WEATHER 175 



vapor at 12 C. is 10.5 millimeters. Hence the air con- 

 tains 10.5/23.5 or .447 as much water vapor as it is capable 

 of holding. The relative humidity is 44.7 per cent. 



Humidity determinations have a practical value in that 

 they enable us better to predict weather conditions and to 

 have proper conditions in public buildings, schools, and 

 homes. For the most healthful conditions the relative 

 humidity should be from 50 per cent to 60 per cent. 



Dew and Frost. Dew is a deposit of moisture from 

 the air upon the ground, grass, stones, and other objects 

 near the ground. After sunset these objects radiate their 

 heat very rapidly and soon become quite cold. The air 

 which comes in contact with them is cooled below the dew 

 point and some of its moisture is deposited. If their 

 temperature is below the freezing point of water, frost is 

 formed instead of dew. Dew and frost are formed most 

 rapidly on the surfaces of substances which are the best 

 radiators of heat. 



Clouds are not favorable to the formation of dew, since 

 they act as a blanket over the earth and prevent the 

 radiation of its heat. Some of the rural weather prophets 

 consider, the absence of dew as a " sign " of rain. It is an 

 indication that there were clouds during the night or a 

 relatively high humidity of the atmosphere. Strong 

 winds also are unfavorable to the formation of dew, since 

 the air does not remain long enough in contact with the cool 

 objects to be cooled to its dew point and deposit moisture. 



Clouds. Clouds are formed by the condensation of 

 the moisture in the air. If a cloud meets the surface of 

 the earth, it is called a fog, and other names are applied 

 to it according to the manner of its formation, its shape, 

 its color and appearance, and its elevation. The more 

 common forms are the cumulus, the cirrus, the stratus, 



