WEATHER AND CLIMATE 139 



to northwesterly winds, and there is a succession of cloudi- 

 ness and possible precipitation followed by clearing skies, 

 the winds from the north being cool. The rate at which 

 the changes occur, and the intensity of the storm, is fore- 

 casted by the rapidity and extent of fall of the mercury in 

 the barometer. 



During the summer the paths of the lows across the United 

 States are likely to be farther to the north, and in winter 

 farther to the south. For an observer in any one place there 

 is likelihood of a repetition in summer of warm winds from a 

 southerly direction, and in winter of cold winds from a 

 northerly direction. 



Some or many of these features of weather changes may 

 be absent in the passing eastward of any cyclonic area, 

 and the intensity of these features may vary widely. But so 

 largely are they characteristic of cyclones that without re- 

 course to weather maps and barometers close observers of 

 changes in the appearance of the sky, in the directions of the 

 wind, and in thermometer readings, are able to foretell 

 weather changes somewhat accurately. The direction of 

 the centre of any area of low pressure from an observer will 

 in general be somewhat to the left of the direction toward 

 which the wind is blowing. 



SUMMARY 



The advancing edge of a low commonly brings to each region over 

 which it passes more or less of cloudiness and of precipitation. Where 

 this occurs it may be explained as due to an expansion of the moisture- 

 laden air passing into the low. By reason of this expansion the tem- 

 perature of the air is lowered sufficiently to condense more or less com- 

 pletely the moisture it contains. 



When the low has passed on eastward across the United States the 

 westerly winds blowing toward it are relatively cool and dry, coming 

 as they do over the central continental areas northward into Canada. 

 As these winds become warmed from the regions over which they pass 



