MR. W. H. DINES ON THE VERTICAL TEMPERATURE 



270 



does elsewhere and, as the earth and lower layers of air are warmer there, it should 

 receive more radiation from below. It is true that the greater amount of water 

 vapour may cut off radiation from the earth, hut if so this radiation must warm up 

 the strata that absorb it, and they in turn should radiate upwards. 



The question naturally arises whether there is any appreciable interchange of air 

 between the isothermal region and the strata below it. The diffusion over the whole 

 earth of the dust thrown up into the very highest strata by the Krakatoa eruption in 

 1883 proves that the air at these levels circulates between the equator and the poles, 

 and if, as we have good reason to think, the air above 16 km. is much colder over the 

 ..quator than over Europe we should expect this circulation. It is certain, too, that 

 the trades and anti-trades above them do not form a closed system confined to the 

 region between 35 S. and 35 N. This may be proved as follows. The sum total of 

 the angular momentum of the earth's atmosphere about the axis of rotation remains 

 approximately constant from year to year, and hence the moment of the couple acting 

 upon the atmosphere is on the average zero. The only forces that form this couple 

 are forces due to friction at the earth's surface, for the mutual reactions of the 

 atmosphere as a whole cannot alter the angular momentum. From the fact that a 

 persistent wind produces a powerful ocean current we know that the frictional force 

 between the atmosphere and the earth must be considerable. Such a force is 

 persistently exerted in the region of the trades over nearly half the earth's surface, 

 and is constantly acting so as to give an easterly directed momentum to the 

 atmosphere. It is balanced, as FERREL pointed out, by the westerly directed 

 component produced by the friction of westerly winds in temperate latitudes. This 

 requires an interchange of angular momentum between the tropics and temperate 

 latitudes, and this can hardly occur in any way save by the interchange of masses of 

 air which must have both a N.-S. and an E.-W. component. Hence at some level 

 across parts of the boundary either N.E. and S. W. or N. W. and S.E. winds must blow. 



The Local Circulation. 



When it first became possible to obtain correct observations of the temperature up 

 to 5 or 10 km. height, it was supposed that a few years' observations would show the 

 mechanism of a cyclonic storm, and enable us to say to what it was due. This 

 expectation has not been fulfilled. 



There were two theories to account for these storms one, due to FERREL, that a 

 cyclone consisted of a current of warm ascending air at the centre, which current 

 drew in the air from neighbouring districts and, aided by the directive force of the 

 earth's rotation, produced the characteristic whirl; the other, due I think to 

 Dr. HANN, was that they were eddies in the general circulation, just as small water 

 eddies are formed in a swiftly flowing stream. The facts brought to light by 

 registering balloons do not, so far as Europe is concerned, favour either hypothesis. 

 The cyclone is cold, not warm, and the strong upper westerly current in which the 



