IMPORTANCE OF WINDS AND RAINS 197 



are dark and of irregular shape. The clouds of thunder- 

 storms seem to combine the qualities of both these kinds; 

 they are called cumulo-nimbus clouds (see Fig. 72). 

 Cirrus clouds are feathery (see Fig. 73). They seem the 

 smallest clouds, and are highest in the air; five miles or 

 more. Usually they occur in large groups. When they 

 are abundant, the sky is said to be a "mackerel sky." 

 It is believed that this kind of cloud is composed of fine 

 particles of snow and ice. Stratus clouds are the low-lying, 

 horizontal clouds that seem to lie in layers (see Fig. 74). 

 Of course there are many gradations between these various 

 kinds of clouds, which really merge one into the other. 



Tornadoes. These destructive storms, usually called 

 cyclones, are due to very strong convection currents with 

 very small diameters. They are like the little dust whirl- 

 winds, multiplied millions of tunes in power. Like the 

 little whirlwinds, a tornado cloud has a funnel shape which 

 spreads out more or less widely at the top. In the center 

 of this funnel, due to the great uprush of air, the atmos- 

 pheric pressure may be much less than normal atmospheric 

 pressure. So we see why houses in the path of a tornado 

 sometimes "blow up" from the inside. It is because the 

 pressure of the outer air is temporarily removed, and the 

 inner air in consequence expands with such violence as to 

 rend walls asunder. Here is an excellent proof of the real- 

 ity of this atmospheric pressure which we do not actually 

 feel. 



Tornadoes are rarely more than a thousand feet in diam- 

 eter at the bottom, usually much less. So we can see why 

 the path of destruction which they leave in their wake is 

 narrow. Within this path, however, judged by the damage 



