12 Transactions of the South African Philosophical Society. 



equality. Acting upon this principle the temperature of the dew- 

 point has been tabulated, in each month, at the end of any hour 

 in which there was a rain exceeding *10 inch. The results are 

 arranged in Table 22 : Column 1 contains the months ; Column 2 

 the mean monthly temperature of the air for the five years 1898- 

 1902 ; ' Column 3 the mean monthly temperature of the dewpoint ; 

 'Column 4 the mean temperature of the dewpoint immediately after 

 rain, for each month ; Column 5 the frequency, i.e., the number of 

 •observations from which Column 4 has been deduced. We may 

 from these results form some idea of the altitudes from which the 

 rain has fallen. For taking the adiabatic rate of cooling of the air as 

 l°-6 F. for each 300 feet of ascent, and the lowering of the dewpoint 

 as o, 3 for the same space due to the expansion of the air, we have 

 this formula for the height h of the lower surface of the rain clouds — 



h = 3000(£ - d)/13, 



where t is the normal temperature of the air and d that of the dew- 

 point after rain at the earth's surface." Whence we get the relative 

 heights of the rain clouds in Column 6. Of course these values can 

 make no claim to any great precision, because while on the one 

 hand the falling rain must notify the temperature of the air through 

 which it passes, and thus also the mean temperature of the month, 

 ■on the other hand it is certain that the rain mostly comes not so 

 much from the cooling of ascending currents of moist air as from 

 horizontal streams bringing moisture from the ocean. The average 

 of Column 6 for the summer half, September to March, is 2,560 feet, 

 that for the winter 1,060 feet. The August value, being obtained 

 from only one observation, is of no great consequence. That October 

 has a lower cloud level than either September or November is 

 probably a fact, and may be directly connected with the absence of 

 very heavy thunderstorms characteristic of that month, so plainly 

 indicated in Table 16. Moreover, the October clouds tend, perhaps 

 more than those of any other month, to a stratiform type, sug- 

 gestive of the plane of contact of two humid air-strata at different 

 temperatures.! The smallness of the April value is remarkable. 



In the absence of direct measures the formula just quoted may 

 •be used for the purpose of approximately determining the altitudes 

 at Kimberley of clouds generally. It gives a monthly average series 

 ranging from upwards of 6,000 feet in November to 3,000 feet in 

 April — this last value confirming the April minimum of Table 22. 

 A useful development is a comparison between the mean monthly 



* W. M. Davis, " Elementary Meteorology," p. 163. 



f See the remarks by F. Waldo, " Modern Meteorology," p. 255. 



