The Weather 31 



they acquire tremendous rotary velocity. The winds of hurri- 

 cane force are usually restricted to the central area or to a diam- 

 eter of two hundred fifty to three hundred miles, and they blow 

 with a velocity ranging from eighty to more than one hundred 

 miles an hour. The circular form of the tropical cyclone is 

 somewhat distorted and its force is greatly diminished in pass- 

 ing over the land surface. The entire diameter of the storm 

 may exceed a thousand miles. Exceptionally heavy rainfall 

 accompanies these storms, often as much as an inch of rain 

 falling in an hour. 



One of the greatest services of the United States Weather 

 Bureau is to forecast and issue warning to shipping and other 

 interests concerning the approach of these destructive hurri- 

 canes. Prediction has attained a high degree of probability. 



22. Anticyclones. The anticyclonic area is quite the 

 opposite of the cyclonic. The pressure of the anticyclone is 

 greatest at the center and gradually diminishes with more and 

 more widely spaced isobars toward the circumference. With 

 the anticyclone, light winds may blow almost parallel with the 

 outlying isobars for a considerable distance, but they become 

 increasingly variable or shifting as the center approaches. 

 With the very marked high-pressure areas in winter strong, 

 cold winds are occasionally associated, which, with the precipita- 

 tion due to sudden condensation of the water vapor in the air, 

 give rise to blizzards or to driving rain. The winds of an anti- 

 cyclone may reach Texas, where they produce a rapid lowering 

 of the temperature and sometimes precipitation, either rain 

 or snow. These are known throughout the Southwest as 

 " Northers." There are also the cold waves, which precede 

 the high centers and may extend over nearly all the territory 

 east of the Rocky Mountains. 



Anticyclones move more slowly than cyclones in crossing the 

 continent, their rate of movement averaging from six to eight 

 miles an hour less. Most of the anticyclones enter through 

 British Columbia, moving in a southeasterly direction until 



