THE AIR 157 



105. Rain, Snow, Dew, and Frost. The heat of the 

 sun causes continuous evaporation of the water of the 

 oceans, lakes, and rivers, and some moisture evaporates 

 from the land and from plants. This water vapor is 

 carried by the air to various parts of the earth. Warm or 

 hot air can carry more water vapor than cool or cold air 

 can. The amount of water vapor in the air varies from 

 day to day. Sometimes the air is very dry and at other 

 times it has more moisture than it can carry. 



To prove that there is moisture in the air when there 

 seems to be none, let us recall some of our experiences. 

 In summer the air is warm and usually has a great amount 

 of moisture in vapor form which is invisible. If this air 

 is cooled, it cannot hold its moisture in the invisible form. 

 The vapor will collect in drops on cool objects like the 

 outside of a vessel containing ice water. Cool water 

 just taken from a well in summer and poured into a metal 

 bucket will cause drops of water to collect on the outside 

 of the bucket. These drops are formed from the con- 

 densed vapor of the air. If you want to see these drops 

 of water form, pour a quantity of ice water into a metal 

 vessel. When the water vapor of the air condenses on 

 the cool grass or other objects at night, we call it dew. 

 If it is sufficiently cold to freeze the dew, we have 

 white frost. 



When air has all the water vapor that it can hold 

 without condensing it, it is said to be saturated. If warm 

 air is cooled enough, the saturation point will be reached, 

 and if the cooling is then continued the vapor will collect 

 in small drops. If these drops collect on objects they 

 form dew, but if they continue to float about in the air 

 they form mist, fog, or clouds. When air is saturated, 

 it is at the dew point. Dew point is that condition of 



