156 



SCIENCE 



[N. S. Vol. XLIX. No. 1259 



ates highly trained in the subjects with which 

 they have to deal. And, finally, it is insisted 

 that the physics of the air offers many op- 

 portimities to the creative scholar, and every 

 university must realize that its paramount 

 duty is the fostering of research and the train- 

 ing of investigators, for in no other way can 

 it meet the growing and compelling demands 

 of a progressive civilization. 



It must be admitted, however, that it is not 

 now easy to give a connected course on atmos- 

 pheric physics, for there is no suitable test 

 and the isolated articles upon which such a 

 course must needs be based are scattered 

 through the journals from Dan to Beersheeba 

 and buried mider a babel of tongues. But 

 this is only a difficulty, and not, in the face of 

 imperative needs, an excuse. A far greater 

 and very real difficulty has, it is true, con- 

 fronted most of us, for, until the last decade, 

 or less, several important lectures in such a 

 course would of necessity have bepi restricted 

 to the same brevity as characterizes Horre- 

 bow's famous chapter on snakes in his " ifat- 

 ural History of Iceland " — there aien't any. 



Some of these lectures are still unwritten — 

 tantalizing challenges to the skill of the ex- 

 perimentalist and acumen of the analyst — ■ 

 while others have been at least partially sup- 

 plied, a few of which it will be interesting to 

 review in what follows. 



TEMPERATURE OF THE FREE AIR 



Although efforts to determine the tempera- 

 ture of the free air by means of thermometers 

 carried by kites were made as early as 1749, 

 the experiments being conducted at Glasgow 

 by Alexander Wilson and his pupil Thomas 

 Melville; and although, beginning with Jef- 

 fries in his ascent from London in 1784, 

 balloonists have often carried thermometers 

 on their flights, it was only after the develop- 

 ment of self-recording instruments and the 

 sounding balloon — both at the very end of the 

 last century — that the vertical distribution of 

 temperature up to even 7 or 8 kilometers be- 

 came at all accurately known. As is now 

 known, and as shown in Fig. 1, the average 



temperature decreases slowly with elevation 

 near the surface, then more and more rapidly 

 to a maximum at some such considerable alti- 

 tude as 7 to 9 kilometers, where it roughly ap- 

 proaches the adiabatic rate for dry air of ap- 

 proximately 1° C. i)er 103 meters. 



These are the observed facts; but here too, 

 as in the investigations of other physical phe- 

 nomena, a knowledge of what happens is only 

 so much raw material out of which some one 

 happily may fashion the finished product — 

 why it happens. In this case the why is 

 found in the presence of water vapor in the 

 air, its condensation and the latent heat thus 

 rendered sensible. As air is carried to higher 

 levels by vertical convection it progressively 

 expands against the continuously decreasing 

 pressure, and thereby does work at the ex- 

 pense of its own heat. During the dry stage 

 of this convection, that is, until saturation is 

 attained, the cooling is roughly at the rate of 

 1° C. per 103 meters increase of elevation. 

 Immediately condensation begins, however, la- 

 tent heat is set free and the rate of cool- 

 ing with elevation correspondingly decreased. 

 But as the amoimt of vapor condensed per 

 degree drop in temperature decreases with the 

 temperature, it follows that the latent heat 

 set free and the corresponding check in the 

 rate of cooling with elevation also decreases. 

 Hence the continuous temperature-elevation 

 coordinates of a rising mass of saturated air 

 form a curved line. Furthermore, the curve 

 thus formed approximately coincides with the 

 average temperature-altitude curve of the free 

 air throughout all cloud levels, or from 0.5 

 kilometer, say, to 9 kilometers, or thereabouts, 

 above sea level. This agreement necessarily 

 occurs more or less closely during every rain 

 and in all deep clouds and, therefore, very 

 frequently. l^Tor can there often be much de- 

 parture from it between such occasions for 

 during these intervals the whole of this por- 

 tion of the atmosphere is, as a rule, simul- 

 taneously warmed or cooled, and thus the 

 curve in question usually shifted essentially 

 parallel to itself. 



It appears, then, that the average tempera- 



