Most of the death and destruction from natural disasters comes via severe weather.
Solar radiation is received in abundance between 32 N and 34S latitudes. Much of this heat is transferred in water vapor by winds moving aloft toward the poles. Cold polar air is dense and flows equatorward. The midlatitudes are the transfer zone between the equatorial and polar air masses; they have the most severe weather.
Where tropical air in the heat-expanded troposphere meets the cold, compressed polar air, a west-to-east high-level air flow exists-the fast- moving, polar-front jet stream. The position of the polar jet migrates across the united States with the seasons. In the summer, it is over Canada: in the winter, it is near the Gulf of Mexico. The polar jet stream plays a large role in moving heat and air masses and is involved in many severe weather situations.
The paths of large, moving air and water masses are deflected by the force of the Earth’s rotation. In the Northern Hemisphere, moving masses veer to their right, in the direction of movement; this is the Coriolis effect. In the Southern Hemisphere, moving objects veer to their left.
Water has a remarkable ability to absorb and release heat. Energy is absorbed in water vapor during evaporation (latent heat of vaporization) and released during precipitation. Energy is absorbed in liquid water during melting latent heat of vaporization) and released during precipitation. Energy is absorbed in liquid water during melting (latent heat of fusion) and released during precipitation as snow.
Air masses vary in their temperature and water-vapor content. Different air masses do not readily mix; they are separated along boundaries called fronts. Much severe weather occurs along fronts.
Rotating air bodies create some of the most severe weather via thunderstorms, tornadoes, and hurricanes. In the Northern Hemisphere, rotation is counterclockwise as cyclonic circuation. Cyclones have a low-pressure core, so surface winds flow inward toward toward the core, feeding a large updraft of rising air that cools to form clouds and sometimes rain. Many of the largest cyclonic circulations are linked to troughs (large bends concave toward the North Pole) in the polar jet stream. Intense cyclones a few miles across can be thunderclouds commonly producing heavy rain, lightning, thunder, and hail, and sometimes spinning off even smaller-radius cyclones-tornadoes. The smaller the redius of a rotating air mass, the faster its wind speeds. Tornado winds can exceed 300 mph.
Heat waves are silent killers that prey on the elderly. Droughts are years of rainfall shortage. In the central United States, droughts have resulted from high-pressure ridges (large bends convex toward the North Pole) in the polar-front jet stream that foster anticyclonic circulation. An anticyclone rotates clockwise with dry air descending down its core, warming further, and evaporating moisture from the lands below.
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