PRESIDENTIAL ADDRESS. LXI 



ascent, does such an inversion layer of warm air suddenly mani- 

 fest itself ? Two possible causes, which I would briefly mention, 

 are the following : — 



First, it is well known that the ultra violet rays of sunligb^ 

 are largely absorbed in the higher atmosphere. In this process 

 of absorption they appear to accomplish two kinds of work — the 

 first, the conversion of some of the high-level oxygen into ozone; 

 and as ozone is able to absorb heat even when dry, whereas oxygen 

 does not, in the second place they warm the ozone. 



Next it is pointed out that with one exception all the sub- 

 stances and conditions which lead to the lov/er atmosphere being 

 alternately warmed or chilled disappear at a height of from 5 to 

 6 or 7 miles above the earth's surface. These substances, as 

 already mentioned, are aqueous vapour, carbon dioxide, and dust, 

 and the condition which obtains at the earth's surface, but not at 

 the isothermal layer, is the conduction of some solar heat by 

 contact with the land or water to those substances, and the con- 

 sequent warming of the layer of the atmosphere next to the earth 

 by the long waves of radiant energy, which are practically trans- 

 formed waves from sunlight. ^ Of the three substances which go 

 to bring about changes of temperature in the lower atmosphere, 

 and so produce convection currents, only dust is met with above 

 the base of the isothermal layer. This dust is excessively tenvious 

 and minute. It is not of ordinary terrestrial nor of volcanic 

 origin, but at this high level is probably wholly derived from the 

 small amount of material resulting from the condensation of in- 

 candescent gases given off during the combustion of meteors and 

 meteorites. Such dust would settle at an extremely slow rate 

 through the atmosphere commencing at the point known as the 

 radiant point of meteors, usually about 50 miles above the earth's 

 surface. In this slow process of settling down, the dust becomes 

 denser as the density of the atmosphere increases, and thus con- 

 tact with this dust tends to slightly warm the higher atmosphere 

 in the isothermal layer, the warming effect increasing downwards, 

 but it is so slight that it is insufScient to produce effective con- 

 vection. It is indeed the same process which results from warming 

 of dust in the lower atmosphere, but in the latter case the dust is 

 present in such appreciable quantity that its heating effect on the 

 atmosphere is sufficient to bring about definite convection currents. 



' Foi- fiii-th^r infoi-nntion on the cause of the isothermal layer or upper inversion, see 

 Mount Weather Bulletin, vol. ii, Papers by Humphreys, and Proc. Roy. Soc. Series A, vol. 

 Ixxxii, p. 43, Gold, also Nature, 1908-1909. 



