Storms

(Excerpted from "World of Scientific Discovery," available online in Discovering Collection.)

The central United States is a breeding ground for storm systems, and it will serve here as a typical cyclone model. The jet stream—a rapidly moving wind current about six miles above Earth’s surface—snakes its way across North America, marking the boundary between warm, moist air from the Gulf of Mexico and cold, dry Arctic air. Low pressure cells travel eastward using the jet stream as a conveyor belt. A cold front moves eastward across the central states around a low pressure center located, for instance, over Iowa.

The front represents the western boundary of a warm air mass occupying the southern quarter of the low pressure cell. The eastern boundary is represented by a warm front. At both fronts, the warm air rides over the cold air, interfacing at a low angle to the ground. The warm front angles into the direction of movement, the pressure gradient is low, and the result may be widespread showers from low-ceiling clouds (such as altostratus).

The angle of the cold front opposes the direction of movement, creating friction and a steeper pressure gradient. The result is a narrower band of rain from taller, more active clouds (such as cumulonimbus) which are capable of producing violent storms.

Why we need storms
Without storms, people would not have much of the water that sustains life on Earth. Precipitation is a crucial link in the planet’s water cycle. Nonetheless, some storms are very destructive, and researchers continue to search for ways to retain the benefits of storms while minimizing their dangers.

For example, cloud seeding is a procedure in which a substance such as dry ice or silver iodide is dispersed into clouds. By encouraging the formation of precipitation, this technique can drain energy from storms; it is commonly used in the United States to decrease the size of hailstones that form in thunderstorms.