50 



Introduction to the Study of Science 



Cloud. The warm, moisture-laden air blowing usually 

 from the southeast or south toward a low-pressure area is lifted 

 or forced upward as it approaches the center by the cold air 

 blowing from the northeast or north. The air as it rises ex- 

 pands and cools. When it reaches the temperature of satura- 

 tion or the dew point, the 

 relative humidity is 100 per 

 cent. As it passes below 

 this point, condensation be- 

 gins or water vapor is con- 

 verted into visible moisture 

 deposited upon dust motes 

 in the air. This is cloud. 

 Fog is formed when the 

 moist air is cooled near the 

 earth. 



Rain. Continued reduc- 

 tion of temperature below 

 the dew point causes the 

 condensation of an increas- 

 ing quantity of water vapor. The water particles combining 

 become large and heavy enough to fall through the cloud, and 

 coalescing with smaller particles on the way may eventually 

 become heavy enough to fall to the earth as rain. Sometimes 

 it happens that raindrops formed very high in the air have to 

 pass through a warm'er and less humid layer of air, where they 

 may be evaporated and thus fail to reach the earth. 



Cyclonic storms, which come from the northwestern section 

 of our country, distribute but little rain until they enter the 

 region of moisture-laden winds from the Great Lakes. The 

 lows that cross the southwestern section of the country may 

 yield comparatively little rain over the eastern part of the 

 country, unless they are deepened by the moist, warm winds 

 from the Gulf and the Atlantic Ocean. Such deepened storms 

 usually distribute heavy rainfall. Note the deepening of the 



FIG. 16. Diagram of the water cycle. 



