cold outbreaks colliding with warm air from the ate CAT. Mountain wave CAT may extend from south intensify weather systems in the vicinity of the mountain crests to as high as 5,000 feet above the jet stream along the boundary between the the tropopause, and can range 100 miles or more cold and warm air. CAT develops in the turbulent downstream from the mountains. energy exchange between the contrasting air masses. CAT can be encountered where there seems to Cold and warm advection along with strong wind be no reason for its occurrence. Strong winds may shears develop near the jet stream, especially where carry a turbulent volume of air away from its source curvature of the jet stream sharply increases in region. Turbulence intensity diminishes down­ deepening upper troughs. CAT is most pronounced stream, but some turbulence still may be encoun­ in winter when temperature contrast is greatest be­ tered where it normally would not be expected. tween cold and warm air. CAT forecast areas are sometimes elongated to in­ A preferred location of CAT is in an upper dicate probable turbulence drifting downwind from trough on the cold '( polar) side of the jet stream. the main source region. Another frequent CAT location, shown in figure A forecast of turbulence specifi es a volume of 129, is along the jet stream north and northeast of airspace which is quite small when compared to the a rapidly deepening surface low. total volume of airspace used by aviation, but is Even in the absence of a well-defined jet stream, relatively large compared to the localized extent of

FIGURE 129. A frequent CAT location is along the jet stream north and northeast of a rapidly deepening surface low.

142 the hazard. Since turbulence in the forecast vol. obtain these parameters. If impractical to avoid ume is patchy, you can expect to encounter it only completely an area of forecast turbulence, proceed intermittently and possibly not at all. A flight with caution. You will do well to avoid areas where through forecast turbulence, on the average, en· vertical shear exceeds 6 knots per 1,000 feet or hor­ counters only light and annoying turbulence 10 izontal shear exceeds 40 knots per 150 miles. to 15 percent of the time; about 2 to 3 percent of What can you do if you get into CAT rougher the time there is a need to have all objects secured; than you care to fly? If near the jet core, you could the pilot experiences control problems only about climb or descend a few thousand feet or you two·tenths of 1 percent of the time-Ddds of this could move farther from the jet core. If caught in genuinely hazardous turbulence are about 1 in 500. CAT not associated with the jet stream, your best Look again at figure 126. Where are the most bet is to change altitude since you have no positive probable areas of CAT? Turbulence would be way of knowing in which direction the strongest greatest near the windspeed maxima, usually on shear lies. Pilot reports from other flights, when the polar sides where there is a combination of available, are helpful. strong wind shear, curvature in the flow, and cold Flight maneuvers increase stresses on the aircraft air advection. These areas would be to the north­ as does turbulence. The increased stresses are cu­ west of Vancouver Island, from north of the Great mulative when the aircraft maneuvers in turbu­ Lakes to east of James Bay and over the Atlantic lence. Maneuver gently when in turbulence to east of Newfoundland. Also, turbulence in the form minimize stress. The patchy of CAT makes of mountain waves is probable in the vicinity of current pilot reports extremely helpful to observers, the jet stream from southern California across the briefers, forecasters, air traffic controllers, and, Rockies into the Central Plains. most important, to your fellow pilots. Always, if at In flight planning, use upper air charts and fore­ all possible, make inflight weather reports of CAT casts to locate the jet stream, wind shears, and areas or other turbulence encounters; negative reports of most probable turbulence. AVIATION WEATHER also help when no CAT is experienced where it SERVICES (AC 00-45) explains in detail how to normally might be expected.

CONDENSATION TRAILS

A condensation trail, popularly contracted to nuclei are relatively large. Recent experiments, "," is generally defined as a -like however, have revealed that visible exhaust con­ streamer which frequently is generated in the wake trails may be prevented by adding very minute of aircraft flying in clear, cold, humid air, figure nuclei material (dust, for example) to the exhaust. 130. Two distinct types are observed-exhaust trails Condensation and sublimation on these smaller nu­ and aerodynamic trails. "Distrails," contracted from clei result in contrail particles too small to be visible. dissipation trails, are produced differently from ex­ haust and aerodynamic trails. AERODYNAMIC In air that is almost saturated, aerodynamic EXHAUST CONTRAILS pressure reduction around airfoils, engine nacelles, The exhaust contrail is formed by the addition and propellers cools the air to saturation leaving to the atmosphere of sufficient from condensation trails from these components. This aircraft exhaust gases to cause saturation or super­ type of trail usually is neither as dense nor as per­ saturation of the air. Since is also added to the sistent as exhaust trails. However, under critical atmosphere in the wake of an aircraft, the addition atmospheric conditions, an aerodynamic contrail of water vapor must be of such magnitude that it may trigger the formation and spreading of a deck saturates or supersaturates the atmosphere in spite of cirrus clouds. of the added heat. There is evidence to support the Contrails create one problem unique to military idea that the nuclei which are necessary for con· operations in that they reveal the location of an densation or sublimation may also be donated to aircraft attempting to fly undetected. A more gen­ the atmosphere in the exhaust gases of aircraft eral operational problem is a cirrus layer some­ engines, further aiding contrail formation. These times induced by the contrail. The induced layer

143 FIGURE 130. Contrails. The thin contrail is freshly formed by an aircraft (not visible) in the lower right center of the photograph.

may make necessary the strict use of instruments aircraft flying in a thin cloud layer. The exhaust by a subsequent flight at that altitude. gases sometimes warm the air to the extent that it is no longer saturated, and the affected part of the DISSIPATION TRAILS (DISTRAILS) cloud evaporates. The cloud must be both thin and The term dissipation trail applies to a rift in relatively warm for a distrail to exist; therefore, clouds caused by the heat of exhaust gases from an they are not common.

HAZE LAYERS

Haze layers not visible from the ground are, at winter. Sometimes ice crystals restrict visibility times, of concern at high altitude. These layers are from the surface to the tropopause. really cirrus clouds with a very low of ice Visibility in the haze sometimes may be near crystals. Tops of these layers generally are very zero, especially when one is facing the sun. To definite and are at the tropopause. High level haze avoid the poor visibility, climb into the lower strato­ occurs in stagnant air; it is rare in fresh outbreaks sphere or descend below the haze. This change may of cold polar air. Cirrus haze is common in be several thousand feet.

144 r

Canopy static, similar to the static radio reception. Discharges can occur in such rapid sometimes encountered at lower levels, is produced succession that interference seems to be continuous. by particles brushing against plastic-covered air­ Since dust and ice crystals in cirrus clouds are the craft surfaces. The discharge of static electricity primary producers of canopy static, usually you results in a noisy disturbance that interferes with may eliminate it by changing altitude.

ICING

Although icing at high altitude is not as common mountains. Therefore, icing is more likely to occur or extreme as at low altitudes, it can occur. It can and to be more hazardous over mountainous areas. form quickly on airfoils and exposed parts of jet Because ice generally accumulates slowly at high engines. Structural icing at high altitudes usually altitudes, anti-icing equipment usually eliminates is rime, although clear ice is possible. any serious problems. However, anti-icing systems High altitude icing generally forms in tops of currently in use are not always adequate. If such is tall cumulus buildups, anvils, and even in detached the case, avoid the icing problem by changing alti­ cirrus. Clouds over mountains are more likely to tude or by varying course to remain clear of the contain liquid water than those over more gently clouds. Chapter 10 discusses aircraft icing in more sloping terrain because of the added lift of the detail.

THUNDERSTORMS

A well-developed thunderstorm may extend up­ Thunderstorm avoidance rules given in chapter ward through the troposphere and penetrate the 11 apply equally at high altitude. When flying in lower stratosphere. Sometimes the main updraft in the clear, visually avoid all thunderstorm tops. In a a thunderstorm may toss out the top or the severe thunderstorm situation, avoid tops by at upper portions of the . An aircraft may en­ least 20 miles. When you are on instruments, counter hail in clear air at a considerable distance weather avoidance radar assures you of avoiding from the thunderstorm, especially under the anvil thunderstorm hazards. If in an area of severe thun­ cloud. Turbulence may be encountered in clear derstorms, avoid the most intense echoes by at least air for a considerable distance both above and 20 miles. Most air carriers now use this distance as around a growing thunderstorm. the minimum for thunderstorm avoidance.

145

Chapter 14 ARCTIC WEATHER

The Arctic, strictly speaking, is the region shown Your most valuable source of information con­ in figure 131 which lies north of the Arctic Circle cerning flying the Arctic is the experienced Arctic

(66~ 0 latitude). However, this chapter includes flyer. To introduce you to Arctic flying weather, Alaskan weather even though much of Alaska lies this chapter surveys , air masses, and fronts south of the Arctic Circle. of the Arctic; introduces you to some Arctic weather Because of the lack of roads over most Arctic peculiarities; discusses weather hazards in the Arc­ areas, aviation is the backbone of transportation tic; and comments on Arctic flying. between communities. As the economy expands,- so will air transportation.

147 FIGURE 131. The Arctic. The Arctic Circle is at 66~o N latitude.

CLIMATE, AIR MASSES, AND FRONTS

Climate of any region is largely determined by and winter days when the sun stays all day the amount of energy received from the sun; but below the horizon and days in and summer local characteristics of the area also influence with 24 hours of sunshine. The number of these climate. days increases toward the North Pole; there the sun stays below the horizon for 6 months and shines LONG DAYS AND NIGHTS continuously during the other 6 months. A profound seasonal change in length of day and Twilight in the Arctic is prolonged because of night occurs in the Arctic because of the 's the shallow angle of the sun below the horizon. In tilt and its revolution around the sun. Figure 132 more northern latitudes, it persists for days when shows that any point north of the Arctic Circle has the sun remains just below the horiwn. This

148 SUMMER

24

18

12 SUNSHINE

6 I 80° NORTH LAT. 0 24

18 SUNSHINE HOURS 12

6 I BARTER ISLAND 70° 8' NORTH LAT. 0 24

18

12 SUNSHINE

6 I ANCHORAGE 61° 10' NORTH LAT. 0 JAN FEB MAR APR MAY JUN AUG SEPT OCT NOV DEC

FIGURE 132. Sunshine in the . The sun shines a full 24 hours on the entire area north of the Arctic Circle (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. 149 abundance of twilight often makes visual reference the year, the ice and the water below contain more possible at night. heat than the surrounding cold land, thus moderat­ ing the climate to some extent. Oceanic and coastal LAND AND WATER areas have a milder climate during winter than Figure 131 shows the water and land distribution would be expected and a cool climate in summer. in the Arctic. Arctic mountain ranges are effective As opposed to large water bodies, large land areas barriers to air movement. Large masses of air stag­ show a more significant seasonal temperature nate over the inland continental areas. Thus, the variation. Arctic continental areas are source regions. A large portion of the Arctic is covered TEMPERATURE throughout the year by a deep layer of ice-the As one would expect, the Arctic is very cold in permanent ice pack as shown in figure 133. Even winter; but due to local terrain and the movement though the ocean is ice-covered through much of of pressure systems, occasionally some areas are sur-

IOELAND

...... f r ,/1 / ! ____~.~ ______~~~ __J

FIGURE 133. The pennanent Arctic ice pack.

150 prisingly warm. During winter, coastal areas aver­ days. During summer afternoons, scattered cumulus age about 20 degrees warmer than the interior. clouds forming over the interior occasionally grow During summer, interior areas are pleasantly warm into thundershowers. These thundershowers, usually with many hours of sunshine. Coastal areas have circumnavigable, move generally from northeast to relatively cool short due to their proximity southwest in the which is opposite to water. the general movement in midlatitudes. Precipitation in the Arctic is generally light. An­ CLOUDS AND PRECIPITATION nual amounts over the ice pack and along the Cloudiness over the Arctic is at a minimum dur­ coastal areas are only 3 to 7 inches. The interior is ing winter reaching a maximum in summer and somewhat wetter, with annual amounts of 5 to 15 fall, figure 134. Spring also brings many cloudy inches. Precipitation falls mostly in the form of

CAPE , \. CHELIUSKIN "''', \. )' ! I 1 J t. / .j

I " 1/""" 'j/1 I i I ",1 I // to' W,ARM .MAY I rHROUGH OCTOBER ! I l :, I to'COLD SEASON·NOVEMBER y.l I'" '1/'/ ~ THROUGH APRIL ./ 'i t~~~( 4 z .' '.; ,

FIGURE 134. Average number of cloudy days per month. Note that most stations show the greatest number of cloudy days in the warmer season.

151 over ice caps and oceanic areas and mostly as infrequent wintertime cloudiness and precipitation summer over interior areas. in the Arctic. AIR MASSES-SUMMER WIND During the summer, the top layer of the Arctic Strong winds occur more often along the coasts permafrost layer melts leaving very moist ground, than elsewhere. The frequency of high winds in and the open water areas of the Polar Basin in­ coastal areas is greatest in fall and winter. Wind crease markedly. Thus, the entire area becomes speeds are generally light in the continental interior more humid, relatively mild, and semimaritime in during the entire year, but are normally at their character. The largest amount of cloudiness and strongest during summer and fall. precipitation occurs inland during the summer months. AIR MASSES-WINTER In winter, air masses form over the expanded ice FRONTS pack and adjoining snow-covered land areas. These Occluded fronts are the rule. Weather conditions air masses are characterized by very cold surface with occluded fronts are much the same in the air, very low , and strong low-level tem­ Arctic as elsewhere-low clouds, precipitation, poor perature inversions. Occasionally, air from unfrozen visibility, and sudden formation. Fronts are ocean areas flows northward over the Arctic. These much more frequent over coastal areas than over intrusions of moist, cold air account for most of the the interior.

ARCTIC PECULIARITIES

Several Arctic phenomena are peculiar to that frequency of observations is over the northern region. At times, they have a direct bearing on and northward. Displays of Arctic flying. vary from a faint glow to an illumination of the Earth's surface equal to a full moon. They fre­ EFFECTS OF TEMPERATURE INVERSION quently change shape and form and are also called The intense low-level inversion over the Arctic dancing lights or northern lights. during much of the winter causes sound-including people's voices-to carryover extremely long dis­ LIGHT REFLECTION BY tances. Light rays are bent as they pass at low an­ SNOW-COVERED SURFACES gles through the inversion. This bending creates an Much more light is reflected by snow-covered effect known as looming-a form of mirage that surfaces than by darker surfaces. Snow often re­ causes objects beyond the horizon to appear above flects Arctic sunlight sufficiently to blot out shad­ the horizon. Mirages distorting the shape of the ows, thus markedly decreasing the contrast between sun, moon, and other objects are common with objects. Dark distant mountains may be easily rec­ these low level inversions. ognized, but a crevasse normally directly in view AURORA BOREALIS may be undetected due to lack of contrasts. In theory, certain energy particles from the sun strike the Earth's magnetic field and are carried LIGHT FROM CELESTIAL BODIES along the lines of force where they tend to lower Illumination from the moon and stars is much and converge near the geomagnetic poles. The en­ more intense in the Arctic than in lower latitudes. ergy particles then pass through rarefied gases of Pilots have found that light from a half-moon over the outer atmosphere, illuminating them in much a snow-covered field may be sufficient for landing. the same way as an electrical charge illuminates Even illumination from the stars creates visibility neon gas in neon signs. far beyond that found elsewhere. Only under heavy The Aurora Borealis takes place at high altitudes overcast skies does the night darkness in the Arctic above the Earth's surface and thus has been ob­ begin to approach the degree of darkness in lower served as far south as Florida. However, the highest latitudes.

152 WEATHER HAZARDS

Weather hazards include visibility restricting phe­ heating as the air descends downslope. Icing in ad­ nomena, blowing snow, icing, frost, and lack of vection fog is in the form of rime and may become contrast-whiteout. quite severe. FOG BLOWING SNOW Fog limits landing and takeoff in the Arctic more Over the frozen Arctic Ocean and along the than any other visibility restriction. Water-droplet coastal areas, blowing snow and strong winds are fog is the main hazard to aircraft operations in common hazards during autumn and winter. Blow­ coastal areas during the summer. Ice fog is the ing snow is a greater hazard to flying operations in major restriction in winter. the Arctic than in mid latitudes because the snow is «dry" and fine and can be picked up easily by light Ice Fog winds. Winds in excess of 8 knots may raise the Ice fog is common in the Arctic. It forms in snow several feet off the ground obliterating ob­ moist air during extremely cold, calm conditions in jects such as runway markers as illustrated in figure winter, occurring often and tending to persist. Ef­ 135. A sudden increase in surface wind may cause fective visibility is reduced much more in ice fog an unlimited visibility to drop to near zero in a few when one is looking toward the sun. Ice fog may minutes. This sudden loss of visibility occurs fre­ be produced both naturally and artificially. Ice fog quently without warning in the Arctic. Stronger affecting aviation operations most frequently is pro­ winds sometimes lift blowing snow to heights above duced by the combustion of aircraft fuel in cold 1,000 feet and produce drifts over 30 feet deep. air. When the wind is very light and the temper­ ature is about -300 F or colder, ice fog often ICING forms instantaneously in the exhaust gases of auto­ Icing is most likely in spring and fall, but is also mobiles and aircraft. It lasts from as little as a few encountered in winter. During spring and fall, icing minutes to days. may extend to upper levels along frontal zones. While icing is mostly a problem over water and Steam Fog coastal areas, it does exist inland. It occurs typically Steam fog, often called "sea smoke," forms in as rime, but a combination of clear and rime is not winter when cold, dry air passes from land areas unusual in coastal mountains. over comparatively warm ocean waters. Moisture evaporates rapidly from the water surface; but FROST since the cold air can hold only a small amount of In coastal areas during spring, fall, and winter, water vapor, condensation takes place just above heavy frost and rime may form on aircraft parked the surface of the water and appears as "steam" outside, especially when fog or ice fog is present. rising from the ocean. This fog is composed entirely This frost should be removed; it reduces lift and is of water droplets that often freeze quickly and fall especially hazardous if surrounding terrain requires back into the water as ice particles. Low level tur­ a rapid rate of climb. bulence can occur and icing can become hazardous. WHITEOUT Advection Fog "Whiteout" is a visibility restricting phenomenon Advection fog, which may be composed either of that occurs in the Arctic when a layer of cloudiness water droplets or of ice crystals, is most common in of uniform thickness overlies a snow or ice-covered winter and is often persistent. Advection fog forms surface. Parallel rays of the sun are broken up and along coastal areas when comparatively warm, diffused when passing through the cloud layer so moist, oceanic air moves over cold land. If the land that they strike the snow surface from many angles. areas are hilly or mountainous, lifting of the air The diffused light then reflects back and forth results in a combination of low stratus and fog. The countless times between the snow and the cloud stratus and fog quickly diminish inland. Lee sides eliminating all shadows. The result is a loss of of islands and mountains usually are free of advec­ depth perception. Buildings, people, and dark­ tion fog because of drying due to compressional colored objects appear to float in the air, and the

153 FIGURE 135. Visibility reduced by blowing snow. Common in Arctic regions since wind easily picks up the dry, powder-like snow. horizon disappears. Low level flight over icecap gerous. Disastrous accidents have occurred as a terrain or landing on snow surfaces becomes dan- result of whiteouts.

ARCTIC FLYING WEATHER A great number of pilots who fly Alaska and the mostly natural landmarks to guide you as illustrated Arctic are well seasoned. They are eager to be of in figure 136. Before flying in the Arctic, be sure to help and are your best sources of information. learn all you can about your proposed route. Alaska and the Arctic are sparsely settled with Generally, flying conditions in the Arctic are

FIGURE 136. A typical frozen landscape of the Arctic.

154 good when averaged over the entire year; how­ frontal passages frequent the coastal areas accom­ ever, areas of Greenland compete with the Aleu­ panied by turbulence, especially in the coastal tians for the world's worst weather. These areas mountains. are exceptions. Icing is most frequent in spring and fall and Whiteouts, in conjunction with overcast skies, may extend to high levels in active, turbulent often present a serious hazard especially for visual frontal zones. Fog is also a source of icing when flight. Many mountain peaks are treeless and temperature is colder than freezing. rounded rather than ragged, making them un­ usually difficult to distinguish under poor visibility CONTINENTAL AREAS conditions. Over the continental interior, good flying weather prevails much of the year; although during winter, OCEANIC AND COASTAL AREAS ice fog often restricts aircraft operations. In terms In oceanic and coastal areas, predominant haz­ of ceiling and visibility, the summer months pro­ ards change with the . In summer, the main vide the best flying weather. However, the number hazard is fog in coastal areas. of cloudy days during the summer exceeds those in In winter, ice fog is the major restriction to air­ winter. develop on occasion during craft operation. Blowing and drifting snow often the summer, but they usually can be circumnav­ restrict visibility also. and well-defined igated without much interference with flight plans.

IN CLOSING

If one were to summarize general weather condi­ are usually circumnavigable and generally tions and flight precautions over Alaska, northern move from northeast to southwest. , and the Arctic, he would say: 7. Always file a flight plan. Stay on regularly 1. Interior areas generally have good flying traversed routes, and if downed, stay with weather, but coastal areas and Arctic slopes your plane. often are plagued by low ceiling, poor vis­ 8. If lost during summer, fly down-drainage, ibility, and icing. that is, downstream. Most-airports are lo­ 2. "Whiteout" conditions over ice and snow cated near rivers, and chances are you can covered areas often cause pilot disorienta­ reach a landing strip by flying downstream. tion. If forced down, you will be close to water 3. Flying conditions are usually worse in moun­ on which a rescue plane can land. In sum­ tain passes than at reporting stations along mer, the tundra is usually too soggy for the route. landing. 4. Routes through the mountains are subject 9. Weather stations are few and far between. to strong turbulence, especially in and near Adverse weather between stations may go passes. undetected unless reported by a pilot in 5. Beware of a false mountain pass that may flight. A report confirming good weather lead to a dead-end. between stations is also just as important. 6. Thundershowers sometimes occur in the in­ Help yourself and your fellow pilot by terior during May through August. They reporting weather en route.

155

Chapter 15 TROPICAL WEATHER

Technically, the Tropics lie between latitudes and driest regions of the world. This chapter de­ 23!;'2° Nand 23!;'2° S. However, weather typical of scribes the circulation basic to the Tropics, terrain this region sometimes extends as much as 45 0 from influences that determine arid and wet regions, and the Equator. One may think of the Tropics as uni­ transitory systems that invade or disturb the basic formly rainy, warm, and humid. The facts are, tropical circulation. however, that the Tropics contain both the wettest

157 CIRCULATION

In chapter 4, we learned that wind blowing out infrequent but fog is common along the coast. Con­ of the subtropical high pressure belts toward the taminants trapped along with fog under the strong Equator form the northeast and southeast trade inversion may persist for days creating "smog." winds of the two hemispheres. These In winter, the subtropical high pressure belts converge in the vicinity of the Equator where air shift southward. Again, let's consider southern rises. This convergence zone is the "intertropical California as an example. In winter, the area convergence zone" (ITCZ). In some areas of the comes under the influence of midlatitude circu­ world, seasonal temperature differences between lation which increases frequency of rain. Also, an land and water areas generate rather large circula­ occasional wintertime outbreak of polar air brings tion patterns that overpower the trade wind circu­ clear skies with excellent visibility. lation; these areas ate "" regions. Tropical The situation on eastern continental coasts is just weather discussed here includes the subtropical the opposite. The inversion is weakest and highest high pressure belts, the trade wind belts, the inter­ where the west side of an overlies the tropical convergence zone, and monsoon regions. eastern coast of a . can pene­ trate the inversion, and showers and thunderstorms SUBTROPICAL HIGH PRESSURE BELTS often develop. Precipitation is generally sufficient If the surface under the subtropical high pres­ to support considerable vegetation. For example, in sure belts were all water of uniform temperature, the United States, Atlantic coastal areas at the the high pressure belts would be continuous highs same latitude as southern California are far from around the globe. The belts would be areas of de­ arid in summer. scending or subsiding air and would be character­ Low ceiling and fog often prevent landing at a ized by strong temperature inversions and very little west coast destination, but a suitable alternate gen­ precipitation. However, land surfaces at the lati­ erally is available a few miles inland. Alternate tudes of the high pressure belts are generally warm­ selection may be more critical for an eastern coast er throughout the year than are water surfaces. destination because of widespread instability and Thus, the high pressure belts are broken into semi­ associated hazards. permanent high pressure over with troughs or lows over as shown in Weather over Open Sea figures 23 and 24, chapter 4. The subtropical highs Under a subtropical high over the open sea, shift southward during the Northern Hemisphere cloudiness is scant. The few clouds that do develop winter and northward during summer. The sea­ have tops from 3,000 to 6,000 feet depending on sonal shift, the height and strength of the inversion, height of the inversion. Ceiling and visibility are and terrain features determine weather in the generally quite ample for VFR flight. subtropical high pressure belts. Island Weather Continental Weather An island under a subtropical high receives very Along the west coasts of continents under a sub­ little rainfall because of the persistent temperature tropical high, the air is stable. The inversion is inversion. Surface heating over some larger islands strongest and lowest where the east side of an anti­ causes light convective showers. Cloud tops are overlies the west side of a continent. Mois­ only slightly higher than over open water. Temper­ ture is trapped under the inversion; fog and low atures are mild, showing small seasonal and diurnal stratus occur frequently. However, precipitation is changes. A good example is the pleasant, balmy rare since the moist layer is shallow and the air is climate of Bermuda. stable. Heavily populated areas also add contami­ nants to the air which, when trapped under the TRADE WIND BELTS inversion, create an problem. Figures 138 and 139 show The extreme southwestern United States, for throughout the Tropics for July and January. Note example, is dominated in summer by a subtropical that trade winds blowing out of the subtropical high. We are all familiar with the semi-arid sum­ highs over ocean areas are predominantly north­ mer climate of southern California. Rainfall is easterly in the Northern Hemisphere and south-

158 easterly in the . The inversion they always strike the same side of the island; this from the subtropical highs is carried into the trade side is the windward side. The opposite side is the winds and is known as the "trade wind inversion." leeward side. Winds blowing up the windward side As in a subtropical high, the inversion is strongest produce copious and frequent rainfall, although where the trades blow away from the west coast cloud tops rarely exceed 10,000 feet. Thunderstonns of a continent and weakest where they blow onto are rare. Downslope winds on the leeward slopes an eastern continental shore. Daily variations from dry the air leaving relatively clear skies and much these prevailing directions are small except during less rainfall. Many islands in the trade wind belt tropical storms. As a result, weather at any specific have lush vegetation and even rain forests on the location in a trade wind belt varies little from day windward side while the leeward is semiarid. For to day. example, the island of Oahu, , is about 24 miles wide in the direction of the trade winds. An­ Weather over Open Sea nual rainfall averages from about 60 inches on the In the trade wind belt, skies over open water are windward coast to 200 inches at the mountain tops, about one-half covered by clouds on the average. decreasing to 10 inches on the leeward shore. Tops range from 3,000 to 8,000 feet depending on The greatest flying hazard near these islands is height of the inversion. Showers, although more obscured mountain tops. Ceiling and visibility oc­ common than under a subtropical high, are still casionally restrict VFR flight on the windward side light with comparatively little rainfall. Flying in showers. IFR weather is virtually nonexistent weather generally is quite good. on leeward slopes. Islands without mountains have little effect on Continental Weather cloudiness and rainfall. Afternoon surface heating Where trade winds blow offshore along the west increases convective cloudiness slightly, but shower coasts of continents, skies are generally clear and activity is light. However, any island in either the the area is quite arid. The Baja Peninsula of Lower subtropical high pressure belt or trade wind belt California is a well-known example. Where trade enhances cumulus development even though tops winds blow onshore on the east sides of continents, do not reach great heights. Therefore, a cumulus rainfall is generally abundant in showers and oc­ top higher than the average tops of surrounding casional thunderstonns. The east coast of Mexico cumulus usually marks the approximate location of is a good example. Rainfall may be carried a con­ an island. If it becomes necessary to "ditch" in the siderable distance inland where the winds are not ocean, look for a tall cumulus. If you see one, head blocked by a mountain barrier. Inland areas blocked for it. It probably marks a land surface, increasing by a mountain barrier are deserts; examples are your chances of survival. the Sahara Desert and the arid regions of south­ western United States. Afternoon convective cur­ THE INTERTROPICAL CONVERGENCE rents are common over arid regions due to strong ZONE nTCZ) surface heating. Cumulus and cumulonimbus Converging winds in the intertropical conver­ clouds can develop, but cloud bases are high and gence zone (ITCZ) force air upward. The inver­ rainfall is scant because of the low moisture sion typical of the subtropical high and trade wind content. belts disappears. Figures 138 and 139 show the Flying weather along eastern coasts and moun­ ITCZ and its seasonal shift. The ITCZ is well tains is subject to the usual hazards of showers and marked over tropical oceans but is weak and ill­ thunderstorms. Flying over arid regions is good defined over large continental areas. most of the time but can be turbulent in afternoon convective currents; be especially aware of dust Weather over Islands and Open Water devils. Blowing sand or dust sometimes restricts Convection in the ITCZ carries huge quantities visibility. of moisture to great heights. Showers and thunder­ storms frequent the ITCZ and tops to 40,000 feet Island Weather or higher are common as shown in figure 137. Pre­ Mountainous islands have the most dramatic ef­ cipitation is copious. Since convection dominates fect on trade wind weather. Since trade winds-are the ITCZ, there is little difference in weather over consistently from approximately the same direction, islands and open sea under the ITCZ.

159 45,000'

30,000'

15,000'

a lOON

FIGURE 137. Vertical cross section illustrating convection in the Intertropical Convergence Zone.

Flying through the ITCZ usually presents no Summer or Wet Monsoon Weather great problem if one follows the usual practice of During the summer, the low over central avoiding thunderstorms. He usually can find a safe draws warm, moist, unstable maritime air from corridor between storms. the southwest over the continent. Strong surface Since the ITCZ is ill-defined over continents, we heating coupled with rising of air flowing up the will not attempt to describe ITCZ continental higher terrain produces extensive cloudiness, copi­ weather as such. Continental weather ranges from ous rain, and numerous thunderstorms. Rainfall at arid to rain forests and is more closely related to some stations in India exceeds 400 inches per year the monsoon than to the ITCZ. with highest amounts between June and October. MONSOON The monsoon is so pronounced that it influences circulation many miles out over the ocean. Note in If you refer again to figures 23 and 24 in chapter figure 138 that in summer, prevailing winds from 4, you can see that over the large land mass of the Equator to the south Asian coast are southerly Asia, the subtropical high pressure breaks down and southeasterly; without the monsoon influence, completely. Asia is covered by an intense high dur­ these areas would be dominated by northeasterly ing the winter and a well-developed low during the trades. Islands within the monsoon influence receive summer. You can also see the same over Australia frequent showers. and central Africa, although the seasons are reversed in the Southern Hemisphere. The cold, high in winter cause wind to Winter Monsoon Weather blow from the deep interior outward and offshore. Note in figure 139 how the winter flow has re­ In summer, reverses and warm versed from that shown in figure 138. Cold, dry air moist air is carried far inland into the low pres­ from the high plateau deep in the interior warms sure area. This large scale seasonal wind shift is adiabatically as it flows down the southern slopes the "monsoon." The most notable monsoon is that of the Himalayan Mountains. Virtually no rain falls of southern and southeastern Asia. in the interior in the dry winter monsoon. As the

160 FIGURE 138. Prevailing winds throughout the Tropics in July. Remember that in the Southern Hemisphere, circulation around pressure centers is opposite that in the Northern Hemisphere.

FIGURE 139. Prevailing winds in the Tropics in January.

161 dry air moves off shore over warmer water, it and jungles. Note in figures 138 and 139 that pre­ rapidly takes in more moisture, becomes warmer in vailing wind is onshore much of the year over low levels and, therefore, unstable. Rain is frequent these regions. Some regions are wet the year round; over off-shore islands and even along coastal areas others have the seasonal monsoon shift and have after the air has had a significant over-water a summer and a winter . trajectory. Climate of Africa is so varied that only a detailed The Philippine Islands are in an area of special area-by-area study can explain the climate typical interest. During the summer, they are definitely in of each area. southerly monsoon flow and are subjected to abun­ In the Amazon Valley of during dant rainfall. In the winter, wind over the Phil­ the Southern Hemisphere winter (July), southeast ippines is northeasterly-in the transition zone be­ trades, as shown in figure 138, penetrate deep into tween the northeasterly trades and the monsoon the valley bringing abundant rainfall which con­ flow. It is academic whether we call the phenom­ tributes to the jungle climate. In January, the enon the trade winds or monsoon; in either case, ITCZ moves south of the valley as shown in figure it produces abundant rainfall. The Philippines 139. The northeast trades are caught up in the have a year-round humid, tropical climate. monsoon, cross the Equator, and also penetrate the Amazon Valley. The jungles of the Amazon result Other Monsoon Areas largely from monsoon winds. Australia in July (Southern Hemisphere winter) is an area of high pressure with predominantly off­ Flying Weather in shore winds as shown in figure 138. Most of the During the winter monsoon, excellent flying continent is dry during the winter. In January, weather prevails over dry interior regions. Over figure 139, winds are onshore into the continental water, one must pick his way around showers and low pressure. However, most of Australia is rimmed thunderstorms. In the summer monsoon, VFR by mountains, coastal regions are wet where the flight over land is often restricted by low ceilings onshore winds blow up the mountain slopes. The and heavy rain. IFR flight must cope with the interior is arid where down-slope winds are warmed hazards of thunderstorms. Freezing level in the and dried. Tropics is quite high-14,000 feet or higher-so Central Africa is known for its humid climate icing is restricted to high levels.

TRANSITORY SYSTEMS

So far, we have concentrated on prevailing cir­ able thunderstorm and rain shower activity occurs culations. Now, let's turn to migrating tropical along a shear line. weather producers--the shear line, trough aloft, , and . TROUGH ALOFT SHEAR LINE Troughs in the atmosphere, generally at or above A wind shear line found in the Tropics mainly 10,000 feet, move through the Tropics, especially results from midlatitude influences. In chapter 8 along the poleward fringes. Figure 141 shows such we stated that an air mass becomes modified when a trough across the Hawaiian Island chain. As a it flows from its source region. By the time a cold trough moves to the southeast or east, it spreads air mass originating in high latitudes reaches the middle and high cloudiness over extensive areas to Tropics, temperature and moisture are virtually the east of the trough line. Occasionally, a well­ the same on both sides of the front. A shear line, developed trough will extend deep into the Tropics, or wind shift, is all that remains. A shear line also and a closed low forms at the equatorial end of the results when a semi-permanent high splits into two trough. The low then may separate from the trough cells inducing a trough as shown in figure 140. and move westward producing a large amount of These shear lines are zones of convergence creat­ cloudiness and precipitation. If this occurs in the ing forced upward motion. Consequently, consider- vicinity of a strong subtropical jet stream, extensive

162 HIGH POLAR

---SH£A.!.------HIGH

SUBTROPICAL

HIGH SUBTROPICAL

FIGURE 140. A shear line and an induced trough caused by a polar high pushing into the sUbtropics. and sometimes dense cirrus and some convective ern perimeter of the subtropical high pressure sys­ and clear air turbulence often develop. tems. They travel from east to west around the Troughs and lows aloft produce considerable southern fringes of these highs in the prevailing amounts of rainfall in the Tropics, especially over easterly circulation of the Tropics. Surface winds land areas where mountains and surface heating in advance of a wave are somewhat more northerly lift air to saturation. Low pressure systems aloft than the usual trade wind direction. As the wave contribute significantly to the record 460 inches approaches, as shown in figure 142, pressure falls; average annual rainfall on Mt. Waialeale on as it passes, surface wind shifts to the east-southeast Kauai, Hawaii. Other mountainous areas of the or southeast. The typical wave is preceded by very Tropics are also among the wettest spots on earth. good weather but followed by extensive cloudiness, as shown in figure 143, and often by rain and TROPICAL WAVE thunderstorms. The weather activity is roughly in Tropical waves (also called easterly waves) are a north-south line. common tropical weather disturbances, normally Tropical waves occur in all seasons, but are more occurring in the trade wind belt. In the Northern frequent and stronger during summer and early Hemisphere, they usually develop in the southeast- fall. Pacific waves frequently affect Hawaii; Atlan-

163 HAWAIIAN ISLANDS

FIGURE 141. A trough aloft across the Hawaiian Islands. Extensive cloudiness develops east of the trough. tic waves occasionally move into the Gulf of Mex­ cyclone in the Atlantic and eastern Pacific is a ico, reaching the U.S. coast. "hurricane"; in the western Pacific, ""; near Australia, "willy-willy"; and in the Indian TROPICAL CYCLONE Ocean, simply "cyclone." Regardless of the name, Tropical cyclone is a general term for any low these tropical produce serious aviation that originates over tropical oceans. Tropical cy­ hazards. Before we delve into these aspects, let's clones are classified according to their intensity look at the development, movement, and decay of based on average one-minute wind speeds. Wind these cyclones. gusts in these storms may be as much as 50 percent higher than the average one-minute wind speeds. Development Tropical cyclone international classifications are: Prerequisite to tropical cyclone development are ( 1) Tropical Depression-highest sustained optimum sea surface tempe~ature under weather winds up to 34 knots (64 km/h), systems that produce low-level convergence and (2) Tropical Storm-highest sustained winds cyclonic wind shear. Favored breeding grounds are of 35 through 64 knots (65 to 119 km/ tropical (easterly) waves, troughs aloft, and areas h), and of converging northeast and southeast trade winds (3) Hurricane or Typhoon-highest sustained along the intertropical convergence zone. winds 65 knots (120 km/h) or more. The low level convergence associated with these Strong tropical cyclones are known by different systems, by itself, will not support development of names in different regions of the world. A tropical a tropical cyclone. The system must also have

164 30 0 ______~

100 ______~ ______; LOW

FIGURE 142. A Northern Hemisphere easterly wave. Progressing from (A) to (B), note that winds shift generally from northeasterly to southeasterly. The wave moves toward the west and is often preceded by good weather and followed by extemive cloudiness and precipitation.

25,000'

20,000'

15,000'

10.000'

5.000'

150 o 150 300 450 _ MILES

FIGURE 143. Vertical cross section along line A-B in figure 142.

165 horizontal outflow-divergence-at high tropo­ ically and may even reverse course, or circle. Final­ spheric levels. This combination creates a "chim­ ly, the prevailing westerlies gain control and the ney," in which air is forced upward causing clouds storm recurves toward the north, then to the north­ and precipitation. Condensation releases large quan­ east, and finally to the east-northeast. By this time tities of which raises the temperature of the storm is well into rriidlatitudes. the system and accelerates th~ upward motion. The rise in temperature lowers the surface pressure Decay which increases low-level convergence .. This draws As the storm curves toward the north or east, it more moisture-laden air into the system. When usually begins to lose its tropical characteristics and these chain-reaction events continue, a huge vortex acquires characteristics of lows in middle latitudes. is generated which may culminate in hurricane Cooler air flowing into the storm gradually weak­ force winds. ens it. If the storm tracks along a coast line or over Figure 144 shows regions of the world where the open sea, it gives up slowly, carrying its fury tropical cyclones frequently develop. Notice that to areas far removed from the Tropics. However, they usually originate between latitudes 50 and if the storm moves well inland, it loses its moisture 200. Tropical cyclones are unlikely within 50 of the source and weakens from starvation and increased Equator because the is so small near surface friction, usually after leaving a trail of the Equator that it will not turn the winds enough destruction and flooding. for them to flow around a low pressure area. Winds When a storm takes on middle latitude charac­ flow directly into an equatorial low and rapidly teristics, it is said to be "extratropical" meaning fill it. "outside the Tropics." Tropical cyclones produce weather conditions that differ somewhat from those Movement produced by their higher latitude cousins and invite Tropical cyclones in the Northern Hemisphere our investigation. usually move in a direction between west and north­ west while in low latitudes. As these storms move Weather in a Tropical Depression toward the midlatitudes, they come under the in­ While in its initial developing stage, the cyclone fluence of the prevailing westerlies. At this time the is characterized by a circular area of broken to storms are under the influence of two wind systems, overcast clouds in multiple layers. Embedded in i.e., the trade winds at low levels and prevailing these clouds are numerous showers and thunder­ westerlies aloft. Thus a storm may move very en'at- storms. Rain shower and thunderstorm coverage

FIGURE 144. Principal regions where tropical cyclones form and their favored directions of movement.

166 varies from scattered to almost solid. Diameter of radars, and weather satellites is fed into the center. the cloud pattern varies from less than 100 miles The center forecasts the development, movement, in small systems to well over 200 miles in large ones. and intensity of tropical cyclones. Forecasts and warnings are issued to the public and aviation in­ Weather in Tropical Storms and Hurricanes terests by field offices of the . As cyclonic flow increases, the thunderstorms and rain showers form into broken or solid lines paral­ Flying leling the wind flow that is spiraling into the center All pilots except those especially trained to of the storm. These lines are the spiral rain bands explore tropical storms and hurricanes should frequently seen on radar. These rain bands con­ AVOID THESE DANGEROUS STORMS. Oc­ tinually change as they rotate around the storm. casionally, jet aircraft have been able to fly over Rainfall in the rain bands is very heavy, reducing small and less intense storms, but the experience ceiling and visibility to near zero. Winds are usual­ of weather research aircraft shows hazards at all ly very strong and gusty and, consequently, gen­ levels within them. erate violent turbulence. Between the rain bands, Tops of thunderstorms associated with tropical ceilings and visibilities are somewhat better, and cyclones frequently exceed 50,000 feet. Winds in turbulence generally is less intense. a typical hurricane are strongest at low levels, de­ The "eye" usually forms in the tropical storm creasing with altitude. However, research aircraft stage and continues through the hurricane stage. In have frequently encountered winds in excess of 100 the eye, skies are free of turbulent cloudiness, and knots at 18,000 feet. Aircraft at low levels are ex­ wind is comparatively light. The average diameter posed to sustained, pounding turbulence due to the of the eye is between 15 and 20 miles, but some­ surface friction of the fast-moving air. Turbulence times is as small as 7 miles and rarely is more than increases in intensity in spiral rain bands and be­ 30 miles in diameter. Surrounding the eye is a comes most violent in the surrounding wall of cloud that may extend above 50,000 feet. the eye. This "wall cloud" contains deluging rain and the An additional hazard encountered in hurricanes strongest winds of the storm. Maximum wind is erroneous altitude readings from pressure altim­ speeds of 175 knots have been recorded in some eters. These errors are caused by the large pressure storms. Figure 145 is a radar display and 146, a difference between the periphery of the storm and satellite photograph of a mature hurricane. Note its center. One research aircraft lost almost 2,000 the spiral rain bands and the circular eye. Notice feet true altitude traversing a storm while the pres­ the similarity between these two figures. sure altimeter indicated a constant altitude of 5,000 feet. Detection and Warning In short, tropical cyclones are very hazardous, The National Weather Service has a specialized so avoid them! To bypass the storm in a min­ hurricane forecast and warning service center at imum of time, fly to the right of the storm to take Miami, Florida, which maintains constant watch advantage of the tailwind. If you fly to the left of for the formation and development of tropical cy­ the storm, you will encounter strong headwinds clones. Weather information from land stations, which may exhaust your fuel supply before you ships at sea, reconnaissance aircraft, long range reach a safe landing area.

167 FIGURE 145. Radar photograph of hurricane "Donna" observed at Key West, Florida.

168 FIGURE 146. A hurricane observed by satellite.

169