See the lightning flash? Hear the thunder boom? A dangerous storm is on its way! Thunderstorms feature thunder and lightning, but also heavy rain, strong winds, and sometimes hail. When the cold upper air sinks and the warm moist air rises, storm clouds or 'thunderheads' develop, resulting in thunderstorms.
Thunderstorms are produced by cumulonimbus clouds. Because it is very turbulent inside a cumulonimbus cloud, the water droplets and ice crystals in it break up and become electrically charged. The upper portion of the cloud is positively charged, while the lower portion is negatively charged. When the electric voltage between the positive and negative charges is large enough, discharges take place between clouds or between cloud and the earth's surface, and lightning occurs. The electric current in a lightning discharge generates a large amount of heat. This causes a very rapid expansion of air, resulting in rumbles of thunder.
Thunderstorms can be extremely dangerous. Rainfall from thunderstorms can cause flash flooding, which kills more people each year than hurricanes, tornadoes or lightning. Lightning is responsible for many fires and fatalities. Hail up to the size of softballs damages cars and windows, and kills livestock caught out in the open. Strong winds knock down trees, power lines and mobile homes. Tornadoes (with winds up to about 300 mph) can destroy all but the best-built man-made structures.
Lightning and thunder occur at the same time. You see the lightning before you hear the thunder because light travels faster than sound. Count the number of seconds after you see lightning until you hear thunder. Divide the number of seconds by five. The storm is that many miles away.