Rainfall increases in frequency and amount. The average rainfall of Copiapé may be given as about 20mm. The average for the 26 year period 1888-1913 was 22 mm.[1] I copied the records of the Meteorological Observatory of Copiapé for this period and give them in Table I.
East of the Copiapé valley the rainfall increases with ele- vation on the western flank of the Andes. Bordering the valley are the outer ranges of the cordillera, which in summer have heavier rainfall than the lower desert and which occasionally have snow in the winter season. The effect is clearly seen in the vegetation and belts of settlement. The desert sands and bare rock surfaces or pebble pavements (Figs. 4, 5, and 15) give way to grass-covered tracts where the mountain streams debouch (Fig. 16); and higher up are the pajonales where bunch grass and shrubs and a thin scattered growth of succulent grasses come in after the summer rains (Fig. 17). Each important stream has its clump of huts, and the largest streams are marked by villages or towns no matter how remote the situation may be. Each pasture tract is annually invaded and explored by the migratory shepherds in the summer season.
In addition, there is a heavy belt of fog which during most of the year hangs on the western slope of the Coast Range and even extends inland, so that a certain amount of moisture is collected mechanically from the fog and furnishes additional moisture for lowly desert shrubs and grasses. South of Copiapé the rains increase in number, and the rainfall in- creases in amount per shower as well, so that in the one hun- dred miles from Copiapé to Vallenar the rainfall has increased to 80 mm. a year. The latter place has at least twice as many showers and four times as much rain as the former.
Even in the northern section of this transition zone, between Copiapé and Huasco, it is unusual to find two successive years absolutely rainless, although there may be a period of six or seven years with very little rain; but it must be impressed that
- ↑ As already indicated widcly different “averages” will be obtained for desert rainfalls according to the observation period. The official figure for the average rainfall of Copiapé for 23 years between 1870 and Lor§ is 17.8 mm. (Recopilaciin de sumas de agua caida en Chile 1849-1915, Inst. Meteorol. y Geofisico de Chile, Seccién Liuvias, Publ. No. 20, Santiago, 1917-)
