140 INTRODUCTION TO GENERAL SCIENCE 



Water vapor, when cooled to its point of saturation, called 

 the dew point, condenses into liquid water. Due to radia- 

 tion, the surface of the ground, and especially the vegetation, 

 becomes cool after sunset, and rapidly lowers the temperature 

 of the surrounding air. This causes the invisible water vapor 

 to condense as dew. 



Whenever water vapor condenses, it gives out large quan- 

 tities of heat, as we have learned in the case of condensation 

 of steam in Section 25. If water vapor condenses at a lower 

 temperature than 100 C., it gives out more heat than it does 

 at the boiling point. Each gram of water vapor, when it con- 

 denses, gives out a number of calories which can be estimated 

 roughly by multiplying the temperature by six tenths and 

 subtracting this result from 596. For the above reason, if 

 the relative humidity is great, the temperature will remain 

 more constant during the night than it would if the air were 

 very dry. 



References : 



1. 1103:123-124. The Dew Point and the Measurement 



of Humidity. 



2. 1103:162. Dew. 



3. 1304:246. Dew. 



4. 1803 : 97-99. Condensation of Water from the Air. 



5. Weather Bureau Bulletin No. 235. Psychrometric Tables. 



a. 1102 : 155-157. Formation of Dew and Frost. 



6. 1301 : 130-131. Dew and its Formation. 



c. 1302 : 285-286. Dew and Frost. 



d. 1303:61-62. Dew and Frost. 



e. 1305 : 75. Natural Formation of Dew. 

 /. 1306:107-108. Dew. 



g. 1307 : 232-234. Dew Point and Dew. 



h. 1309:241-242. How Dew is Formed. 



i. 1310:372-373. Dew and Frost. 



j. 1311:227-228. Dew and Frost. 



