z g ELEMENTARY SCIENCE 



and this shows how much water evaporates. For all the 

 water that falls as rain must have gone up by evaporation. 

 There is no other way. 



Condensation. After the water particles get up in the 

 air, a number of things may happen to them. At first 

 they are lighter than the air and keep moving up, some- 

 what as a balloon moves up. But presently they come to 

 where the air is cooler. The cool air affects them. They 

 begin to crowd together. This is what is called condensa- 

 tion. The particles condense into small drops. You can 

 see how this happens by holding a cool piece of glass in 

 the steam that comes from the spout of a kettle. 



We cannot see the particles of water when they evapo- 

 rate. They are much too small. But after they have con- 

 densed into little drops they form clouds. When the sun 

 is shining on them the clouds are white and beautiful. 

 They drift with the wind. When a cloud forms close to 

 the surface of the earth we call it fog. 



Sometimes the air just over the earth is moist when 

 evening comes. Then, if the night is cool, this moisture 

 condenses into little drops that we see shining on the grass 

 when the sun comes up. We call it dew. If the night is 

 so cold that this moisture condenses in the form of tiny 

 crystals of ice, we call it frost. 



The tiny drops of water that form the clouds are no 

 heavier than the air, so they stay up in the air and float 

 along like tiny balloons. But sometimes they come to 

 where it is cooler. Then they condense more and more, 

 until they form big drops that are heavier than air. The 

 drops begin to fall. The clouds are breaking up in rain. 



The warmer the air the higher the light clouds go up in 



