18-4
18.3.5 Icing in Stratiform Clouds. Icing in middle and low-level stratiform clouds is confined, on the average, to a layer between 3,000 and 4,000 feet thick. Thus, a change in altitude of only a few thousand feet may take the aircraft out of icing conditions, even if it remains in clouds. Icing intensity generally ranges from a trace to light, with the maximum values occurring in the cloud’s upper portions. Both rime and mixed are found in stratiform clouds. The main hazard lies in the great horizontal extent of stratiform clouds layers. High-level stratiform clouds (i.e., at temperatures colder than -20 °C) are composed mostly of ice crystals and produce little icing.
18.3.6 Icing in Cumuliform Clouds. The icing layer in cumuliform clouds is smaller horizontally, but greater vertically than in stratiform clouds. Icing is more variable in cumuliform clouds because many of the factors conducive to icing depend on the particular cloud’s stage of development. Icing intensities may range from a trace in small cumulus to severe in a large towering cumulus or cumulonimbus. Although icing occurs at all levels above the freezing level in a building cumuliform cloud, it is most intense in the upper portion of the cloud where the updraft is concentrated and SLDs are plentiful. Icing can extend to great heights in towering cumulus and cumulonimbus where strong updrafts allow SLDs to exist at temperatures as cold as -40 °C. Icing in a cumuliform cloud is usually clear or mixed with rime in the upper levels.
18.3.7 Icing with Fronts. Most icing reports occur in the vicinity of fronts. This icing can occur both above and below the front (see Figure 18-1).
For significant icing to occur above the front, the warm air must be lifted and cooled to saturation at temperatures below zero, making it contain supercooled water droplets.
The supercooled water droplets freeze on impact with an aircraft. If the warm air is unstable, icing may be sporadic; if it is stable, icing may be continuous over an extended area. A line of showers or thunderstorms along a cold front may produce icing, but only in a comparatively narrow band along the front.
18-5 Figure 18-1. Icing with Fronts
A favored location for severe clear icing is freezing rain and/or freezing drizzle below a front. Rain forms above the frontal surface at temperatures warmer than freezing.
Subsequently, it falls through air at temperatures below freezing and becomes supercooled. The SLDs freeze on impact with an aircraft. If the below freezing layer is shallow, freezing rain will occur at the surface. If the below freezing layer is deep, the supercooled droplets may freeze into ice pellets. Ice pellets indicate icing above.
The icing can be severe because of the large amount of supercooled water. Icing in freezing precipitation is especially dangerous because it often extends horizontally over a broad area and a pilot may be unable to escape it by descending to a lower altitude.
18.3.8 Icing with Mountains. Icing is more likely and more severe in mountainous regions.
Mountain ranges cause upward air motions on their windward side. These vertical currents support large supercooled water droplets above the freezing level.
The movement of a front across a mountain range combines frontal lift with the mountain’s upslope flow effect to create extremely hazardous icing zones (see Figure 18-2).
18-6 Figure 18-2. Icing with Mountains
The most severe icing occurs above the crests and on the ridges’ windward side. This zone usually extends to about 5,000 feet above the mountaintops, but can extend much higher if cumuliform clouds develop.
Icing with mountains can be especially hazardous because a pilot may be unable to descend to above freezing temperatures due to terrain elevation. If a pilot approaches a mountain ridge from the windward side, his aircraft may be unable to climb above the mountaintops, or even maintain altitude due to severe ice accumulation. The end result may be a crash.
18.3.9 Icing Hazards. Structural icing degrades an aircraft’s performance. It destroys the smooth flow of air, increasing drag while decreasing the ability of the airfoil to create lift.
The actual weight of ice on an airplane is insignificant when compared to the airflow disruption it causes. As power is added to compensate for the additional drag, and the nose is lifted to maintain altitude, the angle of attack is increased. This allows the underside of the wings and fuselage to accumulate additional ice.
Wind tunnel and flight tests have shown that frost, snow, and ice accumulations (on the leading edge or upper surface of the wing) no thicker or rougher than a piece of coarse sandpaper can reduce lift by 30 percent and increase drag up to 40 percent. Larger accretions can reduce lift even more and can increase drag by 80 percent or more.
Ice accumulates on every exposed frontal surface of the airplane: wings, propeller, windshield, antennas, vents, intakes, and cowlings. It can build in flight where no heat or boots can reach it. It can cause antennas to vibrate so severely that they break.
In moderate to severe icing, a light aircraft could be subject to enough ice accumulation
18-7 or accretion that continued flight is impossible. The airplane may stall at much higher speeds and lower angles of attack than normal. It can roll or pitch uncontrollably, and recovery might be impossible.
Regardless of anti-ice or deice protection offered by the aircraft, a pilot’s first course of action should be to leave the area of visible moisture. This might mean descending to an altitude below the cloud bases, climbing to an altitude that is above the cloud tops, or turning to a different course. If this is not possible, then the pilot must move to an altitude where the temperature is above freezing.
18.4 Engine Icing.
18.4.1 Carburetor Icing. In an aspirated engine, the carburetion process can lower the temperature of the incoming air by as much as 33 °C. If the moisture content is high enough, ice will form on the throttle plate and venturi, gradually shutting off the supply of air to the engine. Even a small amount of carburetor ice will result in power loss and may make the engine run rough. It is possible for carburetor ice to form even when the skies are clear and the outside temperature is as high as 33 °C (90 °F) if the relative humidity is 50 percent or more.
18.4.2 High Ice Water Content (HIWC). High Ice Water Content (HIWC) is a relatively new icing hazard, at least from the standpoint of research and understanding. HIWC refers to high altitude ice crystals, which may exist in the tops and anvils of cumulonimbus clouds and thunderstorms. Under certain HIWC conditions, turbine engine performance can be affected, including flameouts. Research into HIWC conditions continues as of the writing of this section.
19-1 CHAPTER 19. THUNDERSTORMS
19.1 Introduction. A thunderstorm is a local storm, invariably produced by a cumulonimbus cloud, and always accompanied by lightning and thunder, usually with strong gusts of wind, heavy rain, and sometimes with hail. There are as many as 40,000 thunderstorm occurrences each day worldwide, and the United States certainly experiences its share.
Thunderstorms are barriers to air traffic because they are usually too tall to fly over, too dangerous to fly through or under, and can be difficult to circumnavigate.
19.2 Necessary Ingredients for Thunderstorm Cell Formation. Thunderstorm cell formation requires three ingredients: sufficient water vapor, unstable air, and a lifting mechanism (see Figure 19-1). Sufficient water vapor (commonly measured using dewpoint) must be present to produce unstable air. Virtually all showers and thunderstorms form in an air mass that is classified as conditionally unstable.
A conditionally unstable air mass requires a lifting mechanism strong enough to release the instability. Lifting mechanisms include: converging winds around surface lows and troughs, fronts, upslope flow, drylines, outflow boundaries generated by prior storms, and local winds, such as sea breeze, lake breeze, land breeze, and valley breeze circulations.
Figure 19-1. Necessary Ingredients for Thunderstorm Cell Formation
19.3 Thunderstorm Cell Life Cycle. A thunderstorm cell is the convective cell of a cumulonimbus cloud having lightning and thunder. It undergoes three distinct stages during its life cycle (see Figure 19-2): towering cumulus, mature, and dissipating.
The total life cycle is typically about 30 minutes.
The distinguishing feature of the towering cumulus stage is a strong convective updraft.
The updraft is a bubble of warm, rising air concentrated near the top of the cloud which leaves a cloudy trail in its wake. Updraft speeds can exceed 3,000 feet per minute.
19-2 Figure 19-2. Thunderstorm Cell Life Cycle
The cell transitions to the mature stage when precipitation reaches the surface.
Precipitation descends through the cloud and drags the adjacent air downward, creating a strong downdraft alongside the updraft. The downdraft spreads out along the surface, well in advance of the parent thunderstorm cell, as a mass of cool, gusty air.
The arc-shaped leading edge of downdraft air resembles a miniature cold front and is called a gust front. Uplift along the gust front may trigger the formation of new cells, sometimes well ahead of the parent cell. Cumulonimbus tops frequently penetrate into the lower stratosphere as an overshooting top, where strong winds aloft distort the cloud top into an anvil shape. Weather hazards reach peak intensity toward the end of the mature stage.
The dissipating stage is marked by a strong downdraft embedded within the area of precipitation. Subsiding air replaces the updraft throughout the cloud, effectively cutting off the supply of moisture provided by the updraft. Precipitation tapers off and ends. Compression warms the subsiding air and the relative humidity drops.
The convective cloud gradually vaporizes from below, leaving only a remnant anvil cloud.
19.4 Thunderstorm Types. There are three principal thunderstorm types: single cell, multicell (cluster and line), and supercell. All thunderstorms are hazardous to aircraft.
A single cell (also called ordinary cell) thunderstorm consists of only one cell. Its life cycle was covered in the previous section. It is easily circumnavigated by pilots, except at night or when embedded in other clouds. Single cell thunderstorms are rare; almost all thunderstorms are multicell.
19-3 A multicell cluster thunderstorm (see Figure 19-3 and Figure 19-4) consists of a cluster of cells at various stages of their life cycle. With an organized multicell cluster, as the first cell matures, it is carried downwind, and a new cell forms upwind to take its place.
A multicell cluster may have a lifetime of several hours (or more). New cells will continue to form as long as the three necessary ingredients exist (see paragraph 19.2).
Its size and persistence make it a bit tougher to circumnavigate than a single cell thunderstorm. An area of multicell cluster thunderstorms can be like a mine field for air traffic.
Figure 19-3. Multicell Cluster Thunderstorm
Sometimes thunderstorms will form in a line that can extend laterally for hundreds of miles. New cells continually re-form at the leading edge of the system with rain, and sometimes hail, following behind. Sometimes storms which comprise the line can be supercells. The line can persist for many hours (or more) as long as the three necessary ingredients continue to exist. These squall lines are the thunderstorm type which presents the most effective barrier to air traffic because the line is usually too tall to fly over, too dangerous to fly through or under, and difficult to circumnavigate. About 25 percent of all U.S. tornadoes are spawned by squall lines.
19-4 Figure 19-4. Multicell Line Thunderstorm
A supercell thunderstorm (see Figure 19-5) is an often dangerous convective storm that consists primarily of a single, quasi-steady rotating updraft that persists for an extended period of time. It has a very organized internal structure that enables it to produce especially dangerous weather for pilots who encounter them. Updraft speeds may reach 9,000 feet per minute (100 knots). This allows hazards to be magnified to an even greater degree. Nearly all supercells produce severe weather (e.g., large hail or damaging wind) and about 25 percent produce a tornado. A supercell may persist for many hours (or longer). New cells will continue to form as long as the three necessary ingredients exist (see paragraph 19.2).
A supercell’s size and persistence make it a bit tougher to circumnavigate than a single cell thunderstorm. Also, multicell clusters and lines may have supercells incorporated as part of the system as well.
19-5 Figure 19-5. Supercell Thunderstorm
19.5 Factors that Influence Thunderstorm Motion. A thunderstorm is a process, not a solid object or block of wood. Storm motion equals the combined effects of both advection and propagation (see Figure 19-6). Advection is the component of storm motion due to individual cells moving with the average wind throughout the vertical depth of the cumulonimbus cloud. The wind at FL180 (500 millibars) usually provides a good approximation. Propagation is the component of storm motion due to old cell dissipation and the new cell development. Storm motion may deviate substantially from the motion of the individual cells which comprise the storm.
Figure 19-6. Factors that Influence Thunderstorm Motion
Individual cells which comprise the storm move northeast (advection), but dissipate and are replaced by new cells (propagation). Storm motion equals the combined effects of both advection and propagation.
19-6
19.6 Hazards. A thunderstorm can pack just about every aviation weather hazard into one vicious bundle. These hazards include: lightning, adverse winds, downbursts, turbulence, icing, hail, rapid altimeter changes, static electricity, and tornadoes.
19.6.1 Lightning. Every thunderstorm produces lightning and thunder by definition. Lightning is a visible electrical discharge produced by a thunderstorm. The discharge may occur within or between clouds, between the cloud and air, between a cloud and the ground, or between the ground and a cloud.
Lightning can damage or disable an aircraft. It can puncture the skin of an aircraft. It can damage communication and electronic navigational equipment. Nearby lightning can blind the pilot, rendering him or her momentarily unable to navigate either by instrument or by visual reference. Nearby lightning can also induce permanent errors in the magnetic compass. Lightning discharges, even distant ones, can disrupt radio communications on low and medium frequencies. Lightning has been suspected of igniting fuel vapors, causing explosion; however, serious accidents due to lightning strikes are extremely rare.
19.6.2 Adverse Wind. Adverse winds are always found within thunderstorms and often many miles away from the precipitation area. Crosswinds, gusts, and variable winds/sudden wind shifts can lead to a crash during takeoffs, approaches, and landings. The area along and immediately behind the gust front is particularly dangerous because this is where rapid and sometimes drastic changes in surface winds occur.
19.6.3 Downburst. Shower and thunderstorm cells sometimes produce intense downdrafts called downbursts that create strong, often damaging winds. Downbursts (see Figure 19-7) can create hazardous conditions for pilots and have been responsible for many low-level wind shear accidents. Smaller, shorter-lived downbursts are called microbursts.
A downburst is especially dangerous to airplanes when it is encountered when climbing from takeoff or approaching to land. During this phase, the aircraft is operating at relatively slow speeds. A major change of wind velocity can lead to loss of lift and a crash.
Figure 19-7. Downburst Life Cycle (1-10 km)
FORMATION - IMPACT - DISSIPATION - Evaporation and Downdraft quickly Downdraft moves precipitation drag accelerates and away from point of forms downdraft strikes the ground impact A microburst is particularly dangerous during landing if the pilot has reduced power and lowered the nose in response to the headwind shear (see Figure 19-8). This leaves the aircraft in a nose-low, power-low configuration when the tailwind shear occurs, which makes recovery more difficult. It can cause the airplane to stall or land short of the runway.
Figure 19-8. Landing in a Microburst Glideslope At point X, the aircraft enters the microburst zone where a headwind causes it to balloon above the normal glideslope. At the center of the microburst, point Y, there is a downdraft which causes the aircraft to sink. At point Z, the aircraft enters the most lethal zone where a sudden tailwind causes the aircraft to lose airspeed.
19-7
19-8 Pilots should be alert for indications of a microburst early in the approach phase, and ready to initiate a missed approach at the first indication. However, it may be impossible to recover from a microburst encounter at low altitude.
19.6.4 Turbulence. Turbulence is present in all thunderstorms. Severe or extreme turbulence is common. Gust loads can be severe enough to stall an aircraft at maneuvering speed or to cause structural damage at cruising speed. The strongest turbulence occurs with shear between updrafts and downdrafts. Outside the cumulonimbus cloud, turbulence has been encountered several thousand feet above, and 20 miles laterally from, a severe storm.
The wind-shear zone between the gust front and surrounding air is very turbulent airspace. Oftentimes, the surface position of the gust front is denoted by a line of dust or debris along the ground, or a line of spray along bodies of water. Sometimes the gust front shear zone is denoted by a shelf cloud (see Figure 19-9), which forms as warm, moist air is lifted by the gust front. Shelf clouds are most common with multicell line thunderstorms.
Figure 19-9. Thunderstorm with Shelf Cloud
19.6.5 Icing. Thunderstorms produce abundant supercooled liquid water above the freezing level. When temperature in the upward current cools to about -15 °C, much of the remaining water vapor sublimates as ice crystals; and above this level, the amount of supercooled water decreases. However, supercooled water can exist at temperatures as cold as -40 °C in the presence of vigorous upward vertical motion, such as in a thunderstorm updraft.