Because of the fact that light is transmitted almost instantaneously, while sound travels at a speed of eleven hundred feet per second, the sound will not reach the ear for some few seconds after the flash of lightning. Average space of time between a flash and a report is about twelve seconds. The longest interval is seventy-two seconds and the shortest one second. Prolonged peals of thunder are, in some cases, due to the effect of echoes. These peals are especially noticeable in mountainous countries. The echoes are also produced by the reflection of sound from the clouds. Thunder storms are distributed over certain sections of the globe, occurring most frequently in the equatorial regions and diminishing as we approach the polar regions. Within the tropics, where there are trade winds, thunder storms are rare. Thunder storms are common in warm climates because evaporation supplies electricity in great abundance, and thus precipitation of the air is brought about. [Illustration: Fig. 11 ] TORNADOES Tornadoes are caused by the air becoming abnormally heated over certain areas. Likewise, caused by a difference in pressure. Tornadoes are local whirlwinds of great energy, generally formed within thunder storms. They are most easily distinguished by a funnel-shaped cloud that hangs from the bottom of the larger thunder cloud mass above it. The funnel is formed around a violent ascending mass of whirling winds; its diameter sometimes reaching several hundred feet, being larger above than below, the winds themselves covering a greater space. [Illustration: Fig. 12 ] The whirling funnel advances generally to the east or northeast at a rate of twenty to forty miles an hour, accompanied by a deafening noise, destroying everything in its path. The path is usually less than a quarter of a mile in width. The winds in the vortex (the apparent cavity or vacuum formed in the center of the whirling winds) of the tornado attain an incredible violence, and due to this fact houses are shattered, trees uprooted, and human lives lost, besides other devastation of property and animal life. It is, therefore, the vorticular whirl that causes the destruction produced by tornadoes. Tornadoes are more frequent in the southern states than anywhere else in the country, and occur in the warmer months.