Elements

Jeffreys' detailed analysis of land and sea breezes, and of moun tain breezes, show that they are mainly antitriptic.

The General Circulation.

The main features of the dis tribution of pressure and winds over the earth are best shown by maps. (See section on Distribution of Winds over the Globe.) When we come to consider the theoretical explanation of the main features of the earth's circulation, we find that no satisfac tory theory has been advanced to explain these phenomena. Two papers by Jeffreys in the Q.J. of the R. Met. Soc., Jan. 1926 and Oct. 1927, indicate the most hopeful lines of future attack upon the problem. Jeffreys shows that no circulation which is com pletely symmetrical about the poles could maintain itself against friction, and concludes that the cyclones of middle latitudes are to be regarded as essential features of the general circulation rather than as disturbances superposed upon the general circulation.

Surfaces of Discontinuity.

The possibility of having two currents of different densities flowing side by side with different velocities, separated by a definite surface of discontinuity, was first demonstrated by von Helmholtz. The slope of the surface

of discontinuity to the surface of the earth can be derived from the equations (4), (5), (6) of the section Dynamical Aspects.

The lines of the discussion would follow those laid down by Helmholtz and Margules. The two currents are assumed to be separated by a clearly defined surface of negligible thickness. In the atmosphere, however, this condition is never accurately reproduced, and the mathematical surface of separation is re placed by a layer of transition in which both the velocity and temperature change over gradually from one set of conditions to the other. Pilot balloon observations frequently display this layer of transition, whose thickness is usually of the order of 50o metres. It is not probable that the slope of the layer of transition differs much from the computed slope of the mathe matical surface of discontinuity. The chief limitation met in the application of the formulae consists rather in the fact that the motions of the two currents are not always parallel to the surface line of separation, so that the warm current climbs up over the cold current.