METEOROLOGY 



39 



liy introducing a little water into the Torricellian vacuum of a 

 barometer tube. The aqueous vapour which is formed drives the 

 mercury down by an amount which measures the aqueous vapour 

 pressure. If sufficient liquid is present the vapour pressure reaches 

 a maximum, called the saturation pressure, for a given temperature 

 and increases with the temperature. There is always a certain 

 amount of aqueous vapour present in the atmosphere, and it becomes 

 of importance to measure the amount present at any given time. 



By hygrometry is meant the study of the amount of aqueous 

 vapour present at any time in the atmosphere, and instruments 

 \\-hich enable this to be carried out are termed hygrometers. 



By Relative Humidity or Hygrometric State of the Atmosphere 

 is meant the ratio of the actual mass of water vapour present in any 

 volume of the air at any temperature to that mass which would 

 be present if that volume of air were saturated at that temperature. 

 The Dew Point is that temperature at which the water vapour then 

 present in the atmosphere would be sufficient to saturate it. 



Since the vapour pressure approximately follows Boyle's Law 

 we see that : 



Saturated vapour pressure at dew point 

 Humidlty = Saturated vapour pressure at air temperature' 



and it is usually expressed as a percentage. 



The commonest type of hygrometer is that 

 known as the Dry and Wet Bulb. It consists of 

 two thermometers, one, the dry bulb, being simply an 

 ordinary thermometer while the wet bulb has muslin 

 wound round the bulb; this muslin is attached to 

 threads of cotton which dip into a small vessel of 

 water and is thus kept " wet." The combination of 

 thermometers is also called the Psychrometer. The 

 wet bulb temperature is generally less than that given 

 by the dry bulb for evaporation is constantly going 

 on, unless the surrounding air is saturated with 

 water vapour, and part of the heat of vaporisation 

 (latent heat) is taken from the thermometer in 

 question, thus lowering its temperature. 



The readings of the temperatures of the two 

 thermometers together with a suitable set of tables 

 will enable the dew point, aqueous vapour pressure 

 and relative humidity to be determined, for if the 

 difference between the temperatures of the dry and 

 wet bulb or " depression of the wet bulb " be 



31 



a 



V 



FIG. 3. A Dry and 

 Wet Bulb 

 Thermometer. 



