16 Dew Point: Fogs and Clouds. 



Condensation has the opposite effect, and the substance condensed 

 becomes ■warm. It is by many astronomers believed that the sun's 

 heat is caused by the condensation of gases going on upon or within 

 its surface. Evaporation, or the passing of a liquid into gaseous 

 form, is notably an expanding, and consequently a cooling process. 

 The leaves of trees in the growing season evaporate abundantly, 

 and hence the coolness of groves in summer. The condensation of 

 vapors in the form of dew or rain, is always the effect of a cooling 

 down of the atmosphere to below the Dew Point [§ 55], and is al- 

 ways attended with a diminution in the previous volume of the air 

 from which it forms. 



64. Dew, is the moisture deposited from the air when cooled at 

 night by the radiation of heat from the earth's surface. It can only 

 occur when the temperature is reduced to the "dew point,'' and is 

 greatest iu still clear nights. When covered with clouds, the radi- 

 ations of heat are returned to the earth. The same effect is often 

 seen immediately under a tree, which will remain dry, while all 

 around it the dew on the grass may be heavy. This shows that the 

 air under the tree has been a little warmer than in the open space 

 around it, and that the general percentage of moisture in the air is 

 relatively high. 



65. On the contrary, we sometimes see the grass, boards, etc., 

 under a tree wet with the dew, when the ground around it is dry. 

 This occurs from the greater humidity of the air under the tree in 

 consequence of the evaporation of its foliage, and is seen only in 

 a calm night, when the general humidity of the air is less. These 

 effects are sometimes seen where a plank walk extends along under 

 an avenue of large trees standing widely apart. The open portions 

 may be white with hoar-frost (frozen dew), while the parts covered 

 by the trees are bare, or the sheltered portions may be wet, as if 

 rained upon, while the open spaces are dry. 



66. Fogs and Chads, are formed only when the air is at or below 

 the dew point, showing that it can hold the moisture no longer, 

 and the excess becomes visible, and may, in certain cases, descend 

 as rain. They show a reduced temperature, and common summer 

 clouds are often formed by the unequal heating of a portion of the 

 earth's surface by the sun. 



67. The air in contact with these heated portions expands, and, 

 becoming lighter, rises — the air from surrounding spaces coming in 



