July 21, 192 1] 



NATURE 



653 



The Air and its Ways.^ 



Bv SiK Napier Shaw, F.R.S. 



THE physical problems of the weather map have 

 not been solved, for the subject is inherently 

 difficult. In the first place, the atmosphere is on 

 such an immense scale that its behaviour is not 

 to be brought under the principles of physics 

 without much trouble, and, I may add, many mis- 

 takesi The most confident theories of the past 

 are flatly contradicted by facts which have come 

 to light since the investigation of the atmosphere 

 was extended to the upper air by balloons, kites, 

 kite-balloons, and more recently by airships and 

 aeroplanes. We have now many facts about the 

 atmosphere up to 20 kilometres at our disposal. 

 They are, of course, not necessary for the forma- 

 tion of a correct theory, because no new principles 

 are involved, but they are invaluable for the 

 purpose of the verification or contradiction by 

 which hypotheses get moulded into consistent 

 theory. 



The behaviour of air in bulk is so entirely dif- 

 ferent from that of the laboratory sample that the 

 ways of the air are, indeed, as peculiar as those 

 of "the heathen Chinee." The air as we know it 

 in the laboratory is a very mobile fluid, yet in the 

 atmosphere it manages to take on a sufficiency 

 of the character of an elastic solid. It does not 

 go in the way it is pushed ; pushed north it goes 

 east, and pushed east it goes south. The con- 

 dition for getting it to go north is that it should 

 be pushed west. If you blow a jet of air straight 

 upward you may find that part of the effect is a 

 vortex whirling around you. In front of its fire 

 — the sun — the air will very likely get colder in- 

 stead of warmer ; losing heat by exposure to the 

 clear sky on a cold night, it may get warmer. In 

 spite of all that is taught in the laboratory about 

 the levitating effect of warmth, cold air floats 

 above us with warmer air beneath. If you tell 

 the air that warm air rises, it winks an eye and 

 interjects an "if " and a "when." If the Olympian 

 gods felt cold and thought to make themselves 

 warmer bv stirring up their chilly air with the 

 warmer air enjoved by mortals down below them, 

 thev would be disappointed. Stirring would make 

 them colder and us warmer. Shake air up violently, 

 water falls out of it ; and if the shaking went_ on 

 long enough the air would become intolerably dry, 

 verv cold at the top and. very warm at the bottom. 

 \ot only has the air the innate capacity for these 

 conjuring tricks, but it never, or scarcely ever, fails 

 to use them. 



The General Problem of the Science of Meteor- 

 ology. 



Yet, underlying the work that is done in 

 meteorology officially or unofficially, there is, and 

 has been all the time, a definite purpose to bring 

 our knowledge of the air into relation with the 

 laws of physics as established in the laboratory, 



I Abridged from the Rede Lecture at Cambridge on June 9. 



NO. 2699, VOL. 107] 



and, therefore, particularly with the laws of 

 energy. 



The Fundamental Facts. 



There are two sides to the study of the air and 

 its ways which can be pursued by difterent people 

 who may never meet each other. One is the 

 observation and collection of the facts about the 

 weather from every part of the world ; the other 

 is the interpretation of facts by dynamical and 

 physical reasoning. Nothing — at least nothing 

 useful — can be done without real facts ; but real 

 facts do not, as a rule, explain themselves. The 

 composition of air at different levels has been 

 computed, and the results for one hundred kilo- 

 metres are different according to Humphreys, 

 Wegener, and Chapman. Below the level of 

 20 km. we are not troubled with changes of com- 

 position except those in the amount of water- 

 vapour. The meteorological facts may be ex- 

 pressed by maps showing coast lines and oro- 

 graphic features, surface-temperature in January 

 and July and its discontinuities at the coast 

 lines, water-vapour at the surface in July, cloud, 

 rainfall over the land, winds over the sea, and 

 pressure over the globe in th^ same month. 



Winds and temperatures in the upper air can 

 be illustrated by models in cardboard. That for 

 temperature shows the general run of the iso- 

 thermal surfaces and the modifications caused by 

 the introduction of local cyclones and anticyclones. 



The Atmosphere a Great S.team Engine. 



We are all agreed that the atmosphere is in 

 reality a great engine, partly an air engine, but 

 more effectively a steam engine, or at least a 

 moist-air engine. Now the essential parts of a 

 steam engine are a boiler to supply it with heat, 

 a condenser at a lower temperature to absorb the 

 surplus heat, and a fly-wheel to maintain the con- 

 tinuitv and uniformity of its action. We describe 

 the action of the engine as taking a supply of 

 heat from the boiler, giving out heat to the con- 

 denser, and converting into work, useful or other- 

 wise, the difference between the heat taken in and 

 that given out. 



Can we rightly use such language about the 

 atmosphere and usefully contemplate the ways of 

 the air from that point of view? I think we can, 

 though the analysis of the phenomena from that 

 point of view is difficult. The boiler is certainly 

 there ; I have shown it to you in the distribution 

 of temperature with the great warmth of the equa- 

 torial regions. In the map of the distribution of 

 water-vapour I have shown you where the steam 

 is raised. The condenser is there also, partly in 

 the shape of the vast cooling surfaces of the high 

 lands of the arctic and antarctic regions, and of 

 snow-covered mountains generally ; but perhaps 

 more effectively in the upper air, particularly in 

 the stratosphere, which at a temperature of 



