Storms | Research & Encyclopedia Articles

Storms

The Earth's lower atmosphere is composed of a global system of cyclones and anticyclones. In the Northern Hemisphere, cyclonic winds move counterclockwise around centers of low pressure, while anticyclones move clockwise around centers of high pressure. In the Southern Hemisphere, the directions are reversed. The air in a high pressure system is relatively stable, that is, it does not tend to rise. Air in a low pressure system is unstable: it rises and as it does so, becomes more buoyant and continues to rise. Storms are the direct result of the rising air associated with cyclonic instability. Generically, any circular air movement around a low pressure center is a cyclone. Thunderstorms, tornadoes, hurricanes, and dust devils are all intense, localized classes of cyclones.

The central United States, a breeding ground for storm systems, will be used here for a typical cyclone model. The jet stream 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 overrides 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. If it were not for the Coriolis force, winds would blow directly into the low pressure center. Instead, they are deflected to the right in the northern hemisphere. In the model cold front, the clouds move neither toward the cell nor toward the front, but in a northeasterly angle. Each storm along either front is its own cyclonic cell. These cells can be isolated, or they can be distributed in liberal numbers throughout the front. The towering thunderstorms that build along a cold front are the best example of this. Warm moist air rises around the low pressure at the center of the storm, building highly energized, moisture-laden clouds. The storms along a cold front can produce heavy rain, hail, high winds, severe lightning and tornadoes.

Tornadoes are highly concentrated cyclones that can form anywhere along a squall line. For their size, they are considered to be the most powerful storms on Earth. Hurricanes are tropical storms that generate over warm ocean waters off West Africa and track westward toward Central America and the eastern United States. The same type of storm in the western Pacific is referred to as a typhoon. Australians call them cyclones. In all cases, the tracks of these powerful storms follow the paths of tropical ocean currents, increasing their size and strength until they reach land or higher latitudes. Tropical storms are classified as hurricanes when their winds reach 75 mph (120 kph). In addition to high, sustained winds, hurricanes deliver heavy rain and devastating ocean surges. They can also produce tornadoes along their outer margins. Monsoons are groups of storms that bring heavy rainsto India, Southeast Asia and Northern Australia for about three months, from June to September. Very hot, humid ocean winds blow landward at this time. During the remainder of the year, dry continental winds dominate. Unlike hurricanes, monsoons do not have steep pressure gradients or high winds. Other types of storms include waterspouts, which are nothing more than tornadoes over water. Dust devils are sudden updrafts of warm air that pick up dust and small objects in a swirling cyclical pattern and occur in hot, dry conditions.

Storms are both beneficial and harmful to life and property. One of the primary purposes for the science of meteorology is to analyze and predict storms. The precipitation from them is essential for agriculture and water supplies. At the same time, people want to know what dangers to anticipate so that they can take the proper safety measures. Until recently not much was known about storms since they were so difficult to approach. Even unmanned balloon soundings were most successful under stable conditions. While the invention of instruments that record the effects of storms can be attributed to various individuals throughout history, the actual analysis of storms has required a more collective effort. Even the Bergen cyclone model developed by Vilhelm Bjerknes in the early 1920s was the work of a group of meteorologists. With the establishment of weather services, such as the U.S. Weather Bureau, and the invention of high-speed communications, data collection could be coordinated to show the regional impact of storm systems. It has been through the use of radar, rockets, high altitude aircraft, and sophisticated instruments that the anatomy of a storm, especially those of a more dangerous variety, could be discerned. Enough has been learned that tornadoes and hurricanes can be predicted by anticipating the conditions that create them, or by watching them begin to develop in other areas. Without storms, the ability to obtain life-sustaining water supplies would be difficult, if not impossible. They are a crucial link in the Earth-to-atmosphere water cycle.