THE ATMOSPHERE 131 



The spiral movement of air upward around the centre of a "low' 

 may be accounted for as the result of differences in the rate of eastward 

 motion of portions of the earth's surface in different latitudes. This 

 velocity varies from seventeen miles a minute at the equator to a zero 

 value at the poles. 



Wind directions in the United States are largely controlled by the 

 changing positions of the atmospheric conditions known as highs and 

 lows in their passage across the country. In a general way the winds 

 at any point are from a high toward a low, and the positions of these 

 highs and lows are ever changing. 



The cyclone of the meteorologist is a condition of the atmosphere, 

 not a destructive storm. The weather changes accompanying it may 

 at times be very marked, and again scarcely noted. Its average rate 

 of progress eastward is about seven hundred miles in twenty-four hours. 



The heat equator is an imaginary line that for sake of simplicity 

 may be considered as connecting all those places where the sun's rays 

 are vertical as the earth makes each day's rotation. Such a line con- 

 tinued day after day would be a spiral, extending as far northward as 

 the Tropic of Cancer on June 21, and as far southward as the Tropic 

 of Capricorn on or about December 21. This involves a shifting of 

 over three thousand miles north and south in six month's time. It 

 practically coincides with the earth's equator on or about the dates 

 September 21 and March 21. Owing to differences in altitude of 

 places along the course of the heat equator, and because of local con- 

 ditions, any line connecting places of highest temperature would be 

 very irregular. 



As the northward moving upper atmosphere gradually settles over 

 the North American continent and comes to the earth's surface in the 

 Rocky Mountain plateau, it may be considered as still retaining more 

 or less of its eastward motion acquired in the equatorial regions. As 

 dry cold air, and therefore dense air, its eastward passage across coun- 

 try will be accompanied by relatively high barometer readings, clear 

 skies, and cooler temperatures. Its advancing eastern edge may so 

 chill the atmosphere of any section to which it comes as to cause 

 cloudiness, with possible precipitation in the form of snow. 



Exercises 



1. What determines the height at which a column of mercury in a barometer 

 tube is sustained? Why would a water column be sustained at a greater 

 height? 



