WEATHER MAPS 305 



and unless they are very strong, they will fall, as if the building 

 had exploded. Sometimes only the weakest part, such as windows, 

 yields. 



Not only is the pressure at the center of the tornado very low, but 

 the area of low pressure is very small. While a cyclone may be 1,000 

 miles or more across, a tornado may be no more than one-eighth 

 of a mile across, or even less. The result is that the pressure 

 gradient in a tornado is very much higher than in a cyclone, and 

 the winds are violent. Their velocities, estimated by the size and 

 height of the objects moved, have been thought to reach 400 or 

 500 miles per hour. With this velocity, or even a velocity which 

 is much less, the destruction is great. Trees are overturned, build- 

 ings unroofed or even blown down, and bridges hurled from their 

 foundations. 



A tornado is often seen first as a funnel-shaped cloud (Fig. 1, 

 PL XLVI, p. 272), the point of which may be far above the ground. 

 As the funnel moves forward, its lower end may rise or fall. The cloud 

 is due to the condensation of the moisture in the sharp convection 

 current, and the funnel shape is due to the expanding and spread- 

 ing of the air as it rises. 



The tornado is, of all storms, the most destructive, but, in most 

 cases, it has a very narrow track, and does not work destruction 

 for a very great distance. After a short course most tornadoes 

 play out, or rise above the land. 



One of the most destructive, though not one of the most violent, 

 tornadoes of recent times was that at St. Louis, May 27, 1896. 

 It accompanied a thunder-storm in the southeastern part of a 

 cyclone, central some distance northwest of the city. One of the 

 extraordinary features of this storm was the fact that its base was 

 about 30 feet above the surface. Trees were twisted off at this level, 

 'and the principal destruction of houses was above the first floor. 



As in other tornadoes, the wind played many curious freaks. 

 Single stones and bricks were picked out of walls, while the walls 

 remained standing. In one case a span of horses attached to a 

 loaded wagon were blown away, though the wagon was not over- 

 turned. The destruction of property in and about St. Louis was 

 estimated at about $13,000,000. 



