20 THE CLIMATOLOGY OF THE UNITED STATES. 



This latent heat plays a most important part in the pro- 

 duction of storms. 



In the above cases we have assumed that the air was dry. 

 We will now suppose it to contain moisture to a considerable 

 amount. Where the layers of air next to the surface of the 

 earth become heated, the air rises and in so doing it ex- 

 pands and cools. The moisture condenses and we haveclouds 

 formed ; but in condensing heat is given out which pre- 

 vents the air from cooling as much as it would otherwise, 

 therefore it continues to rise till it reaches a high alti- 

 tude and overflows, passing off from the central spiral of 

 ascending air ; greater quantities follow and an activity 

 is started by the new supply of heat which maintains the 

 action after the sun has ceased to exert a direct influence ; 

 thus we have the development of a typical storm which 

 continues day and night. Now, if we cut through this 

 storm and take a horizontal section or ground plan, we 

 shall see that in the centre is a calm of ascending air ; 

 about this a rain area and beyond this an area of clouds 

 and we shall see that the winds rush in towards this centre. 

 Therefore, on the north side of the storm we shall notice 

 northerly winds, on the east side, easterly winds and so on, 

 the storm appearing like a great wheel, with the excep- 

 tion that the winds, instead of following radial lines to 

 the centre, as the spokes of a wheel do, tend to reach that 

 centre by a more or less curved line, this curve changing 

 according to the distance of the centre. At great dis- 

 tances from the centre the winds are drawn towards it in 

 nearly radial lines, while of course at the centre the mo- 

 tion is nearly circular. This motion of the winds towards 

 the centre is always in the opposite direction from the 

 hands of a watch (that is, from right to left) in the northern 

 hemisphere and from left to right in the southern hemi- 

 sphere. Such are ordinary storms in all parts of the 



