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CHAPTER 13. SEVERE WEATHER <br />13-2 <br />electrical charges. Ice particles usually have positive charges, and rain droplets usually have negative <br />charges. When the charges build up enough, they are discharged in a bolt of lightning, which causes the <br />sound waves we hear as thunder. Thunderstorms have three stages (see Figure 13-1): <br />• The developing stage of a thunderstorm is marked by a cumulus cloud that is being pushed <br />upward by a rising column of air (updraft). The cumulus cloud soon looks like a tower (called <br />towering cumulus) as the updraft continues to develop. There is little to no rain during this <br />stage but occasional lightning. The developing stage lasts about 10 minutes. <br />• The thunderstorm enters the mature stage when the updraft continues to feed the storm, but <br />precipitation begins to fall out of the storm, and a downdraft begins (a column of air pushing <br />downward). When the downdraft and rain-cooled air spread out along the ground, they form a <br />gust front, or a line of gusty winds. The mature stage is the most likely time for hail, he avy <br />rain, frequent lightning, strong winds, and tornadoes. The storm occasionally has a black or <br />dark green appearance. <br />• Eventually, a large amount of precipitation is produced and the updraft is overcome by the <br />downdraft beginning the dissipating stage. At the ground, the gust front moves out a long <br />distance from the storm and cuts off the warm moist air that was feeding the thunderstorm. <br />Rainfall decreases in intensity, but lightning remains a danger. <br /> <br />Figure 13-1. The Thunderstorm Life Cycle <br />There are four types of thunderstorms: <br />• Single-Cell Thunderstorms—Single-cell thunderstorms usually last 20 to 30 minutes. A true <br />single-cell storm is rare, because the gust front of one cell often triggers the growth of another. <br />Most single-cell storms are not usually severe, but a single-cell storm can produce a brief severe <br />weather event. When this happens, it is called a pulse severe storm. <br />• Multi-Cell Cluster Storm—A multi-cell cluster is the most common type of thunderstorm. <br />The multi-cell cluster consists of a group of cells, moving as one unit, with each cell in a <br />different phase of the thunderstorm life cycle. Mature cells are usually found at the center of <br />the cluster and dissipating cells at the downwind edge. Multi-cell cluster storms can produce <br />moderate-size hail, flash floods and weak tornadoes. Each cell in a multi-cell cluster lasts only <br />about 20 minutes; the multi-cell cluster itself may persist for several hours. This type of storm <br />is usually more intense than a single cell storm.