A28-1146-102-00
Federal Aviation Administration (FAA) Advisory Circulars
A-5
TORNADOES
D The most violent thunderstorms draw into their cloud bases with
greatvigor. Iftheincoming airhas anyinitial rotatingmotion, itoften
forms an extremely concentrated vortex from the surface well into
thecloud. Meteorologists haveestimated thatwind insuch avortex
can exceed 200 knots; pressure inside the vortex is quite low. The
strongwindsgatherdustanddebrisandthelowpressuregenerates
afunnelshapedcloudextendingdownwardfrom thecumulonimbus
base. If the cloud does not reach the surface, it is a funnel cloud;
if it touches the land surface, it is a tornado.
D Tornadoes occur with both isolated and squall line thunderstorms.
Reports for forecasts of tornadoes indicate that atmospheric
conditions are favorable for violent turbulence. An aircraft entering
atornadovortexisalmostcertaintosufferstructuraldamage. Since
the vortex extends wellinto thecloud, anypilot inadvertentlycaught
on instruments in a severe thunderstorm, could encounter a hidden
vortex.
D 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 connected to a severe
thunderstorm carries a threat of violence.
TURBULENCE
D Potentially hazardous turbulence is present in all thunderstorms,
and a severe thunderstorm can destroy an aircraft. Strongest
turbulence within the cloud occurs with shear betweenupdraftsand
downdrafts. Outside the cloud, shear turbulence has been
encountered several thousand feet above and 20 miles laterally
from a severe thunderstorm. Alow levelturbulent area is the shear
zoneassociatedwiththegustfront. Often,arollcloudontheleading
edge of a storm marks the top of the eddies in this shear and it
signifies an extremely turbulent zone. Gust fronts move far ahead
(up to 15 miles) of associated precipitation. The gust front causes
a rapid and sometimes drastic change in surface wind ahead of an
approaching storm. Advisory Circular 00-50A, ”Low Level Wind
Shear,” explains in greater detail the hazards associated with gust
fronts. Figure1 showsa schematiccross sectionof athunderstorm
withareasoutsidethecloudwhereturbulencemaybeencountered.
D It is almost impossible to hold a constant altitude in a thunderstorm,
and maneuvering in an attempt todo soproduces greatlyincreased
stress on the aircraft. It is understandable that the speed of the
aircraft determines the rate of turbulence encounters. Stresses are
least 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.
To Next Page
Loading...
