CHAPTER II 

 MOISTURE COMING FROM THE ATMOSPHERE 



WATER CONDENSING NEAR THE EARTH 



Dew. Put a few pieces of ice in a shiny can half full of water. 

 If there is sufficient moisture in the air of the room, or a relative 

 humidity of about 60 per cent, fine drops of water begin to form on 

 the can, slowly increasing in size until large drops of dew appear. 

 The temperature of the can when the dew begins to form is called 

 dew point. This means that dew would collect on all the objects 

 in the room if they were cooled to the temperature produced by plac- 

 ing ice in the can. 



We have learned that clothes dry better on a warm day because 

 warm air holds more moisture than cold air. As soon as the air 

 of the room comes in contact with the cold can, it is cooled and 

 must drop some of its moisture, since the cold air can not hold as 

 much water vapor as warm air. 



Dew on the Grass. During the night the ground is cooled by 

 radiation. The air next to the ground is chilled by coming in contact 

 with the cold ground. If this air has a great deal of water vapor 

 in.it, moisture will be deposited on the grass just as the water was 

 deposited on the can. 



Dew forms on the grass more quickly than on the wooden side- 

 walk becduse the grass cools off faster than the wood. Some 

 things give up heat very quickly, while others hold it for a long 

 time. 



Frost. Frost is not often frozen dew. Usually it is frozen 

 water vapor. The temperature of the cold object on which dew 

 would form is below 32 F., which means that the frozen vapor is 

 deposited directly on plants before it can form into dew. Frost is 

 deposited in many fantastic crystal forms on our windows* 



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