They develop within low-pressure areas of high winds; the speed of the funnel winds themselves is often placed at more than 480 km/h (more than 300 mph), although speeds of more than 800 km/h (500 mph) have been estimated for extremely strong storms. Damage to property hit by a tornado results both from these winds and from the extremely reduced pressure in the center of the funnel, which causes structures to explode when they are not sufficiently ventilated to adjust rapidly to the pressure difference. The pressure reduction is in keeping with Bernoulli's principle, which states that pressure is reduced as velocity increases.
Tornadoes are most common and strongest in temperate latitudes, and in the U.S. they tend to form most frequently in the early spring; the "tornado season" shifts toward later months with increasing latitude. The number of funnels observed each year could vary greatly in any given region.
Cyclone, in strict meteorological terminology, an area of low atmospheric pressure surrounded by a wind system blowing, in the northern hemisphere, in a counterclockwise direction. A corresponding high-pressure area with clockwise winds is known as an anticyclone. In the southern hemisphere these wind directions are reversed. Cyclones are commonly called lows and anticyclones highs. The term cyclone has often been more loosely applied to a storm and disturbance attending such pressure systems, particularly the violent tropical hurricane and the typhoon, which center on areas of unusually low pressure.
Hurricane, name applied to migratory tropical cyclones that originate over oceans in certain regions near the equator, and particularly to those arising in the West Indian region, including the Caribbean Sea and the Gulf of Mexico. Hurricane-type cyclones in the western Pacific are known as typhoons.
Most hurricanes originate within the doldrums, a narrow equatorial belt characterized by intermittent calms, light variable breezes, and frequent squalls, and lying between the northeast and southeast trade winds. As the doldrums of the Atlantic are situated largely to the north of the equator, hurricanes do not occur in the South Atlantic Ocean. The Pacific doldrums extend north and south of the equator; thus hurricanes occur in the South and North Pacific oceans.
Hurricanes consist of high-velocity winds blowing circularly around a low-pressure center, known as the eye of the storm. The low-pressure center develops when the warm, saturated air prevalent in the doldrums is forced upward by denser, cooler air. From the edge of the storm toward its center, the atmospheric pressure drops sharply and the wind velocity rises. The winds attain maximum force close to the point of lowest pressure (about 724 torr, or about 28.5 in. of mercury). The diameter of the area affected by winds of destructive force may exceed 240 km (150 mi). Gale winds prevail over a larger area, averaging 480 km (300 mi) in diameter. The strength of a hurricane is rated from 1 to 5. The mildest, Category 1, has winds of at least 120 km/h (74 mph). The strongest (and rarest), Category 5, has winds that exceed 250 km/h (155 mph). Within the eye of the storm, which averages 24 km (15 mi) in diameter, the winds stop and the clouds lift, but the seas remain very violent.
Hurricanes generally move in a path resembling the curve of a parabola. In the northern hemisphere the storms usually travel first in a northwesterly direction and in the higher latitudes turn toward the northeast. In the southern hemisphere the usual path of the hurricane is initially to the southwest and subsequently to the southeast. Hurricanes travel at varying rates. In the lower latitudes the rate ranges from 8 to 32 km/h (5 to 20 mph) and in the higher latitudes it may increase to as much as 80 km/h (50 mph). Those areas in which the hurricane winds blow in the same direction as the general movement of the storm are subjected to the maximum destructive violence of the hurricane.
