LX PRESIDENTIAL ADDBESS. 



Next is the factor that in places where the weather is calm, 

 so that no convection currents of air are rising, bringing about 

 this adiabatic fall of temperature, the fall of temperature of the 

 atmosphere is at the rate of about 1 degree F. for ascent of 300 

 feet. 



Next is the factor that outside the ordinary gases, such as 

 oxygen, hydrogen, and carbon dioxide, which enter into the com- 

 position of the earth's lower atmosphere, is the vast vault of the 

 higher atmosphere composed chiefly of hydrogen and helium, with 

 small quantities or argon, neon, and xenon. These extremely 

 light gases, as far as we are at present aware, play no very im- 

 portant part in our atmospheric circulation. 



Next is the great factor, the importance of which has been 

 realized only of late years, that at a height which varies chiefly 

 with altitude, partly with surface atmospliei'ic conditions, of from 

 27,000 feet near the North Pole ^ to over 50,000 feet at the 

 equator and tropics, the temperature of the earth's atmosphere 

 suddenly ceases to fall, and usually commences to rise again, after 

 which, at a much higher altitude still, ^ it may fall again, but 

 at an extremely slow rate. The bottom of this layer, the in- 

 version laver, is now known as the isothermal line, and one of 

 the greatest aims of modern meteorology, when dealing with the 

 higher atmosphere is to determine the height temperature and 

 movement of this isothermal line or surface. Practically it 

 forms a ceiling over the top of the circulating portion of the 

 earth's atmosphere against which all appreciable upward or 

 downward or horizontal moving convection currents practically 

 stop. The great discovery of the existence of the isothermal layer 

 was made by the distinguished French meteorologist, Teisserenc 

 de Bort, at the world-famoiis observatory at Trappes, near Paris, 

 and almost simultaneouslv by Professor Asstnan, in Germany. It 

 is beyond the scope of this address to discuss all the probable 

 reasons for the existence of the isothermal layer. Two only may 

 be given. Obviously the cessation upwards of convection currents 

 from the lower atmosphere, when the isothermal layer is reached, 

 is due to the fact that air which is getting colder by adiabatic 

 expansion cannot force its way above air which is warmer than 

 itself, and at the same time under less pressure than itself, as is 

 the air at the base of the isothermal layer. The fact which most 

 needs explanation is why, after a steady fall in temperature with 



> Darin? thi -lUia-Uir of 19)3 Prf^sor Ht^mUI caaclnl^d that over the Arctic Ocean 

 near lat. 75' tf., th? isothsrmal layer there sunk as low as 23,010 tset. 



" Pi-of<uar Milhirn foaal rfOsQtly hhifc oa •Jth Octobtr, at St LouU, Mo., U 3.A., lat. SB* 

 N., th? ralaimim t3m)'rature recordid ocoirrad at 47.S39 faet, while the sam? day at the 

 fm«imim altituda attaiaad, 54,100 feet, the tempsrature rose to -72° F., a rise of 18' F. 

 10" C). 



