Figure 5-2 shows the nature of the wind outflow and
indicates how it is formed from the settling dome of cold
air that accompanies the rain core during the mature
stage of the thunderstorm. The arrival of this outflow
results in a radical and abrupt change in the wind speed
and direction. It is an important consideration for
aircraft that are landing or taking off.
Wind speeds at the leading edge of the thunderstorm
are ordinarily far greater than those at the trailing edge.
The initial wind surge observed at the surface is known
as the first gust. The speed of the first gust is normally
the highest recorded during thunderstorm passage, and
it may vary as much as 180 degrees in direction from
the surface wind direction that previously existed. The
mass of cooled air spreads out from downdrafts of
neighboring thunderstorms (especially in squall lines),
and often becomes organized into a small, high-pressure
area called a bubble high, which persists for some time
as an entity that can sometimes be seen on the surface
chart. These highs may be a mechanism for controlling
the direction in which new cells form.
The speed of the thunderstorm winds depends upon
a number of factors, but local surface winds reaching 50
to 75 miles per hour for a short time are not uncommon.
Because thunderstorm winds can extend several miles
in advance of the thunderstorm itself, the thunderstorm
wind is a highly important consideration for pilots
preparing to land or take off in advance of a storm’s
arrival.
Also, many thunderstorm winds are strong
enough to do considerable structural damage, and are
capable of overturning or otherwise damaging even
medium-sized aircraft that are parked and not
adequately secured.
The outflow of air ahead of the thunderstorm sets
up considerable low-level turbulence. Over relatively
level ground, most of the significant turbulence
associated with the outrush of air is within a few hundred
feet of the ground, but it extends to progressively higher
levels as the roughness of the terrain increases.
THUNDERSTORM ALTIMETRY
During the passage of a thunderstorm, rapid and
marked surface pressure variations generally occur.
These variations usually occur in a particular sequence
characterized as follows.
l An abrupt fall in surface pressure as the storm
approaches.
l An abrupt rise in surface pressure associated with
rain showers as the storm moves overhead (often
associated with the first gust).
. A gradual return to normal surface pressure after
thunderstorm passage, and the rain ceases.
Such pressure changes may result in significant
pressure altitude errors on landing.
Of greater concern to the pilot are pressure altitude
readings that are too high. If a pilot used an altimeter
setting computed during the maximum pressure, and
then landed after the pressure had fallen, the altimeter
still could read 60 feet or more above the true altitude
after landing.
Here is where you, as a forecaster, can make certain
that timely and accurate altimeter settings are furnished
to the tower for transmission to pilots during
Figure 5-2.-Cold dome of air beneath a thunderstorm cell in the mature stage.
lines indicate rainfall.
5-4
Arrows represent deviation of windflow. Dashed
