ABSTRACT
In earlier chapters diurnal and seasonal rhythms in the behaviour of winds and weather have been noticed. That these are responses to the regular 24-hourly and yearly astronomical variations of the radiation supply needs no elaboration, and the chains of thermal and dynamical causation that produce them can be fairly simply followed. The semi-diurnal pressure wave (see later, pp. 217–18) which is very prominent in low latitudes, where longer-term variations of pressure are slight, is of partly tidal — i.e. gravitational — and partly thermal origin. No other rhythms are known in the atmosphere which could have such straightforward origins and which can be observed to show a similar approach to regularity in operation. Even the semi-diurnal, diurnal and annual (i.e. seasonal) rhythms are liable to fade, and may be lost for a time, especially in those latitudes where their amplitudes are weak — the former two near the poles, the last-named in the tropics. Variations of wind and heat transport, cloudiness, sky haze, etc., due to other causes, may for a time so affect the radiation receipt at the surface as to suppress, or mask and overlay, the expected rhythm.
