The fundamental structural element of the thunderstorm is the unit of convective circulation known as the convective cell. A mature thunderstorm contains several of these cells, which vary in diameter from 1 to 6 miles. By radar analysis and measurement of drafts, it has been determined that, generally, each cell is independ-ent of surrounding cells of the same storm. Each cell progresses through a cycle which lasts from 1 to 3 hours. In the initial stage (cumulus development), the cloud consists of a single cell, but as the development progresses, new cells form and older cells dissipate.

The life cycle of the thunderstorm cell con-sists of three distinct stages; they are the cumulus stage, the mature stage, and the dissipating or anvil stage. (See fig. 5-4-1.)

CUMULUS STAGE. Although most cumu-lus clouds do not become thunderstorms, the in-itial stage of a thunderstorm is always a cumulus cloud. The chief distinguishing feature of this cumulus or building stage is an updraft which prevails throughout the entire cell. Such updrafts vary from a few feet per second in the early cells to as much as 100 feet per second in mature cells.

MATURE STAGE. The beginning of sur-face rain, with adjacent updrafts and downdrafts, initiates the mature stage. By this time the top of the average cell has attained a height of 25,000 feet or more. As the raindrops begin to fall, the frictional drag between the raindrops and the sur-rounding air causes the air to begin a downward motion. Since the lapse rate within a thunderstorm cell is greater than the moist adiabatic rate, the

Figure 5-4-2.Downdraft beneath a thunderstorm cell in the mature stage. Arrows represent wind flow. Dashed lines indicate rainfall.

descending saturated air soon reaches a level where it is colder than its environment; conse-quently, its rate of downward motion is ac-celerated, resulting in a downdraft. (See fig. 5-4-2.)

A short time after the rain starts its initial fall, the updraft reaches its maximum speed. The speed

Figure 5-4-1.-Life cycle of a thunderstorm cell.

of the updraft increases with altitude. Downdrafts are usually strongest at the middle and lower levels, although the variation in speed from one altitude to another is less than in the case of up-drafts. Downdrafts are not as strong as updrafts; downdraft speeds range from a few feet per sec-ond to 40 feet per second or more. Significant downdrafts seldom extend to the top of the cell because, in most cases, only ice crystals and snowflakes are present, and their rate of fall is insufficient to cause appreciable downdrafts. 

The mature cell, then, generally extends far above 25,000 feet, and the lower levels consist of sharp updrafts and downdrafts adjacent to each other. Large water droplets are encountered suspended in the updrafts and descending with the downdrafts as rain.

DISSIPATING (ANVIL) STAGE. Through-out the life span of the mature cell, more and more air aloft is being dragged down by the falling rain-drops. Consequently, the downdraft spreads out to take the place of the dissipating updraft. As this process progresses, the entire lower portion of the cell becomes an area of downdraft. Since this is an unbalanced situation and since the descending motion in the downdraft effects a dry-ing process, the entire structure begins to dissipate. The high winds aloft have now carried the upper section of the cloud into the anvil form, indicating that gradual dissipation is overtaking the storm cell.

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