1bow Dew fa formed. 119 



dew. For the same reason no dew will form 

 under a light covering spread over the ground 

 even at some distance above it. The covering 

 acts as a screen, which prevents the heat from 

 radiating to the dew point. From what has 

 gone before it will be seen that if the atmos- 

 phere is not charged with moisture up to the 

 point of saturation it will require a greater 

 amount of depression of temperature to cause 

 condensation, and this is why we usually have 

 heavier dews in June when the air is more 

 highly charged with moisture than we do in 

 August when it is dry. This also accounts for 

 the ice clouds, called cirrus, being formed so 

 high up in the atmosphere during dry weather. 

 There is so little moisture in the air that it re- 

 quires a very great difference of temperature 

 to cause condensation to take place, and the 

 necessary depression is not reached in these 

 cases except at an altitude of several miles. 



Dr. Wells has shown that if we take the 

 reading of two thermometers on a clear sum- 

 mer night, one of them lying on the grass and 

 the other suspended two feet above it, we shall 

 find that the one lying on the grass will read 

 8 or 10 degrees lower than the one suspended 

 in the air. If the night is still there will be 

 a cold stratum of air next to the earth, which 

 will not tend to diffuse itself to a very great 

 degree and dew will form. If, however, it is 

 cloudy or the wind is blowing there is rarely 



