JPATS Weather - NETC

JPATS AVIATION WEATHER BOOKLET
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 ¼ to
2½ miles in diameter (Figures 4-5 and 4-6). Microbursts may emanate from any convective
cloud, not just cumulonimbus clouds. Another unique aspect of a microburst is its short life
span–usually only 5 to 10 minutes after reaching the ground–which makes the study, and hence
the prediction, of microbursts a difficult task.
Figure 5-5 — Vortex Ring of a Microburst
Figure 5-6 — Cross Section of a Microburst
Version 3.2/Dec 08 5-5