The Weather 25 



the difference in pressure and the wider the spaces between 

 the isobars, the less is the strength of the wind. The difference 

 in pressure for a definite distance, as for a degree of latitude, 

 or a given number of miles, is sometimes called the gradient. 

 The gradient is at once seen to vary from steep to slight, and 

 the wind accordingly from a hurricane or a gale to a very gentle 

 breeze. A gradient of one inch of mercury for five hundred 

 miles indicates a wind with a velocity of approximately fifty 

 miles an hour. A gradient of seven tenths of an inch indicates 

 a wind velocity of thirty to thirty-five miles an hour ; of five 

 tenths of an inch, about twenty miles an hour, and four-tenths, 

 about nine or ten miles an hour. Test these figures by com- 

 parison with weather maps and reported wind velocities. 



VI. CYCLONES AND ANTICYCLONES 



16. Paths of Storms. What paths do cyclones or storm 

 centers generally follow? The path of the cyclone just studied 

 for wind force is shown on the map of December 26 (Fig. 3). 

 It was reported as appearing on the morning of the twenty- 

 third over southeastern Idaho. Estimate the distance it 

 traveled in the three days. About how many miles did it 

 move each day? An hour? Did it maintain the same rate 

 of progress on its entire course? Compare with this path the 

 path of the cyclone shown over Maine on December 24 (Fig. 1). 

 A third typical path is shown (Fig. 10) for December 29. 

 This cyclone deepened or gained in intensity during twenty- 

 four hours while over Louisiana ; on the day before it had little 

 force. It entered by way of southern California. 



The general direction of cyclonic storms is from west to east, 

 but mountains and watercourses and the earth's rotation divert 

 as well as distort them. Cyclones seldom pass over the Rocky 

 Mountains. If they do, they lose in intensity and are greatly 

 distorted, as is also the case with those passing over the Appa- 

 lachian ranges. Cyclones developing in the northern part of 

 the Rocky Mountain system or entering from the North Pacific 



