| FREQUENCY OF THE DIURNAL VARIABILITY, SANTA LUCIA, 1913-14 | ||||||||||||
| Degrees F. | May | June | July | Aug. | Sept. | Oct. | Nov. | Dec. | Jan. | Feb. | March | Total No. of days |
| 0 | — | 2 | 6 | 3 | 4 | 6 | 2 | — | 1 | — | 2 | 26 |
| 0-1 | 2 | 7 | 7 | 5 | 6 | 4 | 8 | 12 | 14 | 9 | 5 | 79 |
| 1-2 | 11 | 5 | 7 | 11 | 7 | 8 | 5 | 5 | 4 | 9 | 13 | 85 |
| 2-3 | 2 | 8 | 8 | 9 | 3 | 7 | 7 | 5 | 5 | 4 | 6 | 64 |
| 3-4 | 4 | 4 | 2 | 1 | 4 | 1 | 3 | 6 | 2 | 4 | 2 | 33 |
| 4-5 | 1 | 3 | 1 | — | 2 | 1 | 3 | — | 2 | 1 | 1 | 15 |
| Over 5 | — | 1 | — | 2 | 4 | 4 | 2 | 2 | 3 | 1 | — | 19 |
| Days per month | 20 | 30 | 31 | 31 | 30 | 31 | 30 | 30 | 31 | 28 | 29 | 321 |
If we take the means of the diurnal variations by months we have a still more striking curve showing how little change there is between successive days. June and December are marked by humps in the curve. They are the months of extreme weather when for several weeks the temperatures drop to their lowest or climb to their highest levels. Moreover, there is at these lofty stations no pronounced lag of the maximum and minimum temperatures for the year behind the times of greatest and least heating such as we have at lower levels in the temperate zone. Thus we have the highest temperature for the year on December 2, 70.4° F. (21.3° C.), the lowest on June 3, 0.2° F. (—17.7° C.). The daily maxima and minima have the same characteristic. Radiation is active in the thin air of high stations and there is a very direct relation between the times of greatest heat received and greatest heat contained. The process is seen at its best immediately after the sun is obscured by clouds. In five minutes I have observed the temperature drop 20° F. (11.1° C.) at 16,000 feet (4,877 m.); and a drop of 10° F. (5.6° C.) is common anywhere above 14,000 feet (4,267 m.). In the curves of daily maximum and minimum temperatures we have clearly brought out the uniformity with which the maxima of high-level stations rise to a mean level during the winter months (May-August). Only at long intervals is there a short series of cloudy days when the maximum is 10°-12° F. (5.6°-6.7° C.) below the normal and the minimum stands at abnormally high levels. Since clouds form at night in quite variable amounts—in contrast to the nearly cloudless days—there is a far greater variability among the minimum temperatures. Indeed the variability of the winter minima is greater than that of the summer minima, for at the latter season the nightly cloud cover imposes much more stable atmospheric temperatures. The summer maxima have a greater degree of variability. Several clear days in succession allow the temperature to rise from 5°-10° F. (2.8°-5.6° C.) above the winter maxima. But such extremes are rather strictly confined to the height of the summer season—December and January. For the rest of the summer the maxima rise only a few degrees above those of the winter. This feature of the climate combines with a December maximum of rainfall to limit the period of most rapid plant growth to two months. Barley sown in late November could scarcely mature by the end of January, even if growing on the Argentine plains and much less at an elevation which carries the night temperatures below freezing at least once a week and where the mean temperature hovers about 47° F. (8.3° C.). The proper conditions for barley growing are not encountered above 13,000 to 13,500 feet and the farmer cannot be certain that it will ripen above 12,500 feet in the latitude of Santa Lucia.
The curve of mean monthly temperatures expresses a fact of great importance in the plant growth at high situations in the Andes—the sharp break between the winter and summer seasons. There are no real spring and autumn seasons. This is especially well shown in the curve for non-periodic mean monthly range of temperature for the month of October. During the half of the year that the sun is in the southern hemisphere the sun’s noonday rays strike Santa Lucia at an angle that varies between 0° and 16° from the vertical. The days and nights are of almost equal length and though there is rapid radiation at night there is also rapid insolation by day. When the sun is in the northern hemisphere the days are shortened from one to two hours and the angle of insolation decreased, whence the total amount of heat received is so diminished that the mean monthly temperature lies only a little above freezing point. In winter the quiet pools beside the springs freeze over long before dark as the hill shadows grow down into the high-level valleys, and by morning ice also covers the brooks and marshes. Yet the sun and wind-cured ichu grass lives here, pale green in summer, straw-yellow in winter. The tola bush also grows rather abundantly. But we are almost at the upper limit of the finer grasses and a few hundred feet higher carries one into the realm of the snowline vegetation, mosses and lichens and a few sturdy flowering plants.
For convenience in future comparative studies the absolute extremes are arranged in the following table:
