During the southern winter the cloud bank of the coast is best developed and precipitation is greatest. At Lima, for instance, the clear skies of March and April begin to be clouded in May, and the cloudiness grows until, from late June to September, the sun is invisible for weeks at a time. This is the period of the garua (mist) or the “tiempo de lomas,” the “season of the hills,” when the moisture clothes them with verdure and calls thither the herds of the coast valleys.
Fig. 84—Cloudiness at Callao. Figures are drawn from data in the Boletín de la Sociedad Geográfica de Lima, Vols. 7 and 8, 1898-1900. They represent the conditions at three observation hours during the summers (Dec., Jan.) of 1897-1898, 1898-1899, 1899-1900 and the winters (June, July) of 1898 and 1899.
During the southern summer on account of the greater relative difference between the temperatures of land and water, the sea-breeze attains its maximum strength. It then accomplishes its greatest work in the desert. On the pampa of La Joya, for example, the sand dunes move most rapidly in the summer. According to the Peruvian Meteorological Records of the Harvard Astronomical Observatory the average movement of the dunes from April to September, 1900, was 1.4 inches per day, while during the summer months of the same year it was 2.7 inches. In close agreement are the figures for the wind force, the record for which also shows that 95 per cent of the winds with strength over 10 miles per hour blew from a southerly direction. Yet during this season the coast is generally clearest of fog and cloud. The explanation appears to lie in the exceedingly delicate nature of the adjustments between the various rain-making forces. The relative humidity of the air from the sea is always high, but on the immediate coast is slightly less so in summer than in winter. Thus in Mollendo the relative humidity during the winter of 1895 was 81 per cent; during the summer 78 per cent. Moreover, the temperature of the Coast Range is considerably higher in summer than in winter, and there is a tendency to reëvaporation of any moisture that may be blown against it. The immediate shore, indeed, may still be cloudy as is the case at Callao, which actually has its cloudiest season in the summer but the hills are comparatively clear. In consequence the sea-air passes over into the desert, where the relative increase in temperature has not been so great (compare Mollendo and La Joya in the curve for mean monthly temperature), with much higher vapor content than in winter. The relative humidity for the winter season at La Joya, 1895, was 42.5 per cent; for the summer season 57 per cent. The influence of the great barrier of the Maritime Cordillera, aided doubtless by convectional rising, causes ascent of the comparatively humid air and the formation of cloud. Farther eastward, as the topographic influence is more strongly felt, the cloudiness increases until on the border zone, about 8,000 feet in elevation, it may thicken to actual rain. Data have been selected to demonstrate this eastern gradation of meteorological phenomena.