Thunderstorms
This page contains information reguarding thunderstorms and the hidden
dangers involved in them.
Thunderstorms, which exist in cumulonimbus clouds, deserve special treatment
from pilots for several reasons. Often, you may hear pilots on the radio referring
to "towering cues to the south" or "several CBs to the west". They are talking about
clouds which indicate severe weather.
Three elements must be present for thunderstorm development:
- Warm air
- Moisture
- Unstable air mass
If all three of these elements are present, it takes very little to begin the process--merely
some form of uplifting. Uplifting may be caused by several sources such as a frontal passage, the sun heating the ground and causing
the warm air by the ground to rise, or even wind blowing up the side of a mountain.
Once this uplifting has begun, the warm moist air is carried upwards until the temperature
surrounding the parcel of air drops below the dew point for that parcel of air. At that time,
the moisture inside of the parcel of air will begin to condense out and form the fluffy cumulous
clouds we're all familiar with seeing forming on sunny afternoons. As this moisture condenses,
it releases latent heat and warms the air parcel more, causing more uplifting. Thus, the
thunderstorm, which began as a typical cumulous cloud, continues to become larger and more
buoyant. This stage of formation is known as the cumulous stage and is illustrated in
the following picture.
The cumulous stage of thunderstorm formation.
If conditions for growth remain favorable, the cumulous stage of the thunderstorm will become
even stronger with a strengthening updraft drawing in more warm, moist air and releasing more
latent heat. About 15 to 30 minutes after cumulous stage is reached, precipitation begins to fall
from the base of the cloud, marking the transition from cumulous stage to mature stage. The
storm is now a full blown thunderstorm, complete with thunder, lightning, heavy precipitation,
and turbulent winds. In the following picture, showing the mature stage of a thunderstorm, you
can begin to see the anvil shape typical of thunderstorms forming. This is caused by the air in
the updraft reaching the tropopause where upward motion is hindered and the cloud spreads
out horizontally (note, however, that often the air is moving so fast that it overshoots the
tropopause and creates a cloud dome that bulges up into the lower stratosphere).
The mature stage of thunderstorm formation.
As the precipitation falls, it cools the air in two ways: by direct contact and by absorbing
latent heat to cause some precipitation evaporation into the surrounding air. Both of these
result in a cooler, more dense airmass in the downdraft of the storm, making it even stronger.
The presence of these vigorous updrafts and downdrafts is very characteristic of mature stage
thunderstorms. The following picture shows these updraft and downdraft cores. Also seen in
this picture is the gust front which forms by the dense downdraft spreading outward after
reaching the ground.
The updraft and downdraft cores formed in a mature thunderstorm along with the gust
front.
About 15 to 30 minutes after entering the mature stage, a typical thunderstorm enters the
dissipating stage. This is caused by the cold downdraft air spreading out far enough that it
cuts off the storm's supply of warm humid air, as seen in the first figure below. Once
deprived of it's source of heat energy and moisture, the storm weakens. Thunder and
lightning diminish and eventually cease altogether and the storm consists of weak
downdrafts and light precipitation as seen in the second figure below.
The gust front spreads out under the storm and cuts off the updraft and the storms supply
of warm moist air
The dissipating stage of a typical thunderstorm consists of weak precipitation and
downdrafts
This is the lifespan of a typical airmass thunderstorm. Several variations exist for what
causes the initial uplifting and one thunderstorm can also give birth to a daughter storm
resulting in a line of thunderstorms. This is what can cause an effect often seen in the
midwest where a squall line of thunderstorms forms well in advance of any frontal passage.
The basic stages of the storm, however, are the same.
The main danger from thunderstorms to aviation comes from 2 sources. First off, a lightning
strike to an airplane has the potential to cause damage to several systems. Secondly, the
updrafts and downdrafts are a significant source of windsheer. As a pilot, you should take care to avoid any
type of cumulus clouds whenever possible because even gleaming white, puffy clouds with
no outward menace can be in the cumulus stage of thunderstorm formation and may be
hiding destructive forces such as high winds and large windsheers. Also, one must take
care when flying in the vicinity of a thunderstorm to avoid the far-reaching forces involved
such as turbulence which can be expected for up to 20 miles from a severe thunderstorm.
In addition, gusty winds may be experienced near the surface from the gust front.