It is shown that within the stratosphere the humidity mixing ratio should remain constant with height, and a simple formula is derived, by means of which the total water vapour content of the stratosphere can be derived. This formula indicates that the total is 0·62 times the amount which would be derived on the basis of Dalton's law. The actual amount in the stratosphere above the tropics is found to be very small, equivalent to about OOI mm. of precipitable water.

The results of computations of the level above which there will be 0·3 mm. of precipitable water (a) assuming saturation everywhere in the upper troposphere, and (b) assuming relative humidity 50 per cent, indicate that in the first case the level of the base of the layer coincides approximately with the isothermal surface of 232° abs. and in the second case with the isothermal surface of 245° abs. Similar computations are made for 1·2 mm. of precipitable water, assuming 50 per cent relative humidity in the upper troposphere, and again yield the result that the base of the layer so computed coincides with an isothermal surface.

A brief reference is made to the effect of the considerations discussed relative to the humidity mixing ratio being constant everywhere above the tropopause upon the evaluation of the equilibrium conditions in the ozone layer.