574 Principles of Weather Forecasting. 



across the earth's surface and the general direction is that 

 of the prevailing winds of the part of the earth in which 

 they are. That is, in the temperate zones they tend to 

 move away from the equator and toward the east, while in 

 the tropical zones they tend to move toward the equator 

 and toward the west. 



728. Direction of Storms in the United States. In the 

 great majority of cases the storms of the United States 

 travel from some westerly toward some easterly point and 

 the mean direction is a little north of east. Very many 

 of these storms travel for a time from the northwest toward 

 the southeast until they near the longitude of the Missis- 

 sippi river, when they very often turn their course strongly 

 to the northeast, and Fig. 267 represents the course of the 

 storm centers as they traversed the country during March, 

 1900, there heing 13 of them in all. Wherever the storms 

 of the United States originate or enter the territory they 

 nearly all leave it by crossing the New England states. 



729. Rate of Travel of Storms in the United States 

 There is a very wide range in the rate at which the storm 

 centers progress across the United States, but the average 

 is from 26 to 30 miles per hour. The "circles in the paths 

 of the several storm tracks in Fig. 267 mark the positions 

 of the storm centers at intervals of 12 hours. 



730. Diameters of Storms. The diameter of these cy- 

 clonic wind systems in the United States is generally from 

 1,500 to 2,000 miles, the longest diameter being usually 

 from the southwest to the northeast. A typical one of 

 these storms is represented in Fig. 268, where the heavy 

 lines are drawn through places having the same weight of 

 air above them, while the dotted lines are lines of equal 

 temperature. It will be seen that this wind system reaches 

 from north of the Great Lakes to well into Texas and from 

 North Dakota to Tennessee. 



