SCIENCE OF COMMON THINGS. 89 



Breath visible in cold weather. Difference between mist and fog. 



temperature as fast as the air ; in consequence of which 

 they retain their cold after the thaw lias set in. 



567 How does "retaining their cold" account for their being so wet? 



As the vapor of the warm air touches the cold walls, 

 it is chilled and condensed into water, which either 

 sticks to the walls or trickles down in little streams. 



568 Why is our breath visible in winter, and not in summer ? 



Because the intense cold condenses its moisture into 

 visible vapor, but in summer the air is not cold enough 

 to do so. 



569 Why are our hair and the brim of our hat often covered with lit- 

 tle drops of pearly dew in ivinter-time f 



Because the vapor of the breath condenses as it comes 

 in contact with our cold hair or hat, and hangs there 

 in little dew-drops. 



570 What are fogs f 



Fogs are visible vapors that float in the atmosphere 

 near the surface of the earth. 



571 What is the cause of fogs 1 



They originate in the same causes as rain the union 

 of a cool body of air with one that is warm and humid ; 

 when the precipitation of moisture is slight, fogs are 

 produced ; when it is copious, rains are the result. 



573 What distinction is to be made between a mist and a fog ? 



Mist is generally considered to be a fine rain, while 

 fog is vapor not sufficiently condensed to allow of its 

 precipitation in drops. 



The term mist is also generally applied to vapors condensed on marshes, 

 rivers, and lakes, while the name fog is often applied to vapors condensed 

 on land, especially if those vapors are laden with smoke. 



573 Why does not the fog become dew ? 



Because the chill of the air is so rapid that vapor is 

 condensed faster than it can be deposited, and covering 

 the earth in a fog) prevents any further radiation of 

 heat from the earth. 



574 When the earth can no longer radiate heat upwards, does it con- 

 tinue to condense the vapor of the air ? 



Ko ; the air (in contact with the earth) becomes 



