288 Common Science 



The cause of fogs. When warm air is cooled while 

 it is down around us, the water vapor in it condenses 

 into myriads of droplets that float in the air and make 

 it foggy. The air may be cooled by blowing in from the 

 warm lake or ocean in the early morning, for at night 

 the land cools more rapidly than the water does. This 

 accounts for the early morning fogs in many cities that 

 are on the coasts. 



Likewise when the wind has been blowing over a warm 

 ocean current, the surface of the warm water evaporates 

 and fills the air with water vapor. Then when this 

 air passes over a cold current, the cold current cools the 

 air so much that the moisture in it condenses and forms 

 fog. That is why there are fog banks, dangerous to navi- 

 gation, in parts of the ocean, particularly off Labrador. 



Why you can see your breath on cold days. You 

 really make a little fog when you breathe on a cold 

 morning. The air in your lungs is warm. The mois- 

 ture in the lungs evaporates into this warm air, and 

 you breathe it out. If the outside air is cold, your 

 breath is cooled ; so some of the water vapor in it con- 

 denses into very small droplets, and you see your breath. 



Here are two experiments in condensing water vapor 

 by cooling the air with which it is mixed. Both work 

 best if the weather is warm or the air damp. 



Experiment 88. Put the bell jar on the plate of the air 

 pump and begin to pump the air out of it. Watch the air 

 in the jar. If the day is warm or damp, a slight mist will 

 form. 



As part of the air is pumped out, the rest expands and 

 cools, as warm air does when it rises and is no longer 



