THE CLIMATOLOGY OF THE UNITED STATES. 19 



of a great level plain heated by a summer sun till the air 

 at some point commences to rise ; as it rises air will flow 

 in from all sides and will follow the upward current al- 

 ready created ; in ascending, it will assume a spiral mo- 

 tion. This may be illustrated by taking a basin of water 

 and allowing the water to run out through a hole in the 

 bottom ; the water will not flow in radial lines towards 

 the hole, but in a curved line. And, again, if a column 

 of smoke above a hot bonfire is noticed, it will be ob- 

 served generally to rise in a spiral form ; thus in the above 

 case in the open plain, the air will rise in the same man- 

 ner and currents will flow in from all sides, causing slight 

 breezes along the surface of the ground. It is often no- 

 ticed, on windy days, when the streets or roads are dusty, 

 that little whirls of dust arise and travel for some dis- 

 tance ; the action here is similar, but these, instead of 

 being caused in a calm by the sun's action, are caused by 

 conflicting currents of air ; these of course being origi- 

 nally caused by the heat of the sun. 



Atmospheric disturbances, similar to the above cases, 

 occurring when the air is dry, will continue only so long 

 as the sun remains above the horizon to heat the surface 

 of the earth and the air. After sunset these will cool and 

 the air will have no tendency to rise, hence such disturb- 

 ances cannot develop into storms ; but where there is 

 moisture in the air the case is different. 



It is a well-known fact that when water evaporates, heat 

 is absorbed ; hence we say evaporation causes cold. When 

 the molecules of water separate and assume the vaporous 

 condition, they need energy and so absorb all the heat 

 they can. When condensation takes place, the molecules 

 come together again and assume the cold, sluggish condi- 

 tion of a liquid, hence they do not need the energy to 

 keep them in activity and the heat is given off again. 



