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CHAPTER FOUR
AVIATION WEATHER
402.
REFERENCES
Weather for Aircrews, AFH 11-203, Volume 1, Chapters 10 and 13
403.
STUDY ASSIGNMENT
Review Chapter Four and answer the Study Questions.
404.
THUNDERSTORM DEVELOPMENT
The basic requirements for thunderstorm formation are moisture, unstable air, and some type of
lifting action. Lifted air does not always result in thunderstorm activity. Air may be lifted to a
point where the moisture condenses and clouds form, but these clouds may not grow
significantly unless the air parcel reaches a point where it will continue to rise freely (recall the
point of free convection from Chapter Two). The higher the moisture content the easier the point
of free convection is reached. One of the four lifting methods (from Chapter Two) is necessary
to force warmer air from its lower level to the point of free convection, which is the trigger to
starting the cumulus cloud through the thunderstorm life cycle. Once moist air is lifted in an
unstable environment, the rapidly rising unstable air quickly forms towering cumulus and
eventual cumulonimbus clouds. The degree of vertical cloud growth often indicates the potential
severity of the thunderstorm.
405.
THE LIFE CYCLE OF A THUNDERSTORM CELL
Thunderstorm cells progress through three stages during their life cycle: the cumulus, mature,
and dissipating stages. A thunderstorm cell is simply an individual cumulonimbus cloud. It is
virtually impossible to visually detect the transition from one stage to another. A thunderstorm
often consists of a cluster of cells in different stages. The life cycle of each thunderstorm cell
ranges from 20 minutes to 1-1/2 hours with a few lasting up to three hours. The life span of a
line of thunderstorms depends on the number of cells contained in the line and their stage of
development.
Cumulus Stage
Most cumulus clouds do not become thunderstorms. However, the initial stage of a
thunderstorm is always a cumulus cloud. The main feature of the cumulus stage is the updraft
which may extend from near the Earth's surface to several thousand feet above the visible cloud
top. The strongest updrafts occur at higher altitudes late in the stage and may be greater than
3000 feet per minute. No precipitation is associated with this stage, however significant
turbulence exists.
Mature Stage
The mature stage is reached when the raindrops and ice particles in the cloud have grown too
large to be supported by the updrafts and begin to fall. Rain and/or hail falling from the cloud
base indicates a downdraft has developed and the cell has entered the mature stages. The
4-2
Thunderstorms


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