turbulence and generally heavy rainfall, this particular
altitude appears to be the most hazardous for aircraft.
associated with thunderstorm activity is the rapid
immediately before storm passage. The strong winds at
the surface accompanying thunderstorm passage are
the result of the horizontal spreading out of downdraft
current from within the storm as they approach the
surface of Earth.
The total wind speed is a result of the downdraft
divergence plus the forward velocity of the storm cell.
Thus, the speeds at the leading edge, as the storm
approaches, are greater than those at the trailing edge.
The initial wind surge, as observed at the surface, is
known as the first gust.
The speed of the first gust is normally the highest
recorded during storm passage, and the direction may
vary as much as 180° from the previously prevailing
surface wind. First-gust speeds increase to an average
of about 16 knots over prevailing speeds, although
gusts of over 78 knots (90 mph) have been recorded.
The average change of wind direction associated with
the first gust is about 40°.
In addition to the first gust, other strong, violent,
associated with the thunderstorm. These winds are
referred to as downbursts. Downbursts are subdivided
into macrobursts and microbursts.
damage similar to tornadoes. These damaging winds
can last 5 to 20 minutes and reach speeds of 130 knots
(150 mph) or more.
downbursts. A microburst can last 2 to 5 minutes and
can also reach wind speeds in excess of 130 knots.
crosswinds and wind shear for aircraft and are difficult
to observe or forecast.
Downbursts are not the same as first gusts. First
gusts occur in all convective cells containing showers
however, do not occur in all convective cells and
All thunderstorms are similar in physical makeup,
but for purposes of identification, they may be divided
into two general groups, frontal thunderstorms and
commonly associated with both warm and cold fronts.
The warm-front thunderstorm is caused when warm,
moist, unstable air is forced aloft over a colder, denser
shelf of retreating air. Warm-front thunderstorms are
generally scattered; they are usually difficult to identify
because they are obscured by other clouds.
The cold-front thunderstorm is caused by the
forward motion of a wedge of cold air, into a body of
warm, moist unstable air. Cold-front storms are
normally positioned aloft along the frontal surface in
what appears to be a continuous line.
Under special atmospheric conditions, a line of
thunderstorms develops ahead of a cold front. This line
of thunderstorms is the prefrontal squall line. Its
distance ahead of the front ranges from 50 to 300 miles.
Prefrontal thunderstorms are usually intense and
appear menacing. Bases of the clouds are very low.
Tornadoes sometimes occur when this type of activity
subdivided into several types. In this text, however,
only two basic types are discussed, the convective
thunderstorm and the Orographic thunderstorm.
occur over land or water almost anywhere in the world.
Their formation is caused by solar heating of various
areas of the land or sea, which, in turn, provides heat to
the air in transit. The land type of convective
thunderstorm normally forms during the afternoon
hours after Earth has gained maximum heating from the
Sun. If the circulation is such that cool, moist,
convective, unstable air is passing over the land area,
heating from below causes convective currents and
results in towering cumulus or thunderstorm activity.
Dissipation usually begins during the early evening
hours. Storms that occur over bodies of water form in
the same manner, but at different hours. Sea storms
usually form during the evening after the Sun has set
and dissipate during the late morning.