The Weather 49 



tion begins and cloud is formed or dew is deposited. Thus the 

 reduction of the temperature below the point at which satura- 

 tion of the air occurs causes the condensation of the invisible 

 moisture, and of course reduces the absolute humidity. The 

 temperature at which condensation begins, as shown by the 

 deposited dew or cloud, is said to be the dew point. With an 

 absolute humidity of .35 of an ounce per cubic yard, the tem- 

 perature 59 F., which is the temperature of saturation, is also 

 the dew point the point at which invisible vapor is converted 

 into visible liquid or water. 



Exercise. If the dew point is 59 F., and the temperature is 77 F., 

 what is the relative humidity? 



When the air has a temperature above that of saturation or the 

 dew point, will a lowering of the temperature increase or decrease 

 the relative humidity? What will be the effect upon absolute hu- 

 midity of any fall in temperature above the dew point? Below the 

 dew point ? 



A simple experiment will illustrate clearly the important facts 

 about the dew point, and its variation with the relative humidity of 

 the atmosphere. Into a bright tin can or a porcelain pitcher about 

 half full of water put some cracked ice or snow, a little at a time, and 

 stir carefully with a thermometer. As soon as condensed moisture 

 begins to appear on the outer surface of the vessel, take the ther- 

 mometer reading. This is the temperature at which condensation 

 occurs, that is, the dew point. Try the same experiment on dif- 

 ferent days, and compare the thermometer readings. What do the 

 results show as to the dew point? What do they show as to relative 

 humidity ? 



The dew point may be found approximately for all temperatures 

 in this way. The result is most nearly correct when the temperature 

 is about 70 F. Multiply the difference between the readings of the 

 wet and dry thermometers by 1.8, and subtract the product from the 

 temperature reading of the dry thermometer. This gives the dew 

 point accurately enough for practical purposes. 



The facts which have been considered in the study of humid- 

 ity and condensation in relation to temperature are directly 

 useful in explaining the characteristic weather changes, as 

 cloud formation and precipitation, dew and frost (Fig. 16). 



