Chapter 13 | High Altitude Weather
Many general aviation as well as air carrier and military aircraft routinely fly the upper troposphere and lower stratosphere. Weather phenomena of these higher altitudes include the tropopause, the jet stream, cirrus clouds, clear air turbulence, condensation trails, high altitude “haze” layers, and canopy static. This chapter explains these phenomena along with the high altitude aspects of the more common icing and thunderstorm hazards.
Why is the high altitude pilot interested in the tropopause? Temperature and wind vary greatly in the vicinity of the tropopause affecting efficiency, comfort, and safety of flight. Maximum winds generally occur at levels near the tropopause. These strong winds create narrow zones of wind shear which often generate hazardous turbulence. Preflight knowledge of temperature, wind, and wind shear is important to flight planning.
In chapter 1, we learned that the tropopause is a thin layer forming the boundary between the troposphere and stratosphere. Height of the tropopause varies from about 65,000 feet over the Equator to 20,000 feet or lower over the poles. The tropopause is not continuous but generally descends step-wise from the Equator to the poles. These steps occur as “breaks.” figure 123 is a cross section of the troposphere and lower stratosphere showing the tropopause and associated features. Note the break between the tropical and the polar tropopauses.
figure 123. A cross section of the upper troposphere and lower stratosphere showing the tropopause and associated features. Note the “break” between the high tropical and the lower polar tropopause. Maximum winds occur in the vicinity of this break.
An abrupt change in temperature lapse rate characterizes the tropopause. Note in figure 123 how temperature above the tropical tropopause increases with height and how over the polar tropopause, temperature remains almost constant with height.
THE JET STREAM
Diagrammed in figure 124, the jet stream is a narrow, shallow, meandering river of maximum winds extending around the globe in a wavelike pattern. A second jet stream is not uncommon, and three at one time are not unknown. A jet may be as far south as the northern Tropics. A jet in midlatitudes generally is stronger than one in or near the Tropics. The jet stream typically occurs in a break in the tropopause as shown in figure 123. Therefore, a jet stream occurs in an area of intensified temperature gradients characteristic of the break.
figure 124. Artist's concept of the jet stream. Broad arrow shows direction of wind.
The concentrated winds, by arbitrary definition, must be 50 knots or greater to classify as a jet stream. The jet maximum is not constant; rather, it is broken into segments, shaped something like a boomerang as diagrammed in figure 125.
figure 125. A jet stream segment.
Jet stream segments move with pressure ridges and troughs in the upper atmosphere. In general they travel faster than pressure systems, and maximum wind speed varies as the segments progress through the systems. In midiatitude, wind speed in the jet stream averages considerably stronger in winter than in summer. Also the jet shifts farther south in winter than in summer.
In figure 123 note how wind speed decreases outward from the jet core. Note also that the rate of decrease of wind speed is considerably greater on the polar side than on the equatorial side; hence, the magnitude of wind shear is greater on the polar side than on the equatorial side.
figure 126 shows a map with two jet streams. The paths of the jets approximately conform to the shape of the contours. The northerly jet has three segments of maximum wind, and the southerly one has two. Note how spacing of the height contours is closer and wind speeds higher in the vicinity of the jets than outward on either side. Thus horizontal wind shear is evident on both sides of the jet and is greatest near the maximum wind segments.
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.
Strong, long-trajectory jet streams usually are associated with well-developed surface lows and frontal systems beneath deep upper troughs or lows. Cyclogenesis is usually south of the jet stream and moves nearer as the low deepens. The occluding low moves north of the jet, and the jet crosses the frontal system near the point of occlusion. figure 127 diagrams mean jet positions relative to surface systems. These long jets mark high level boundaries between warm and cold air and are favored places for cirriform cloudiness.
figure 127. Mean 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.
Air travels in a “corkscrew” path around the jet core with upward motion on the equatorial side. Therefore, when high level moisture is available, cirriform clouds form on the equatorial side of the jet. Jet stream cloudiness can form independently of well-defined pressure systems. Such cloudiness ranges primarily from scattered to broken coverage in shallow layers or streaks. Their sometimes fish hook and streamlined, wind-swept appearance always indicates very strong upper wind usually quite far from developing or intense weather systems.
The most dense cirriform clouds occur with well-defined systems. They appear in broad bands. Cloudiness is rather dense in an upper trough, thickens downstream, and becomes most dense at the crest of the downwind ridge. The clouds taper off after passing the ridge crest into the area of descending air. The poleward boundary of the cirrus band often is quite abrupt and frequently casts a shadow on lower clouds, especially in an occluded frontal system. figure 128a is a satellite photograph showing a cirrus band casting a shadow on lower clouds. figure 128b is an infrared photo of the same system; the light shade of the cirrus band indicates cold temperatures while warmer low clouds are the darker shades.
figure 128a. Satellite photograph of an occluded system centered at about 44° N and 137° 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.
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.
The upper limit of dense, banded cirrus is near the tropopause; a band may be either a single layer or multiple layers 10,000 to 12,000 feet thick. Dense, jet stream cirriform cloudiness is most prevalent along midlatitude and polar jets. However, a cirrus band usually forms along the subtropical jet in winter when a deep upper trough plunges southward into the Tropics.
Cirrus clouds, in themselves, have little effect on aircraft. However, dense, continuous coverage requires a pilot's constant reference to instruments; most pilots find this more tiring than flying with a visual horizon even though IFR.
A more important aspect of the jet stream cirrus shield is its association with turbulence. Extensive cirrus cloudiness often occurs with deepening surface and upper lows; and these deepening systems produce the greatest turbulence.
CLEAR AIR TURBULENCE
Clear air turbulence (CAT) implies turbulence devoid of clouds. However, we commonly reserve the term for high level wind shear turbulence, even when in cirrus clouds.
Cold outbreaks colliding with warm air from the south intensify weather systems in the vicinity of the jet stream along the boundary between the cold and warm air. CAT develops in the turbulent energy exchange between the contrasting air masses. Cold and warm advection along with strong wind shears develop near the jet stream, especially where curvature of the jet stream sharply increases in deepening upper troughs. CAT is most pronounced in winter when temperature contrast is greatest between cold and warm air.
A preferred location of CAT is in an upper trough on the cold (polar) side of the jet stream. Another frequent CAT location, shown in figure 129, is along the jet stream north and northeast of a rapidly deepening surface low.
figure 129. A frequent CAT location is along the jet stream north and northeast of a rapidly deepening surface low.
Even in the absence of a well-defined jet stream, CAT often is experienced in wind shears associated with sharply curved contours of strong lows, troughs, and ridges aloft, and in areas of strong, cold or warm air advection. Also mountain waves can create CAT. Mountain wave CAT may extend from the mountain crests to as high as 5,000 feet above the tropopause, and can range 100 miles or more downstream from the mountains.
CAT can be encountered where there seems to be no reason for its occurrence. Strong winds may carry a turbulent volume of air away from its source region. Turbulence intensity diminishes downstream, but some turbulence still may be encountered where it normally would not be expected. CAT forecast areas are sometimes elongated to indicate probable turbulence drifting downwind from the main source region.
A forecast of turbulence specifies a volume of airspace which is quite small when compared to the total volume of airspace used by aviation, but is relatively large compared to the localized extent of the hazard. Since turbulence in the forecast volume is patchy, you can expect to encounter it only intermittently and possibly not at all. A flight through forecast turbulence, on the average, encounters only light and annoying turbulence 10 to 15 percent of the time; about 2 to 3 percent of the time there is a need to have all objects secured; the pilot experiences control problems only about two-tenths of 1 percent of the time—odds of this genuinely hazardous turbulence are about 1 in 500.
Look again at figure 126. Where are the most probable areas of GAT? Turbulence would be greatest near the windspeed maxima, usually on the polar sides where there is a combination of strong wind shear, curvature in the flow, and cold air advection. These areas would be to the northwest of Vancouver Island, from north of the Great Lakes to east of James Bay and over the Atlantic east of Newfoundland. Also, turbulence in the form of mountain waves is probable in the vicinity of the jet stream from southern California across the Rockies into the Central Plains.
In flight planning, use upper air charts and forecasts to locate the jet stream, wind shears, and areas of most probable turbulence. AVIATION WEATHER SERVICES (AC 00-45) explains in detail how to obtain these parameters. If impractical to avoid completely an area of forecast turbulence, proceed with caution. You will do well to avoid areas where vertical 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 rougher than you care to fly? If near the jet core, you could climb or descend a few thousand feet or you could move farther from the jet core. If caught in CAT not associated with the jet stream, your best bet is to change altitude since you have no positive way of knowing in which direction the strongest shear lies. Pilot reports from other flights, when available, are helpful.
Flight maneuvers increase stresses on the aircraft as does turbulence. The increased stresses are cumulative when the aircraft maneuvers in turbulence. Maneuver gently when in turbulence to minimize stress. The patchy nature of CAT makes current pilot reports extremely helpful to observers, briefers, forecasters, air traffic controllers, and, most important, to your fellow pilots. Always, if at all possible, make inflight weather reports of CAT or other turbulence encounters; negative reports also help when no CAT is experienced where it normally might be expected.
A condensation trail, popularly contracted to “contrail,” is generally defined as a cloud-like streamer which frequently is generated in the wake of aircraft flying in clear, cold, humid air, figure 130. Two distinct types are observed—exhaust trails and aerodynamic trails. “Distrails,” contracted from dissipation trails, are produced differently from exhaust and aerodynamic trails.
FIGURE 130. Contrails. The thin contrail is freshly formed by an aircraft (not visible) in the lower right center of the photograph.
The exhaust contrail is formed by the addition to the atmosphere of sufficient water vapor from aircraft exhaust gases to cause saturation or supersaturation of the air. Since heat is also added to the atmosphere in the wake of an aircraft, the addition of water vapor must be of such magnitude that it saturates or supersaturates the atmosphere in spite of the added heat. There is evidence to support the idea that the nuclei which are necessary for condensation or sublimation may also be donated to the atmosphere in the exhaust gases of aircraft engines, further aiding contrail formation. These nuclei are relatively large. Recent experiments, however, have revealed that visible exhaust contrails may be prevented by adding very minute nuclei material (dust, for example) to the exhaust. Condensation and sublimation on these smaller nuclei result in contrail particles too small to be visible.
In air that is almost saturated, aerodynamic pressure reduction around airfoils, engine nacelles, and propellers cools the air to saturation leaving condensation trails from these components. This type of trail usually is neither as dense nor as persistent as exhaust trails. However, under critical atmospheric conditions, an aerodynamic contrail may trigger the formation and spreading of a deck of cirrus clouds.
Contrails create one problem unique to military operations in that they reveal the location of an aircraft attempting to fly undetected. A more general operational problem is a cirrus layer sometimes induced by the contrail. The induced layer may make necessary the strict use of instruments by a subsequent flight at that altitude.
DISSIPATION TRAILS (DISTRAILS)
The term dissipation trail applies to a rift in clouds caused by the heat of exhaust gases from an aircraft flying in a thin cloud layer. The exhaust gases sometimes warm the air to the extent that it is no longer saturated, and the affected part of the cloud evaporates. The cloud must be both thin and relatively warm for a distrail to exist; therefore, they are not common.
Haze layers not visible from the ground are, at times, of concern at high altitude. These layers are really cirrus clouds with a very low density of ice crystals. Tops of these layers generally are very definite and are at the tropopause. High level haze occurs in stagnant air; it is rare in fresh outbreaks of cold polar air. Cirrus haze is common in Arctic winter. Sometimes ice crystals restrict visibility from the surface to the tropopause.
Visibility in the haze sometimes may be near zero, especially when one is facing the sun. To avoid the poor visibility, climb into the lower stratosphere or descend below the haze. This change may be several thousand feet.
Canopy static, similar to the precipitation static sometimes encountered at lower levels, is produced by particles brushing against plastic-covered aircraft surfaces. The discharge of static electricity results in a noisy disturbance that interferes with radio reception. Discharges can occur in such rapid succession that interference seems to be continuous. Since dust and ice crystals in cirrus clouds are the primary producers of canopy static, usually you may eliminate it by changing altitude.
Although icing at high altitude is not as common or extreme as at low altitudes, it can occur. It can form quickly on airfoils and exposed parts of jet engines. Structural icing at high altitudes usually is rime, although clear ice is possible.
High altitude icing generally forms in tops of tall cumulus buildups, anvils, and even in detached cirrus. Clouds over mountains are more likely to contain liquid water than those over more gently sloping terrain because of the added lift of the mountains. Therefore, icing is more likely to occur and to be more hazardous over mountainous areas.
Because ice generally accumulates slowly at high altitudes, anti-icing equipment usually eliminates any serious problems. However, anti-icing systems currently in use are not always adequate. If such is the case, avoid the icing problem by changing altitude or by varying course to remain clear of the clouds. Chapter 10 discusses aircraft icing in more detail.
A well-developed thunderstorm may extend upward through the troposphere and penetrate the lower stratosphere. Sometimes the main updraft in a thunderstorm may toss hail out the top or the upper portions of the storm. An aircraft may encounter hail in clear air at a considerable distance from the thunderstorm, especially under the anvil cloud. Turbulence may be encountered in clear air for a considerable distance both above and around a growing thunderstorm.
Thunderstorm avoidance rules given in chapter 11 apply equally at high altitude. When flying in the clear, visually avoid all thunderstorm tops. In a severe thunderstorm situation, avoid tops by at least 20 miles. When you are on instruments, weather avoidance radar assures you of avoiding thunderstorm hazards. If in an area of severe thunderstorms, avoid the most intense echoes by at least 20 miles. Most air carriers now use this distance as the minimum for thunderstorm avoidance.