I909.J BY MEANS OF KITES AND BALLOONS. 23 



and setting it in motion. Observations upon this distribution show 

 that at 2,500 meters the moisture content of the air is one third 

 what it is at sea level, at 5,000 meters one tenth. Most clouds of 

 the cumulus and stratus types form below the latter level. It is to 

 be expected, therefore, and we are not disappointed in finding, that 

 this lower stratum of air is in continuous and complicated motion, 

 vertical currents as well as horizontal obtaining. Above this stratum 

 the air movement seems to be less complex. 



When an air mass h heated to a temperature higher than that of 

 the air about it, as we now see may be the case near the earth's sur- 

 face, an unstable condition obtains and convection currents set in. 

 A body of air rising to higher levels is cooled by its own expansion 

 as it passes into the rarer atmosphere. This is called adiabatic 

 cooling. If the body of air in question were dry, the rate of adia- 

 batic cooling would be about one degree Centigrade per 100 meters, 

 or one degree Fahrenheit per 180 feet. If it contain moisture, it 

 will not cool so rapidly for the moisture in condensing gives off 

 its latent heat into the air. This effect is a function of the relative 

 humidity and tends to accelerate the upward motion and postpone 

 the return of stable conditions. Sufficient condensation soon takes 

 place, so that heat from this source ceases to offset the adiabatic 

 cooling, and the convection current finds its upper limit. Other 

 moist air coming in from below supports the system thus set up, 

 and the whole moves with the upper westerly wind. This sort of 

 circulation on a larger or smaller scale, more or less modified by 

 other circulations of the same sort, is in progress continuously. An 

 almost unmodified type of it may often be observed during the sum- 

 mer months in the formation of a single cumulus cloud. The cloud 

 formation shows the outlines of the ascending air column. The 

 horizontal air movement is slight at such times and the column 

 nearly vertical. 



We should expect to find then that the change of temperature 

 with altitude is less in the lower moist stratum of the atmosphere 

 than in that immediately above it and always, when mean conditions 

 for a sufficiently long time, say a year, are considered, less than the 

 adiabatic rate of cooling for dry air, some moisture being present at 

 all altitudes. The sounding balloon observations in middle Europe, 



