CHAPTER XIII. 

 WATER IN THE ATMOSPHERE. 



Hygrometry Relative Humidity Dew-Point and its Determination Regnault's 

 Researches on the Density of Water- Vapour Cloud Convective Equi- 

 librium Halos and Parhelia Coronas Rate of Fall of Cloud Drops Hail 

 Fog Dew. 



Hygrometry, or the Measurement of the Amount of Water-Vapour 

 present in the Atmosphere. The atmosphere always consists in part of 

 water- vapour, which is present in varying quantity, the mass in any 

 given mass of air depending on its preceding history. If it has been 

 moving over the sea or over damp ground, meanwhile being at a high 

 temperature, it will contain a large quantity. If it has lately been cooled 

 down, so that much of its vapour has been condensed, and has fallen 

 as snow or rain, and if it has since been moving in dry regions or been 

 maintained at a low temperature, it will contain a small quantity. 



Relative Humidity. It is of great importance in meteorology to 

 measure the quantity present at any given time, for should this approach 

 the quantity required to saturate the air, a slight lowering of temperature 

 may result in condensation. It is usual to determine the amount present 

 in terms of the pressure which it exerts, and the ratio which this bears 

 to the maximum vapour-pressure at the temperature is frequently termed 

 the "relative humidity." 



Dew-Point and its Determination. The simplest mode of 



determining the pressure of the water-vapour present in the air is by 

 means of some form of "dew-point" apparatus in which a surface is 

 gradually cooled down until dew is deposited on it. At the temperature 

 at which deposition begins, the " dew-point," the air is just saturated, 

 and from the tables of the pressure of water-vapour, the pressure for the 

 dew-point may be obtained. Now in merely cooling the air the pressure 

 of the water-vapour in it does not change, since both dry air and vapour 

 contract equally. Hence the pressure of the vapour in the air before 

 cooling is the maximum pressure at the dew-point. 



A very good and easily used form of dew-point apparatus was devised 

 by Regnault. It consists essentially of a tube AB (Fig. 122) like a test- 

 tube, the lower part AB being of very thin highly polished silver. The 

 upper end is closed by a cork through which passes a thermometer T and 

 a narrow glass tube G reaching nearly to the bottom of the silver tube. 

 A side tube DM is connected through the stand MN and by the tube 

 NO with an aspirator. Some ether is placed in the tube up to PQ, and 

 when the aspirator is set working it draws air down through the tube G 

 and up through the ether. The free evaporation into the air-bubbles 

 lowevs the temperature of the ether and of the silver surface, and when 

 the dew-point is passed the bright surface clouds over. The instant this 



