Tornadoes

Tornadoes are violent whirling storms which may occur with severe thunderstorms. They take the form of a funnel or tube building downward from a cumulonimbus cloud. These violently rotating columns of air range in size from a hundred feet to a half mile in diameter. Technically, they are not tornadoes unless they touch the ground, but are referred to as "funnel clouds." When they do reach the ground, they are the most destructive of all atmospheric phenomena on the local scale. They travel with a speed of 25 to 50 m.p.h., usually from southwest to northeast, and often skip along. The length of the path of a single tornado is usually just a few miles, but some tornadoes have remained active for more than a hundred miles-striking the ground for a few miles, skipping an area, then striking the ground again, and so on.

The great destructiveness of tornadoes is caused by the very strong wind and extremely low pressure. Winds in the rapidly spinning vortex have never been measured, but from the destruction it is estimated that winds may exceed 500 m.p.h. The low pressure causes houses and structures to virtually explode when a tornado passes over them. There is a sudden decrease in pressure around the house, while on the inside the pressure changes little. The resulting difference in pressure between the outside and the inside is sufficient to blow the house apart.

Tornadoes have been reported in all of the 48 contiguous States and Southern Canada, but they are rare west of the Rocky Mountains. Maximum occurrence is in the central Midwest, and there is a secondary maximum in the Southeast. In Southern United States tornadoes may occur in any month of the year, but farther north the maximum occurrence is in late spring and early summer. They generally occur with prefrontal squall lines, but they may develop with other violent thunderstorms, including those in hurricanes. Tornadoes usually occur in the late afternoon or evening. Their main effect on the wildland fire problem is the resulting blowndown timber in forested areas that often creates high fire hazard.