294 PHYSIOGRAPHY 



are first known there. Cyclones make their first appearance in 

 various places. More of them originate near the places where 

 anticyclones start than in any other place; but not a few appear 

 first in Colorado, the Great Basin, in Texas, and elsewhere. 



Still another set of lines in Fig. 250, marked 1 day, 2 days, 3 

 days, and 4 days, show the average rate of daily progress of the 

 storms which come in from the northwest on successive days. The 

 average forward movement of cyclones is about 700 miles per day. 



The passage of a cyclone or anticyclone involves a change in 

 the direction of the wind. Thus in Fig. 246, the wind at St. Paul 

 is southeasterly, though this city is in the zone of westerly winds. 

 The next day, after the storm center has moved forward to a 

 position northeast of St. Paul (Fig. 247), the wind is northwesterly. 

 In the zone of westerly winds, an east wind is often the first indica- 

 tion of an approaching cyclone; and since a cyclone often brings 

 rain, the east wind is generally taken as a sign of rain throughout 

 much of the United States. 



Cyclones do not affect the air to great heights. Even when the 

 great whirl or eddy is 2,000 miles across, as is sometimes the case, 

 its height (depth) is rarely more than 4 or 5 miles. 



Winds incidental to cyclones and anticyclones. During the 

 passage of a cyclone, air is often drawn from warmer (lower) to 

 cooler (higher) latitudes. In midsummer this often gives rise to 

 the hot wave, though hot waves are not always closely associated 

 with cyclones. Similar winds are known as the sirocco in the west- 

 ern Mediterranean region, and they go by other names elsewhere. 



Cold waves often attend the anticyclones. These winds are 

 known as northers in the southern part of the United States, and 

 sometimes as blizzards in the northern part, though this name 

 usually implies heavy snow fall and high wind, as well as low tem- 

 perature. Fig. 251 shows the weather map for January 3, 1896. 

 The isotherms bend southward about the high, so that central 

 Texas and Montreal have about the same temperature. On the 

 following day a freezing temperature has been carried down to the 

 orange groves of northern Florida. 



The origin of cyclones and anticyclones of middle latitudes is 

 not well understood. 



