WEATHER 173 



take up moisture instead of precipitating it, thus pro- 

 ducing a period of drouth in the land. Thousands of 

 lives have been lost in the famines following the winds. 



Land and sea breezes occur along the coast of large 

 bodies of water. During the daytime the land is heated 

 more rapidly than the water, the specific heat of water 

 being much greater than that of the earth. The Hot air 

 over the land expands and becomes lighter, and the heavier 

 air from the ocean forces it up and takes its place. This 

 is the sea breeze which blows during the daytime and 

 reaches its maximum strength usually late in the after- 

 noon. At night the earth cools more rapidly than the sea, 

 and in a short time the sea is warmer than the land and the 

 current of air is reversed. This is the land breeze which 

 blows during the night and reaches its maximum toward 

 morning. These winds are more noticeable in the tropics, 

 since the change in temperature is greatest there. 



Humidity and Precipitation. Water vapor is always 

 present in the atmosphere ; the amount, however, varies 

 greatly. The relative humidity, or degree of saturation, 

 of the atmosphere is defined as the ratio between the 

 amount of moisture actually present in a given volume of 

 air and the amount which would be present if the air were 

 saturated. When air contains all the moisture that it can 

 hold at a given temperature, it is said to be saturated. 

 Air over the ocean is usually quite near a state of satura- 

 tion. As a usual thing we can tell something of the 

 humidity of the atmosphere without the aid of scientific 

 instruments. When the air contains a large amount of 

 water vapor and feels damp, we say the humidity is high. 

 When it contains little vapor and feels dry, the humidity 

 is low. Dry air is more invigorating than damp air, since 

 it produces active evaporation of the perspiration of the 



