Thunderstorm Development

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

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Thunderstorms