Fig. 77—To show progressive lowering of saturation temperature in a desert under the influence of the mixing process whereby dry and cool air from aloft sinks to lower levels thus displacing the warm surface air of the desert. The evaporated moisture of the surface air is thus distributed through a great volume of upper air and rain becomes increasingly rarer. Applied to deserts in general it shows that the effect of any cosmic agent in producing climatic change from moist to dry or dry to moist will be disproportionately increased. The shaded areas C and C’ represent the fog-covered slopes of the Coast Range of Peru as shown in [Fig. 92]. X-X’ represents the crest of the Coast Range.
Curves a and b represent the rise of temperature in two contrasted cases of warm and cold sea with the coastal mountains eliminated, so as to simplify the principle applied to A and B. The steeper gradient of b also represents the fact that the lower the initial temperature the dryer will the air become in passing over the warm land. For these two curves the transition line X-X’ coincides with the crest of the Coast Range. It will also be seen that curve a is never so far from the saturation level as curve b. Hence, unusual atmospheric disturbances would result in heavier and more frequent showers.
Fig. 78—Wind roses for Callao. The figures for the earlier period (1897-1900) are drawn from data in the Boletín de la Sociedad Geográfica de Lima, Vols. 7 and 8, 1898-1900: for the latter period data from observations of Captain A. Taylor, of Callao. The diameter of the circle represents the proportionate number of observations when calm was registered.