WINDS AND WEATHER 



41 



What causes land and sea breezes and the various 

 winds of the world? Anyone who has been near large 

 bodies of water knows that during the day breezes 

 blow in from the water to the land and that at night 

 conditions are reversed. As has already been stated, 

 the earth warms up more rapidly than the water; 

 during the day the warmer air over the land is pushed 

 upwards by the cool, heavy air over the water as it 

 comes in, making a sea breeze. Not only does the 

 earth warm up more rapidly during the day, but it 

 also loses its heat more rapidly at night. The land is 

 soon cooler than the water, and the air over it begins 

 to fall and push the warmer air over the water up- 

 wards, making a land breeze toward the sea. A study 

 of Figure 69 will make this clearer to you. 



Since the earth is heated more in regions near the 

 equator, the air over the tropics is warmer and lighter 

 than that over the cold polar regions. This cool polar 

 air comes in under the tropic air and pushes it up- 



DAYTIME TOWARD LAND NIGHT AWAY FROM LAND 



FIG. 69 



ward. This sets up vast convection currents blowing 

 in from the north and south polar regions toward the 

 equator. These currents are called the trade winds and 

 are among the most important of the earth. 



Because of the rotation of the earth these air cur- 

 rents do not run in a north and south direction, but 

 are deflected so that north of the equator they blow 

 from northeast to southwest and south of the equator 

 from southeast to northwest. Figure 70 shows this 

 clearly. 



Sailing vessels formerly took advantage of the 

 trade winds in sailing to America by way of the 

 southwest trades and returned to Europe in the belt 

 of the prevailing westerlies. 



The monsoon winds of the Indian Ocean are caused 

 by the unequal heating of land and ocean areas. The 

 summer weather of India and the countries adjacent 

 to the Indian Ocean is very hot. The air over these 

 land areas therefore becomes heated, expands as a 

 result, and starts to rise. The cooler air from the ocean 

 begins to move in, creating a wind blowing from the 

 water. These winds are very moist, and as a result, 

 the summer monsoons bring plenty of rain. In the 

 winter months the land areas are cooler than the 

 water, and therefore winds are set up which blow 



toward the ocean. Can you predict whether these 

 winds would be moist or dry, and give a reason for 

 your prediction? 



REFERENCES FOR FURTHER STUDY 

 Texts 



Caldwell and Curtis, Science for Today, Chap. 13 



Clement, Collister, and Thurston, Our Surroundings, Chap. 9 



Hunter and Whitman, Science in Our World of Progress, pp. 



67-70 



Lake, Harley, and Welton, Exploring the World of Science, 

 Chap. 10 



FIG. 70. WINDS OF THE WORLD 



Outside arrows represent currents in the upper atmosphere. 



Picper and Beauchamp, Everyday Problems in Science, Unit 



3 



Powers, Neuner, and Bruner, This Changing World, Unit 2 

 Skilling, Tours through the World of Science, Tour 3 

 Van Buskirk and Smith, The Science of Everyday Life, Chap. 



6 



Watkins and Bedell, General Science for Today, Chap. 3 

 Webb and Beauchamp, Science by Observation and Experi- 

 ment, Unit 7 (part) 



Wood and Carpenter, Our Environment: How We Adapt 

 Ourselves to It, Unit 2 



Special references 



Brooks, Why the WeatherT 



Barber, First Course in General Science 



WHAT YOU SHOULD AIM TO ACQUIRE FROM 

 THIS STUDY 



1. The conditions which cause unequal heating of 

 the air over the earth. 



2. The relation of the unequal heating of the air to 

 winds. 



3. The relation of unequal heating of the air to low- 

 and high-pressure areas. 



