LOCAL CLIMATOLOGY. 79 



the polar and the return currents, are for some reason disturbed ; 

 the polar current stiffening and driving })ack the return current 

 until the former — the polar current — prevails, and becomes so 

 strong as to blow, finally, at the moment of the greatest cold, in 

 its normal course, and as it would always blow if there Averc no 

 mountain ranges or other disturbing causes to divert it from its 

 proper path. 



So with " heated terms^^'' as they have been called. So far as I 

 have been able to observe and collect statistics, they occur as fol- 

 lows : First. We have a balancing of winds, or air currents, so 

 that over a large tract, a hundred miles or more in diameter, 

 there are no winds, except slight local currents not passing out 

 of this region, and none from without it passing in. Second. We 

 have a clear sky, with very little vapor or other substance in the 

 air "to absorb," or rather, as I would say, " to reflect back^^ the 

 sun's rays. These conditions, continuing for a day'or two, will pro- 

 duce great heat. If the air continues clear and dry, the heat 

 becomes great, and the direct rays of the sun seem to be intensely 

 scorching. But for the most part, as a third condition, there will 

 arise, by evaporation, within this enclosure of the winds, moisture 

 enough to overspread it like a dome (not perhaps with visible 

 clouds) ; and thus, while it does not obstruct the sun's rays from 

 passing through to the earth, it absorbs and reflects back all of 

 those that are radiated from the earth, and constitutes an oven- 

 like enclosure, with the walls of wind for its sides and this mass 

 of vapor for a dome ; and the weather is not only hot, but sultry. 



I have said there is in such cases much moisture in the air. 

 This usually shows itself, in time, in clouds and a low dew point. 

 But it is a mistake to suppose that when there are no clouds there 

 is but little moisture, or rather water, in the air. Clouds are 

 merely vapor made visible or manifest by contrast of temperature. 

 When the ascending vapor, w^hich is in fact always ascending in 

 some quantity, even in the coldest of weather, reaches a current 

 or stratum of air enough colder than that in which it first became 

 vapor, it is converted into a cloud. This may occur simply by 

 the vapor's ascending to an elevation api)roaching what is called 

 the snow line ; but clouds are formed for the most part at the 

 place where the two currents meet, blowing of course in opposite 

 directions, the upper one being generally the coldest. 



Nor can we doubt for a moment that the meteorological records 

 of any place, if accurately kept and published in detail, would 



