lxil INTRODUCTION. 



pressure e at a lower station and e the vapor pressure at another station 

 at an altitude h meters above it: 



— = 10 6 3°° 

 e 



This is of course an average relation for all times and places from which 

 the actual rate of decrease of vapor pressure in any individual case may 

 widely differ. 



Table 79 gives the values of the ratio — for values of It from 200 to 



6000 meters. An additional column gives the equivalent values of h in feet. 



REDUCTION OF SNOWFALL MEASUREMENT. 



The de'termination of the water equivalent of snowfall has usually been 

 made by one of two methods: (a) by dividing the depth of snow by an ar- 

 bitrary factor ranging from 8 to 16 for snow of different degrees of com- 

 pactness; (b) by melting the snow and measuring the depth of the resulting 

 water. The first of these methods has always been recognized as incapable 

 of giving reliable results, and the second, although much more accurate, 

 is still open to objection. After extended experience in the trial of both these 

 methods, it has been found that the most accurate and most convenient 

 measurement is that of weighing the collected snow, and then converting 

 the weight into depth in inches. The method is equally applicable whether 

 the snow as it falls is caught in the gage, or a section of the fallen snow is 

 taken by collecting it in an inverted gage. 



Table 80. Depth of water corresponding to the weight of a cylindrical snow 

 core, 2.655 inches in diameter. 



This table is prepared for convenience in making surveys of the snow 

 layer on the ground, particularly in the western mountain sections of the 

 country. The weighing method is the only one found to be practicable. 

 Present Weather Bureau practice is to take out a sample by means of a 

 special tube, whose diameter, 2.655 inches, has been selected by reason of 

 convenience in manipulation and simplicity in relation to the pound. 

 Table 80 gives the depth of water in inches and hundredths corresponding 

 to given weights. The argument is given in hundredths of a pound from 

 0.01 pound to 2.99 pounds. 



Table 81 . Depth of water corresponding to the weight of snow {or rain) col- 

 lected in an 8-inch gage. 



The table gives the depth to hundredths of an inch, corresponding to 

 the weight of snow or rain collected in a gage having a circular collecting 

 mouth 8 inches in diameter — this being the standard size of gage used 

 throughout the United States. 



