Fig. 89—Cloudiness at the desert station of [Fig. 88] (near Caraveli), for the summer (January to March) of 1913.
Besides the seasonal variations of precipitation there are longer periodic variations that are of critical importance on the Coast Range. At times of rather regular recurrence, rains that are heavy and general fall there. Every six or eight years is said to be a period of rain, but the rains are also said to occur sometimes at intervals of four years or ten years. The regularity is only approximate. The years of heaviest rain are commonly associated with an unusual frequency of winds from the north, and an abnormal development of the warm current, El Niño, from the Gulf of Guayaquil. Such was the case in the phenomenally rainy year of 1891. The connection is obscure, but undoubtedly exists.
The effects of the heavy rains are amazing and appear the more so because of the extreme aridity of the country east of them. During the winter the desert traveler finds the air temperature rising to uncomfortable levels. Vegetation of any sort may be completely lacking. As he approaches the leeward slope of the Coast Range, a cloud mantle full of refreshing promise may be seen just peeping over the crest (Fig. 91). Long, slender cloud filaments project eastward over the margin of the desert. They are traveling rapidly but they never advance far over the hot wastes, for their eastern margins are constantly undergoing evaporation. At times the top of the cloud bank rises well above the crest of the Coast Range, and it seems to the man from the temperate zone as if a great thunderstorm were rising in the west. But for all their menace of wind and rain the clouds never get beyond the desert outposts. In the summer season the aspect changes, the heavy yellow sky of the desert displaces the murk of the coastal mountains and the bordering sea.