FOGS AND CLOUDS 237 



because the earth and the air cool more rapidly if clouds 

 are not present (cf. 264). A gentle breeze favors dew 

 formation, because it brings fresh supplies of almost 

 saturated air to the cooling objects. A strong wind is 

 not favorable, for it does not allow the air to remain long 

 enough in contact with the cool objects. 



Fruit growers know that there must be air drainage 

 as well as water drainage in their orchards. If an orchard 

 is placed in a hollow, cold air will fall into it as water 

 would, and may cause damaging frosts, 

 while neighboring orchards situated on 

 the tops or slopes of hills may escape 

 injury. 



The dew point may be found at any time by the 

 use of the apparatus of Fig. 217. A polished metal 

 beaker containing water is stirred with a ther- 

 mometer, and bits of ice are added from time to 

 time. The temperature reached when dew is Fig. 217. 



just formed on the outside of the beaker is the . 



dew point. 



We can use the dew point to determine the relative humidity of the 

 air (cf. 266). Such determinations are valuable in greenhouses, in 

 schoolrooms, and in your own house (cf. 249). Let us suppose that 

 the temperature of a room is 23 C. In the Appendix, Table VIII, 

 we find that the pressure of water vapor (cf. 89) at 23 C. is 20.9 mm. 

 This would be the pressure of the water vapor in the air at 23 C., if 

 the air were saturated with it. Suppose that we find the dew point 

 to be 10 C. Table VIII shows that at this temperature the pressure 

 of water vapor is 9.1 mm. The air of the room has, therefore, less 

 than half its capacity of water (9.1 divided by 20.9 is 0.44, or 44 per 

 cent, the relative humidity). 



268. Fogs and Clouds. If the air just above the 

 ground is cooled below the dew point, its water is con- 



