March 25, 2013
by the FAA (From Flight Training, February 1994.)
Annually, and prior to the start of the season, we publish an issue of the Air Traffic Bulletin to focus your attention on the upcoming thunderstorm season and to remind all controllers and specialists of the hazardous weather associated with it. The more we understand the severe atmospheric hazards associated with thunderstorms, the better position we are in to aid the pilot in avoiding these hazards. The article also presents factors that a pilot must consider when flying in the vicinity of, entering or penetrating a thunderstorm. Being familiar with these factors will help you better understand what is going on inside the cockpit.
The latent heat released by a moderate thundercloud is equivalent to the energy of a nuclear explosion of 400 kilotons! Flight through such clouds should be avoided whenever possible and, although most commercial aircraft are equipped with airborne weather radar, a pilot should always request up-to-date information concerning the location and extent of any active thunderstorm area which may affect their fight. Thunderstorms often reach far greater heights than the usual cruising levels of commercial aviation. The hazards involved in penetrating a thunderstorm are severe turbulence, hail, icing, extreme water ingestion and, to a lesser degree, lightning.
Downbursts, the strong descending air current found underneath storm clouds, cause rapid variations in wind speed and wind direction near the ground which have proven to be extremely dangerous to low-flying aircraft. Sometimes violent thunderstorms cause the formation of concentrated powerful vortices, extending from the ground well into the cloud, scattered tornadoes or, when over water, waterspouts. Tornadoes produce the highest wind speeds experienced near the ground (maximum values are estimated at 460 km/h). An aircraft entering a tornado vortex is almost certain to suffer structural damage. Thunderstorm hazards occur simultaneously in numerous combinations. The following discussions examine these and other thunderstorm phenomena.
The most violent thunderstorms draw air into their cloud bases with great vigor. If the incoming air has any initial rotating motion, it often forms an extremely concentrated vortex from the surface well into the cloud. Meteorologists have estimated that wind in such a vortex can exceed 200 knots, and the pressure inside the vortex is quite low. The strong winds gather dust and debris, and the low pressure generates a funnel-shaped cloud extending downward from the cumulonimbus base. If the cloud does not reach the surface, it is a funnel cloud; if it touches the land surface, it is a tornado; if it touches water, it is a water spout.
Tornadoes have, at times, occurred with isolated thunderstorms, but more frequently they form with steady state thunderstorms associated with cold fronts or squall lines. Reports or forecasts of tornadoes indicate that atmospheric conditions are favorable for violent turbulence. Since the vortex extends well into the cloud, any pilot inadvertently caught on instruments in a severe thunderstorm could encounter a hidden vortex.
Families of tornadoes have been observed as appendages of the main cloud extending several miles outward from the area of lightning and precipitation. Thus, any cloud(s) connected to a severe thunderstorm carries a threat of violence. Frequently cumulonimbus mammatus clouds occur in connection with violent thunderstorms and tornadoes. These clouds display rounded, irregular pockets or festoons from its base and are a signpost of violent turbulence. Surface aviation observations specifically mention this and other hazardous clouds.
Tornadoes occur most frequently in the Great Plains states east of the Rocky Mountains; however, they have occurred in every state.
A "squall line" is a nonfrontal, narrow band of active, or very active, thunderstorms. They often develop ahead of a cold front in moist, unstable air, but they may develop in unstable air far from any front. The line may be too long to easily detour and too wide and severe to penetrate. They often contain severe steady state thunderstorms and presents the single most intense weather hazard to aircraft. They usually form rapidly, generally reaching maximum intensity during late afternoon and the first few hours of darkness.
Hazardous turbulence is present in all thunderstorms; in a severe thunderstorm, it can damage an airframe. The strongest turbulence within the cloud occurs with shear between up- and downdrafts. Outside the cloud, shear turbulence has been encountered several thousand feet above and 20 miles laterally from a severe storm. A low level turbulent area is the shear zone between the "plow" wind and the surrounding air. Often, a "roll cloud" on the leading edge of storm marks the eddies in this shear. The roll cloud is most prevalent with cold front or squall line thunderstorms and signifies an extremely turbulent zone. The first gust causes a rapid and sometimes drastic change in surface wind ahead of an approaching storm. It is almost impossible to hold a constant altitude in a thunderstorm! Maneuvering, or attempting to do so, greatly increases the stresses on the aircraft. Stresses will be lessened if the aircraft is held in a constant attitude and allowed to "ride the waves." To date, we have no sure way to pick "soft spots" in a thunderstorm.
Microbursts are small-scale intense downdrafts which, on reaching the surface, spread outward from the downflow center. This causes the presence of both vertical and horizontal wind shear effects that can be extremely hazardous to all types and categories of aircraft, especially at low, critical flight attitudes. Due to their small size, short life-span and the fact that they can occur over areas without surface precipitation, microbursts are not easily detectable using conventional weather radar or wind shear alert systems. Parent clouds producing microburst activity can be any of the low or middle layer convective cloud types.
Size - Approximately 6,000 feet in diameter above the ground with a horizontal extent on the surface spreading to approximately 2 1/2 miles outward from the center.
Intensity - Vertical winds as high as 6,000 feet per minute above the ground becoming strong horizontal winds with as much as an 80 knot variation on the surface. The downward airstream may extend as low as tree top level.
Types -- Wet and dry. In wet areas of the U.S., microbursts are normally accompanied by heavy rain. However, dry areas provide falling raindrops with sufficient time and distance to dissipate before reaching the ground (VIRGA).
Life -- The life-cycle of a microburst from the initial downburst to dissipation will seldom be longer than 10 minutes with maximum intensity winds lasting approximately 2 minutes. Multiple microburst activity in the same area is not uncommon and should be expected.
Signs -- Dry microbursts often generate a ring of dust on the surface. Opposite direction winds over a short distance, accompanied by cell activity is also a clear indication of a microburst.
During landing and takeoff, microburst wind shear effects can cause a sufficient reduction in aircraft performance to create a severe hazard due to the possibility of ground contact. Flight in the vicinity of suspected microburst activity should always be avoided.
Updrafts in a thunderstorm support abundant liquid water. The water becomes supercooled when carried above the freezing level. When temperature in the upward current cools to about -15 degrees Centigrade, much of the remaining water vapor sublimates as ice crystals. Above this level the amount of supercooled water decreases.
Supercooled water freezes on impact with an aircraft. Clear icing can occur at any altitude above the freezing level; but at high levels, icing may be either rime or mixed rime and clear. The abundance of supercooled water makes clear icing occur very rapidly between 0' and -15' C., and encounters can be frequent in a cluster of cells. Thunderstorm icing can be extremely hazardous.
Hail competes with turbulence as the greatest thunderstorm hazard to aircraft. Supercooled drops above the freezing level begin to freeze Once a drop has frozen, other drops latch on and freeze to it, so the hailstone grows - sometimes into a huge iceball. Large hail occurs with severe thunderstorms usually towering to great heights. Eventually the hailstones fall, possibly some distance from the storm core. In fact, hail has been observed in clear air several miles from the parent thunderstorm. As hailstones fall through the melting level, they begin to melt and precipitation may reach the ground as either hail or rain. Rain at the surface does not mean the absence of hail aloft. You should anticipate possible hail with any thunderstorm, especially beneath the anvil of a large cumulonimbus. Hailstones larger than 1/2 inch in diameter can sufficiently damage an aircraft in a few seconds.
Visibility generally is near zero within a thunderstorm cloud. Ceilings and visibility can become restricted in precipitation and dust between the cloud base and the ground. The restrictions create the same problem as all ceiling and visibility restrictions; but the hazards are increased many fold when associated with the other thunderstorm hazards of turbulence, hail, and lighting which make precision instrument fly virtually impossible.
Pressure usually falls rapidly with the approach of a thunderstorm, then rises sharply with the onset of the first just and arrival of the cold downdraft and heavy rain showers, falling back to normal as the storm moves on. This cycle of pressure change may occur in 15 minutes. If the altimeter setting is not corrected, the indicated altitude may be in error by over 100 feet.
Electricity generated by thunderstorms is rare y a great hazard to aircraft, but it may cause damage and is annoying to flight crews. Lighting is the most spectacular of the electrical discharges.
A lightning strike can puncture the skin of an aircraft and can damage communication and electronic navigational equipment. Lightning has been suspected of igniting fuel vapors causing explosions; however, serious accidents due to lightning strikes are believed to be extremely rare. Nearby lightning can blind the pilot, rendering him momentarily unable to navigate either by instrument or by visual reference. Lightning can also induce permanent errors in the compass. Lightning discharges, even distant ones, disrupt radio communications on low and medium frequencies. A few pointers on lightning: The more frequent the lightning, the more severe the thunderstorm. Increasing frequency of lightning indicates a growing thunderstorm. Decreasing lightning indicates a storm nearing the dissipating stage. At night, frequent distant flashes playing along a large sector of the horizon suggest a probable squall line.
Precipitation static, a steady, high level of noise in radio receivers, is caused by intense corona discharges from sharp metallic points and edges of flying aircraft. It is encountered often in the vicinity of thunderstorms. When an aircraft flies through clouds, precipitation, or a concentration of solid particles (ice, sand, dust, etc.), it accumulates a charge of static electricity. The electricity discharges onto a nearby surface or into the air causing a noisy disturbance at lower frequencies. The corona discharge is weakly luminous and may be seen at night. Although it has a rather eerie appearance, it is harmless. It was named 'St. Elmo's Fire" by Mediterranean sailors, who saw the brushy discharge at the top of the ship masts.
Turbine engines have a limit on the amount of water they can ingest. Updrafts are present in many thunderstorms, particularly those in the developing stages. If the updraft velocity in the thunderstorm approaches, or exceeds, the terminal velocity of the falling raindrops, very high concentrations of water may occur. It is possible that these concentrations can be in excess of the quantity of water turbine engines are designed to ingest, resulting in flameout or structural failure of one or more engines.
At the present time, there is no known operational procedure that can completely eliminate the possibility of engine damage or flameout during massive water ingestion. Although the exact mechanism of the water-induced engine stalls has not been determined, it is believed that thrust changes may have an adverse effect on engine stall margins in the presence of massive water ingestion.
Avoidance of severe storm systems is the only measure assured to be effective in preventing exposure to this type of multiple engine damage or flameout. During an unavoidable encounter with severe storms, with extreme precipitation, the best known recommendation is to follow the severe turbulence penetration procedure contained in the approved airplane flight manual with special emphasis on avoiding thrust changes unless excessive airspeed variations occur.
Let's be sure weather dissemination gets proper attention by timely updates of recorded messages on all equipment used to disseminate such information; such as, ATIS, TWEB and HIWAS. An efficient ATC system requires timely communications between all appropriate activities. This includes other facilities, sectors, etc. It may also include other organizations outside the FAA itself, such as the military, the airlines, airport management and other users of our services. Let's not exclude anyone from the communications loop who has a need to know.
Weather radar detects droplets of precipitation size. Strength of the radar return (echo) depends on drop size and number. The greater the number of drops, the stronger the echo; and the larger the drops, the stronger the echo. Drop size determines echo intensity to a much greater extent than does drop number.
Meteorologists have shown that drop size is almost directly proportional to rainfall rate; and the greatest rainfall rate is in thunderstorms. Therefore, the strongest echoes are thunderstorms. Hailstones usually are covered with a film of water and, therefore, act as huge water droplets giving the strongest of all echoes. Showers show less intense echoes, and gentle rain and snow return the weakest of all echoes.
Since the strongest echoes identify thunderstorms, they also mark the areas of greatest hazards. Radar information can be valuable both from ground-based radar for preflight planning and from airborne radar for severe weather avoidance.
Thunderstorms build and dissipate rapidly, and they also may move rapidly. The best use of ground radar information is to isolate general areas and coverage of echoes. Remember that weather radar detects only precipitation drops; it does not detect minute cloud droplets. Therefore, the radar scope provides no assurance of avoiding instrument weather in clouds and fog.
The most intense echoes are severe thunderstorms. Remember that hail may fall several miles from the cloud and hazardous turbulence may extend as much as 20 miles from the cloud. Pilots should request separation from the most intense echoes by 20 miles or more.
Since PIREP information may be such a significant factor in both the pilots' and controllers' operational decisions, the proper application of PIREP procedures should receive special attention. The imminent arrival of thunderstorm season is a good time for all operational personnel to review and refresh their knowledge of PIREP procedures. DO'S AND DON'TS OF
Although the following suggestions are written for pilots, we believe they will be useful information for everyone. Above all, remember this: Never regard any thunderstorm as 'light' even when radar returns show the echoes are of light intensity. Avoiding thunderstorms is still the best policy.
The Following are some Do's and Don'ts of thunderstorm avoidance:
If you cannot avoid penetrating a thunderstorm, the following are some Do's before entering the storm:
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