224 



THE WEATHER 



253. Condensation of Water Vapor ; Dew and Hoar Frost. 



As the ground and vegetation cools by radiation to a tem- 

 perature below the dew-point, a thin layer of air is cooled by 

 contact with the chilled ground or foliage, and dew or frost 

 condenses from a thin film of air next the chilled object. If 

 the temperature is above 32 dewtorms; if 32 or lower, frost 



results. Hoar frost is made of 

 minute crystals of ice which ar- 

 range themselves in beautiful and 

 sometimes fantastic designs (Fig. 

 153). Dew is water and is usually 

 in small droplets. The large 

 drops frequently seen on the 

 edges or points of grass or leaves 



FIG. 153. Frost on vegetation. 

 Frost on windows often shows 

 beautiful designs. 



FIG. 154. A photograph of dew. 



are not always dew, but are often water that has been given 

 off by the plant through its leaves. Figure 154 is a photograph 

 showing both the small droplets of dew and other large drops, 

 some of which may be "false dew" given off by the leaves. 



254. Condensation ; Ground Fog. During the formation of 

 dew or frost the air often remains clear. But the night cooling 

 may go far enough to chill the whole mass of air near the 

 ground below its dew-point. Then a part of the vapor 

 within the chilled air is condensed into very small but vis- 

 ible particles of water that remain suspended in the air as FOG. 



