METEOROLOGY 

 TABLE OF SATURATED VAPOUR PRESSURES. 



41 



Example of the method of determining the relative humidity : 

 e.g., If the dry bulb thermometer records a temperature of 51 F and 

 the wet bulb 49 F, what is the relative humidity? It is first 

 necessary to find the dew point. On referring to the table of 

 Glaisher's factors it will be found that the value of Glaisher's 

 factor for a dry bulb reading of 51 F is 2-04. If the difference 

 between the temperatures of the dry and wet bulb be multiplied by 

 this factor and the result subtracted from the dry bulb temperature, 

 the result will be the dew point, i.e., 5i-{(5i 49) X 2.04 j- =46-9 F. 



The temperature of the dew point is therefore 46-9 F. From 

 the table of saturated vapour pressures it is found that if the air 

 at a temperature of 51 F were saturated with moisture, the tension 

 of aqueous vapour would be 0-374 inch of mercury; the tension 

 of the vapour actually present is the maximum tension corresponding 

 to the dew point (46-9 F), which is 0-322. Therefore the relative 



humidity expressed as a percentage, is : *$*'* x 100 = 86.2. 



-374 

 PRECIPITATION. Rain As has been already stated (see 



Humidity) the atmosphere can only contain a certain maximum 

 amount of water vapour and, on a lowering of temperature taking 

 place, condensation occurs and the formation of large droplets is 



