THE WEATHER 37 



35. Passage of a cyclone. On the average, several cyclones 

 pass eastward over the United States each week. Cyclones 

 and anticyclones commonly follow so closely upon one another 

 that we are practically always in one or the other. Most of 

 the winds which we experience are cyclonic winds, and most 

 of the rainfall in the Mississippi Valley is produced by these 

 cyclonic winds. The events that occur in connection with the 

 passage of a cyclone have been described as follows in a 

 pamphlet issued by the United States Weather Bureau : 



When the wind sets in from points between south and southeast, and 

 the barometer falls steadily, a storm is approaching from the west or 

 northwest, and its center will pass near or to the north of the observer 

 within twelve to twenty-four hours, with winds shifting to northwest 

 by way of southwest and west. When the wind sets in from points be- 

 tween east and northeast, and the barometer falls steadily, a storm is 

 approaching from the south or southwest, and its center will pass near or 

 to the south or east of the observer within twelve to twenty-four hours, 

 with winds shifting to northwest by way of north. The rapidity of the 

 storm's approach, and its intensity, will be indicated by the rate and 

 amount of the fall in the barometer. 



36. Weather in anticyclones. The air 'which flows into a 

 cyclone usually rises when it reaches the center of the storm, 

 and thus the circulation is kept up. An anticyclone is an 

 area in which the cooler air of the upper regions is settling 

 to the surface and flows away in all directions as described. 

 Since this air at the surface of the earth has come from the 

 cooler regions above, it is getting warmer, and as it flows away 

 from the center of the " high " most of it is still going to 

 warmer regions. Since it is becoming warmer, it is relatively 

 dry. For these reasons the weather in a region covered by a 

 high-pressure area is usually cool, clear, and dry, with a west 

 or northwest wind. Most of us have no difficulty in recalling 

 the bracing air of the pleasant days which follow a period of 

 clouds and rain, and this is typical anticyclone weather. 



37. Hot and cold waves. A large low-pressure area some* 

 times draws so much air from the south into the northern part 



