The Weather 27 



through British Columbia move southeastward as far as Mon- 

 tana, where some of great intensity swing east and pass through 

 the Great Lakes and St. Lawrence region. This is one typical 

 path of- storms. Others continue south to Colorado or farther, 

 then swinging northeastward, pass up the Ohio valley over New 

 England, and out by way of the St. Lawrence. Another path 

 has been indicated for the map of December 29. These are 

 the usual paths of cyclones. The path of the West India 

 hurricanes will be considered later. 



17. Progress of cyclones. A mean rate of progress of 

 cyclones is estimated as approximately thirty-five miles an hour 

 for the winter months and twenty-five miles an hour for the 

 summer months. The rate for individual storms varies greatly. 

 A cyclone may maintain a rate of fifteen miles an hour through- 

 out its course and, while not severe, may be very widespread. 

 Another may reach the tremendous speed of seventy miles an 

 hour and be intensely severe but more limited in extent. This 

 fact concerning the rate of progress is significant in connection 

 with the so-called spiral movement (pages 19-21) of the winds 

 about the central low. The center moves more rapidly than the 

 winds, which cannot therefore complete even approximately 

 the spiral course. On the ocean, with no obstacles to circula- 

 tion, and in the tropics, where cyclones move sometimes more 

 slowly, winds attain a very nearly complete spiral course and 

 acquire greater rotary energy. On this point compare the 

 typhoons of the China Sea, the cyclones in the region of the 

 Bay of Bengal, and the West India hurricanes. 



18. Diameter and form of cyclones. The extent of a 

 cyclonic area may be estimated roughly from the weather 

 map. What are the approximate diameters of the cyclones 

 shown on the maps in the text? The form tends generally to 

 be elliptical or oval, with the narrow side usually pointed in 

 the direction in which the cyclone is moving. In the larger 

 number of cases the direction is northeast. The progress of 

 the storm is slower when the broad side faces due north than 



