WEATHER MAPS 289 



is that the inflowing air produces an upward spiral current, and the 

 rising air expands and is cooled (p. 257 and Fig. 242), and so gives 

 up some of its moisture. The prevailing winds which influence 

 the direction of outflow in the upper part of a cyclone (Fig. 242) 

 tend to carry the rainfall to the east of its center. 



In the anticyclone there is a descending spiral movement of 

 air. The descending air comes from an altitude which is colder 

 than that at the bottom of the atmosphere, and hence brings a 

 low temperature. Since the air is condensed and warmed as it 

 comes down, the winds from anticyclones generally bring clear 

 weather; but the cold air moving down and out from an anticy- 

 clone may mingle with the warm air about it, so as to cause some of 

 the moisture of the latter to condense, giving rise to clouds, or even 

 to precipitation. 



Movements of cyclones and anticyclones. The highs and lows 

 do not remain in the same place from day to day. This is shown 

 by Figs. 245 248, which are the weather maps of four successive 

 days. In these figures precipitation is shown by shading. 



In Fig. 245 there is (1) a low along the Atlantic coast; (2) a 

 high central over Iowa; (3) a feeble low north of Montana; and 

 (4) a high in Oregon. The map of the succeeding day (Fig. 246) 

 shows (1) that the low of the St. Lawrence Gulf has disappeared 

 (moved to the east) ; (2) that the high of the interior has moved 

 to West Virginia; (3) that the low which was north of Montana has 

 moved to Dakota; while (4) the high of the Oregon coast remains 

 about where it was. The map of the next day (Fig. 247) shows 

 (1) that the high of the Virginias has moved on, but not so far 

 as on the preceding day; (2) that the low which was over North 

 Dakota is now north of Lake Superior; (3) that the high of Oregon 

 has moved east to Idaho and Montana; and (4) that a weak low has 

 developed in Oklahoma. The map of the 27th (Fig. 248) shows 

 (1) that the high which was over the Virginias has disappeared, 

 presumably to the east; (2) that the low which was north of Lake 

 Superior is now north of Lake Ontario; (3) that the high of Mon- 

 tana has moved southeast to Kansas; (4) that the weak low (of 

 Fig. 247) in Oklahoma has disappeared; and (5) that another feeble 

 low has appeared in southern California, 



