24 



Introduction to the Study of Science 



though on some maps it may be given for certain localities in 

 a marginal table. But the conditions upon which the velocity 

 of the wind depends are represented and may be interpreted 

 with fair accuracy. The chief factors in such interpretation 

 are the differences in pressure between a high and a low center, 



and the relative space between the 

 isobars. Consider these points on 

 the maps given. 



On the map of December 24 (Fig. 

 1), note the pressure and the spaces 

 between the isobars of the low area 

 over Texas and Arkansas. The mean 

 velocity of the wind was not quite 

 twenty miles an hour. The low on 

 the next day's map shows a deepen- 

 ing of the storm center from 29.7 to 

 29.56, and the crowding together of 

 the isobars. The mean wind velocity 

 has increased, though at only a few 

 places did it reach as much as thirty-six miles an hour, as, for 

 example, at Columbus, Ohio, and Erie, Pennsylvania. The 

 same low on the following day is over the southern part of New 

 England. The storm center has deepened, the barometer read- 

 ing is 28.9 inches, and the isobars are crowded much more 

 closely together. The wind velocity in the region of the low 

 was generally in excess of thirty -miles an hour, while in the 

 vicinity of New York City it reached sixty-eight miles an hour. 

 Observe that the strongest wind of a cyclone or storm center is 

 usually northwest of the central low and therefore immediately 

 follows the low. What have you found to be the conditions 

 determining the velocity of the wind, as shown on these maps ? 

 Test your conclusions by study of other weather maps. 



Wind velocity and the gradient. As a general rule the greater 

 the difference in pressure, and the shorter the distance between 

 the isobars, the greater is the velocity of the wind. The less 



FIG. 9. Anemometer. 



