WX BRIEF

Updated essentials about thunderstorms that every pilot flying IFR needs to know

We often share the skies with these giant forces of nature, but complacency can be dangerous and caution is always advisable.




Anatomy of an idealized supercell thunderstorm. Strong storms will often extend to the base of the stratosphere, giving these cumulonimbus clouds a classic anvil-topped shape. Hail and lightning may be encountered 20 miles or more from the storm itself.

Although most updrafts won't exceed about 6000 fpm (60 kts), some supercell storms and meso­scale convective systems occur in environments where the CAPE may exceed 4500 J/kg, meaning their updrafts could possibly reach speeds of more than 95 mps (18,700 fpm or 185 kts).

The ability of an updraft to suspend and loft hailstones is also tied to its maximum speed. Small hail can be suspended by an updraft of as little as 10 mps, while golfball-sized hail or larger requires updrafts above about 29 mps, indicating that even relatively low-energy storms are capable of producing damaging hail.

While an updraft itself is not inherently dangerous to an aircraft, the transition into or out of the updraft is. The air around the updraft is likely moving a lot slower, or may even be sinking, meaning that the speed difference may be extreme. That difference in speed and/or direction means significant windshear around the edges of the updraft.

At the same time, flying an aircraft from a calm vertical wind into air rising at over 100 kts is the equivalent of a 100-kt acceleration of the wind. Since force is defined as mass times acceleration, it is obvious that the force suddenly being applied to the airframe can be extreme.

Downdrafts are also a danger to aircraft. They begin as air, cooled and dried from its ascent, becomes denser than the warm, humid air rising beneath it. As that warmer air rises, so the cooler air begins to descend. Normally, the descending air would quickly warm as it compresses, but as it warms it loses some of that heat energy to evaporation of the liquid water in the cloud.

This keeps the air from warming as quickly as it would otherwise and lets it continue to sink. In addition, falling precipitation in this region of the cloud can enhance the downdraft by drawing the air in its downward wake.

Because downdrafts tend to have less of a temperature difference from their surroundings, they do not have as great a vertical velocity as the updraft. However, downdrafts in the strongest storms are still capable of attaining speeds in excess of 100 kts (10,127 fpm). In a typical airmass type of thunderstorm, the downdraft initiates directly above the updraft, disrupting the updraft and collapsing the storm upon itself.

Flow of air within a strong thunderstorm. Shear aloft tilts the storm, allowing the downdraft to coexist with the updraft. The strongest shear forces within the storm will occur between the updraft and downdraft regions.

But gradually increasing horizontal winds aloft may tilt the storm, allowing the downdraft to fall adjacent to the updraft, and thereby allowing the storm to sustain itself for a longer period of time as a supercell.

An additional danger of downdrafts is that they often emerge from the low base of the thunderstorm cloud at high speed and slam into the ground below. Since they can't continue downward, these downdraft winds spread out in all directions horizontally. They are what create the gust front that normally precedes a storm (and helps to lift the warm, humid surface air into the storm).

Downbursts

Occasionally, a downdraft produces a short-lived pulse of more rapidly moving air. Such pulses, or downbursts, may be exceptionally powerful, with windspeeds of up to 150 kts. Like the downdraft, the downburst will hit the ground and spread out.

If the winds spread out less than 2 km (1.25 miles) from the center of the downburst, it is known as a microburst—otherwise, it is a macroburst. As the downburst spreads out along the surface, it often generates a roll vortex at its leading edge.

This vortex, which often extends only a few hundred feet above the surface, can be extremely violent. While the vortex can cause a loss of control for a landing or departing aircraft, the more common danger from downbursts is low-level windshear.

When a downburst occurs at or near an airport, it is a hazard for arriving or departing aircraft. (Downbursts can even flip aircraft on the ground.) What happens is that the encountering aircraft suddenly registers an increase in airspeed and may balloon above the flightpath as the strong, sudden headwind is encountered.

The normal response would be to retard the throttle and bring the nose down—but, within seconds, the aircraft enters the downburst core and loses altitude rapidly. This may last for only a second or two before the downburst winds attack the aircraft from behind, robbing it of essential airspeed and further decreasing altitude. Pilots who have failed to recognize the symptoms of a downburst encounter now find themselves in a stall situation, sinking and with insufficient power just a few hundred feet above the ground.

Even worse, not all downbursts are accompanied by rain. Some, especially in arid regions, have evaporated all the limited moisture and are simply wind, often visible only as a circle of dust kicked up as the downburst expands.

Fortunately, many airports prone to downbursts and low-level windshear now have alert systems, and most professional pilots receive some level of downburst recovery instruction as part of their recurrent training sessions. Such training boils down to this—at the first sign of an uncommanded increase in airspeed or pitch from an otherwise stable approach in the vicinity of thunderstorms, apply power and climb away from any convection.

Icing, hail and rain

In addition to the raw force of the vertical currents within the storm cell, there are some dangers common to most thunderstorms. Except for tropical or subtropical summer thunderstorms, almost all thunderstorms extend above the atmosphere's freezing level.

This means that even if you manage to avoid the vicious updrafts and downdrafts by climbing higher, you may find yourself in a subfreezing environment surrounded by liquid water droplets. Some of the most severe ice accretions ever experienced have occur­red in the middle levels of thunderstorms, where large supercooled liquid droplets bombard an aircraft in temperatures some 10–20 degrees below freezing.

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