A storm is any disturbance in the atmosphere that has noticeable effects on the earth's surface. The term suggests disagreeable weather conditions that may bring discomfort, inconvenience, economic disaster and loss of human lives. In spite of that fact, storms have a generally positive effect on the environment and on human societies because they are the source of most of the rain and snow on which the planet depends.
Among the many kinds of storms that exist are thunderstorms, hurricanes, tornadoes, blizzards, and ice and hail storms. As different as these types of storms may be, they possess a few common characteristics. They all result from significant atmospheric instabilities, and they all involve dramatic convection currents in which air masses travel upwards at high rates of speed.
The formation of a thunderstorm is typical of the way many storms develop. Imagine that a large mass of air encounters conditions that force it to move upwards. An approaching front or some kind of geographical barrier might produce such an effect. The air mass will continue to rise as long as it is warmer than the atmosphere around it. The upward movement of such an air mass constitutes a convection current.
At some point, moisture within the air mass may begin to condense, releasing heat as it does so. When this happens the convection current may begin to move even more rapidly. The upward movement of air in a thunder cloud has been measured at more than 50 MPH (80 km/h). As the upward movement of air continues, more moisture condenses out of the air mass and a large cloud begins to form. Depending on atmospheric conditions, a thundercloud of this type may rise to a height of anywhere from 6-9 mi (10-15 km). Eventually, ice crystals within the thundercloud will begin to condense as rain, snow, or some other form of precipitation and a thunderstorm will occur.
Some of the most severe types of storms (tornadoes and hurricanes, for example) occur when the upward convention current receives a rotational push. This push converts a purely vertical air movement into a spiraling motion characteristic of these great tropical and mid-latitude storms.