Microbursts - P-3030089

AVIATION WEATHER

CHAPTER FOUR

vapor to air can form a highly explosive mixture. For this reason, as well as for battle

survivability, most military aircraft fuel tanks are pressurized.

Pilots are not immune to the effects of lightning strikes, either. Temporary night vision

degradation can occur due to flash blinding, but turning cockpit lighting to maximum intensity

can minimize this effect. Additionally, some pilots have experienced mild electric shock and

minor burns.

Tornadoes

A tornado is a violent, intense, rotating column of air that descends from cumulonimbus clouds

in funnel-like or tube-like shapes. If the circulation does not reach the surface, it is called a

funnel cloud. If it touches down over the water, it is called a waterspout. A tornado vortex is

normally several hundred yards wide, but some have been measured up to 2 1/2 miles wide.

Within the tornado's funnel-shaped circulation, winds have been measured at speeds over

300 miles per hour, while the forward speed of tornadoes averages 30 to 40 knots.

Observed as appendages of the main cloud, tornadoes often form in groups or families of funnel

clouds, some as far as 20 miles from the lightning and precipitation areas. Innocent looking

cumulus clouds trailing a thunderstorm may mask tornadic activity, and the vortex may not be

visible to warn unwary aircrews. The invisible vortices may be revealed only by swirls in the

cloud base or dust whirls boiling along the ground, but may be strong enough to cause severe

damage to aircraft.

Tornadoes form only with severe thunderstorms. The hazards they present have been chronicled

often by news reports and television documentaries. To avoid tornadoes, avoid areas of severe

thunderstorm activity.

MICROBURSTS

A microburst is an intense, highly localized downward atmospheric flow with velocities of 2000

to over 6000 feet per minute. This downward flow diverges outward, producing a vortex ring of

wind that can produce differential velocities ranging from 20 to 200 knots in an area only 1/4 to

2 1/2 miles in diameter (Figures 4-6 and 4-7). Microbursts may emanate from any convective

cloud, not just cumulonimbus clouds. Another unique aspect of a microburst is its short life

span, usually only five to ten minutes after reaching the ground, which makes the study, and

hence the prediction, of microbursts a difficult task. They are more likely to occur in mid-

afternoons during summer months.

Thunderstorms

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