A semi-diurnal variation appears to some extent in many meteorological phenomena, in particular in barometric pressure, rainfall, visibility, atmospheric electric potential gradient, and atmospheric pollution, even though the variation of temperature, at least in the lower levels of the atmosphere, is very approximately a purely diurnal variation.

The data examined in this paper suggest that, in the forenoon, insolation affecting the ground and the lower layers, and, in the evening, outgoing radiation affecting clouds and the upper part of the troposphere, each lead to a disturbance of the stability of arrangement of the atmosphere in the vertical direction and to a consequent increased mixing of layers. On the other hand, in the late afternoon and in the later part of the night, as the turbulences arising from the above two effects respectively die down, there is an improvement above normal in the laminarity of flow of atmosphere over the earth. The resultant effect is therefore, on the average, a semi-diurnal variation in the vertical structure and horizontal movement of the atmosphere, which variation is reflected in the meteorological phenomena mentioned above. It is shown in the paper that the various individual effects may be greatly exaggerated, or almost eliminated, or accelerated or retarded, at least in temperate latitudes where a suitable variety of types of upper air structure, from which to select, is available.