570 Principles of Weather Forecasting. 



South of the equator, where it is winter, the high pres- 

 sure calm belt has moved nearer the equator so that the air 

 is blowing off the three continents and they are experienc- 

 ing their dry season. 



724. Monsoon Winds. Where the world system of winds 

 is so strongly influenced by the land areas as is the case 

 notably in the region of the Indian Ocean they have been 

 given the special name of monsoons, and these give to In- 

 dia its rainy season, when they blow from the ocean, and 

 its dry season, when they blow from the land. 



ORDINARY STORMS. 



Besides the world system of winds, which have been de- 

 scribed, and the continental winds with their intensified 

 forms called monsoons, which change with the seasons, 

 there are others of smaller magnitude and shorter duration 

 which give rise to our ordinary storms and the still more 

 local tornadoes and thunder storms which are associated 

 with them. These are technically called cyclones or cy- 

 clonic storms. 



725. Cyclones. Most of the rainfall of temperate 

 climates and much of that which falls between the tropics 

 and the equatorial calm belt, occurs during the passage 

 of these cyclonic systems of wind movement, represented 

 in Figs. 265 and 266. 



In these winds the surface air moves spirally about a 

 center, going to the east as it passes toward the poles and 

 to the west of the center when it comes toward the equator. 

 Air coming from the eastward of a cyclonic center always 

 passes to the polar side, while that coining from the west 

 always passes to the equatorial side. 



726. Cause of Wind Directions in Ordinary Storms. The 



cause of the wind directions in ordinary storms is the same 



