Page:Handbook of Meteorology.djvu/174

 warm and moist. Now, if the heavier cold air rests at the surface of the earth no disturbance follows. On the other hand, if the cold air comes to rest on the top of a thick layer of warm air, an unstable condition results. Sooner or later an updraught of warm air takes place and tornado conditions are established. The rapid whirl creates a near- vacuum, and this accounts for the fact that buildings struck bv the funnel-cloud burst outwardly.

The air movement of tornadoes is three-fold—the updraught, the whirl, and the progressive movement. The destructive path of the tornado is as wide as the funnel-cloud, rarely more than a few rods. The entire whirl is not much more than half a mile in diameter; the extent of the path varies from a few miles to about 200 miles. The tornado progresses along its track at a rate varying from 10 or 12 miles an hour to express-train speed. The funnel-cloud is formed at a height of about half a mile.

From the nature of the case, the best values concerning the dynamic force of the tornado are only approximate, but even these are instructive. Normal air pressure is at the rate of 2117 pounds per square foot. Now, if the air pressure within the funnel is only three-fourths normal when the funnel involves a building, the air pressure inside the building will be 530 pounds per square foot greater than on the outside. Such a difference in pressure is sufficient to burst the walls of almost any building.

Tornadoes are most prevalent in May, June and July; the average of these months exceeds that of the rest of the year. They are more common in the United States than in Europe. The regions of greatest frequency are the lower Ohio and Missouri valleys and the Central Mississippi region. Very few occur in the arid region west of the one-hundredth meridian and fewer still are reported north of the fiftieth parallel.

Hail and electrical discharges frequently accompany tornadoes, but they have nothing to do with the cause of them; and although the updraught occurs in thunder-storms—and probably a cyclonic movement of the air within it—one is