TEMPERATURE OF THE AIR 249 



effect on transportation, especially by means of water. Navigation 

 ceases, for example, on the Great Lakes, because ice forms about 

 their borders in winter. 



Temperature and Movement 



When air is heated it expands and a given volume of it becomes 

 lighter. This results in movements of convection (Fig. 212). One 

 of the movements involved in convection is horizontal, and hori- 

 zontal movement of the air is wind. Unequal heating of the air is, 

 therefore, a cause of air movements, and since the air is being un- 

 equally heated all the time, unequal heating is a constant cause of 

 atmospheric movements. Some of the movements are horizontal, 

 and some vertical; some are in the lower part of the air, and some 

 in the upper (Fig. 212). 



The unequal heating of the air is the immediate cause of certain 

 familiar winds and breezes. 



1. Land and sea breezes. In a sunny summer day the land 

 becomes warmer than an adjacent lake or. sea (p. 233). The result 

 is that the air over the land is warmed and expanded more than 

 that over the sea. Movement of the air follows. By day, espe- 

 cially after some hours of heating, the air moves in from the water 

 to the land at the bottom of the atmosphere. This is the sea- 

 breeze or lake-breeze. At night the land cools more than the water, 

 and the movements of air are reversed, giving the land-breeze, which 

 blows from the land to the water at the bottom of the atmosphere. 



The sea-breeze is strongest during the summer in warm regions. 

 When the sea or land breeze has the same direction as the pre- 

 vailing wind, it occasionally, as at Valparaiso (Chile), is so strong 

 that business is stopped and people forced to seek shelter. Along 

 certain coasts fishermen put to sea in the early morning with the 

 land-breeze, and return at night with the sea-breeze. 



2. Monsoons. Some lands near the sea become so much heated 

 in summer that the sea (from-sea) winds continue during the hot 

 season, not merely through the hot part of the day, while the land 

 (from-land) wind holds sway during the winter. This is the case, 

 for example, in India. Such winds, which change their directions 

 with the seasons, are monsoon winds. The monsoon winds of the 



