Fig. 76—Temperatures at Callao, June-September, 1912, from observations taken by Captain A. Taylor, of Callao. Air temperatures are shown by heavy lines; sea temperatures by light lines. In view of the scant record for comparative land and water temperatures along the Peruvian coast this record, short as it is, has special interest.
The hydrographic feature of greatest importance is the Humboldt Current. To its cold waters is largely due the remarkably low temperatures of the coast.[21] In the latitude of Lima its mean surface temperature is about 10° below normal. Lima itself has a mean annual temperature 4.6° F. below the theoretical value for that latitude, (12° S.). An accompanying curve shows the low temperature of Callao during the winter months. From mid-June to mid-September the mean was 61° F., and the annual mean is only 65.6° F. (18° C.). The reduction in temperature is accompanied by a reduction in the vapor capacity of the super-incumbent air, an effect of which much has been made in explanation of the west-coast desert. That it is a contributing though not exclusive factor is demonstrated in Fig. 77. Curve A represents the hypothetical change of temperature on a mountainous coast with temporary afternoon onshore winds from a warm sea. Curve B represents the change of temperature if the sea be cold (actual case of Peru). The more rapid rise of curve B to the right of X-X′, the line of transition, and its higher elevation above its former saturation level, as contrasted with A, indicates greater dryness (lower relative humidity). There has been precipitation in case A, but at a higher temperature, hence more water vapor remains in the air after precipitation has ceased. Curve B ultimately rises nearly to the level of A, for with less water vapor in the air of case B the temperature rises more rapidly (a general law). Moreover, the higher the temperature the greater the radiation. To summarize, curve A rises more slowly than curve B, (1) because of the greater amount of water vapor it contains, which must have its temperature raised with that of the air, and thus absorbs energy which would otherwise go to increase the temperature of the air, and (2) because its loss of heat by radiation is more rapid on account of its higher temperature. We conclude from these principles and deductions that under the given conditions a cold current intensifies, but does not cause the aridity of the west-coast desert.
