. Compendium of meteorology. Meteorology. 316 THE UPPER ATMOSPHERE Except for the shallow layer at 345 mb all the air above 600 mb has a lower humidity mixing ratio tlian would correspond to saturation at the tropopause, while the observations at 345, 700, and 800 mb show that at these levels the air is only slightly more humid. If we assume that all the air was saturated and has moved continuously downward during any recent dy- namic movements, during which entropy and humidity mixing ratio have been conserved, we may compute, for each observation, the level from which the air has subsided. T
. Compendium of meteorology. Meteorology. 316 THE UPPER ATMOSPHERE Except for the shallow layer at 345 mb all the air above 600 mb has a lower humidity mixing ratio tlian would correspond to saturation at the tropopause, while the observations at 345, 700, and 800 mb show that at these levels the air is only slightly more humid. If we assume that all the air was saturated and has moved continuously downward during any recent dy- namic movements, during which entropy and humidity mixing ratio have been conserved, we may compute, for each observation, the level from which the air has subsided. This is shown in Fig. 5a in which the temperature-pressure levels from which the air has sub- sided are shown joined to the corresponding observed temperatures by isentropic lines. It will be seen that there is a sharp discontinuity in the nature of the source air mass between 345 and 400 mb, the air above 345 mb presumably being warm air and the air below 345 mb polar air, most of which has subsided strongly. It should be noted that the demonstration of this feature depends entirely upon the water-vapour measurements. The temperature-height curve does not indicate any significant change in the layer between 345 and 400 mb or in any other layer. The thermal stability of the polar air mass from which the present air mass is derived is to be noted. This stability may be due to the cooling by radiation of the lower parts of the subsided polar air, and radia- tive adjustments of temperature may be the cause of the smoothness of the temperature-height curve. Al- ternatively, the air now found at the level of 600 to 800 mb may have originated from relatively farther north, so that the coldest air has been selected for the greatest subsidence. On thermodynamic grounds it is to be expected that the coldest air would subside most strongly. There is little doubt that all the processes of the atmosphere are not adiabatic, as may be seen from the observations in the stratosphere. The appare
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