Thunderstorms

            Thunderstorms are local storms covering small areas. They are accompanied by strong gusts of winds, heavy rain, lightning, and thunder. Hail may also occur during a thunderstorm.
            Thunderstorms are the result of a strong upward movement of warm, moist air. Cumulus-type clouds 10,000 to 20,000 feet high develop. Rising air currents may exceed a speed of 200 miles per hour. Falling air currents may reach the same speed.
            Thunderstorms develop under different conditions. Some of them occur within an air mass on summer afternoons when warm, moist air is strongly heated at the earth's surface. Powerful upward air currents are created. These thunderstorms are likely to be local or scattered in occurrence. Thunderstorms called frontal thunderstorms may occur at any time of the year in connection with well-developed cold fronts. Lines of thunderstorms called squall lines are formed. These may be several hundred miles long and 40 to 50 miles wide.
            Thunderstorms produce lightning. Lightning is a discharge of static electricity. The violent winds in a thunderstorm tear apart large raindrops. the broken drops of water become electrically charged and are carried to the top of the cloud. The top of the cloud is negatively charged; the base of the cloud is positively charged. When the charges are great enough to overcome the electrical resistance of air, a giant spark is released. When the electricity discharges, either within the cloud itself, to another cloud, or to the earth, a lightning flash is produced. The sudden expansion of the air, heated by the lightning flash, causes air waves which are heard as thunder. So-called "heat lightning" is a flash of lightning from a storm too distant to be heard.

Tornadoes and Waterspouts

            Tornadoes are violent local windstorms that occur over land. They are the smallest and most violent of all storms. Also called twisters and cyclones, tornadoes are narrow, funnel-shaped, spiral whirls in which wind velocities may reach 300 mph. The air pressure at the center of the tornado is very low. A tornado forms a dense, black, funnel-shaped cloud accompanied by heavy rain, hail, lightning, and thunder. Tornado clouds hang low, frequently touching the ground. They cause great destruction through the action of both their violent winds and their suction-producing low-pressure centers.
            Tornadoes vary in diameter from a few hundred feet to a mile. They usually travel from the southwest toward the northeast in erratic paths. They travel at speeds of about 25 to 40 miles per hour, passing any particular point with a deafening roar in but a few seconds. Tornadoes that occur over bodies of water are called waterspouts.

Hurricanes

            Hurricanes are also intense low-pressure wind storms. They are very large whirling storms, often several hundred miles in diameter. They form in tropical or subtropical regions. Hurricanes originate over water but frequently move in over land areas. They then cause great destruction. Like a tornado, the hurricane has two motions – one a whirling motion, the other a forward motion. The whirling motion may cause winds that reach velocities well over a hundred miles an hour.

Other Storms

            There are many kinds of storms. Storms that are made up of wind alone are often called windstorms. And storms that are made up of dust and wind are called duststorms. Most storms have water in them. One such storm is a rainstorm.