. Compendium of meteorology. Meteorology. 314 THE UPPER ATMOSPHERE pause the air normally has a frost point of about 190- 200K and a humidity with respect to ice of 2 or 3 per cent. The effect is normally striking and consistent. An ascent is shown in Fig. 5, and a mean curve (due to Shellard [15]) of the results so far available (southern England) is shown in Fig. ^\ \ % W? 'frost- temp. POINT Fig. 4.—Mean variation of temperature and frost point with height above and below the tropopause, for southern England. All data so far available. {Curve due to H. C. Shellard [15].) This effect had
. Compendium of meteorology. Meteorology. 314 THE UPPER ATMOSPHERE pause the air normally has a frost point of about 190- 200K and a humidity with respect to ice of 2 or 3 per cent. The effect is normally striking and consistent. An ascent is shown in Fig. 5, and a mean curve (due to Shellard [15]) of the results so far available (southern England) is shown in Fig. ^\ \ % W? 'frost- temp. POINT Fig. 4.—Mean variation of temperature and frost point with height above and below the tropopause, for southern England. All data so far available. {Curve due to H. C. Shellard [15].) This effect had been predicted by Jaumotte [11], but other workers on the radiation balance of the lower stratosphere had assumed that the stratospheric air would be at or near saturation. The relative importance of water vapour in the radiation balance of the strato- sphere is therefore much less than has usually been supposed. Dobson [6] has, for example, suggested that as a result of the small amount of water vapour pres- ent, the carbon and the ozone, which also take part in the radiation processes, may be equally impor- tant and that the radiative equilibrium may be deter- mined by the relative amounts of these gases present. If we consider the water vapour as a "tracer," it is difficult to give a satisfactory explanation of the pres- ence and origin of the extremely dry air of the strato- sphere. Since the dry air is consistently found with upper winds from all directions, it seems certain that the effect occurs in all temperate latitudes, and prob- ably in polar regions also. Photolysis of the water vapour at these relatively low levels seems unlikely, for if it occurred, the i-eaction would be well known. If the photolysis occurs at high levels, it is difficult to see why the transition to dry air should be as low as 10 or 12 km. The temperature at the equatorial tropopause, which is the lowest temperature found in the atmosphere, is about 190K and all the results
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