246 THE WEATHER 



ocean furnishes a free, unbroken path for air currents; 

 hence winds that blow over the ocean often keep their 

 direction for weeks and months at a time. Such regular, 

 long-continued winds are the trade winds of temperate 

 latitudes, and the monsoons of the Indian and West 

 Pacific oceans (cf. 272). 



Land and sea breezes are also regular winds, due to the 

 alternation of day and night. By day, the air over the 

 land is warmed more than the air over the water. Con- 

 vection currents are thus set up on the land, which cause 

 an inflow of air from the ocean the sea breeze. At 

 night the land cools more rapidly than the ocean, and as 

 a result the ascending air current is over the water, 

 hence there is a flow of air from the land the land 

 breeze. 



276. Cyclones. If the earth were not rotating on its 

 axis, winds would blow toward the area of low pressure 

 as directly as possible, in straight lines, as the spokes of 

 a wheel come together at the hub. But the rotation of 

 the earth causes the air to flow toward the ' l low " area by a 

 curved route. As a result of the curved paths of the 

 winds, the air at the center is set in a whirl. A dust 

 whirlwind illustrates, on a small scale, this circular move- 

 ment of air; a cyclone is a great whirl that may be 1,000 

 miles in diameter and 5 miles high. 



In the northern hemisphere the direction of this whirl- 

 ing mass of air is opposite that of the hands of a clock. If, 

 in this hemisphere, you stand with your back to the wind, 

 the center of low pressure will be towards the left. In the 

 center of the cyclone there is comparative calm. 



