Fig. 100—Wind roses for Machu Picchu, August 20 to November 6, 1912.
The high percentage of northwest winds during afternoon hours is due to the up-valley movement of the air common to almost all mountain borders. The air over a mountain slope is heated more than the free air at the same elevation over the plains (or lower valley); hence a barometric gradient towards the mountain becomes established. At Machu Picchu the Canyon of Torontoy trends northwest, making there a sharp turn from an equally sharp northeast bend directly upstream. The easterly components are unrelated to the topography. They represent the trades. If a wind rose were made for still earlier morning hours these winds would be more faithfully represented. That an easterly and northeasterly rather than a southeasterly direction should be assumed by the trades is not difficult to believe when we consider the trend of the Cordillera—southeast to northwest. The observations from here down to the plains all show that there is a distinct change in wind direction in sympathy with the larger features of the topography, especially the deep valleys and canyons, the trades coming in from the northeast.
CLOUDINESS
It will be seen that the sky was overcast or a fog lay in the valley 53 per cent of the time at early morning hours. Even at noon the sky was at no time clear, and it was more than 50 per cent clear only 18 per cent of the time. Yet this is the so-called “dry” season of the valleys of the eastern Andes. The rainfall record is in close sympathy. In the 79 days’ observations rain is recorded on 50 days with a greater proportion from mid-September to the end of the period (November 6), a distinct transition toward the wet period that extends from December to May. The approximate distribution of the rains by hours of observation (7 A. M., 1 P. M., 7 P. M.) was in the ratio 4:3:6. Also the greatest number of heavy showers as well as the greatest number of showers took place in the evening. The rainfall was apparently unrelated to wind direction in the immediate locality, though undoubtedly associated with the regional movement of the moist plains air toward the mountains. All these facts regarding clouds and rain plainly show the location of the place in the belt of maximum precipitation. There is, therefore, a heavy cover of vegetation. While the situation is admirable for defence, the murky skies and frequent fogs somewhat offset its topographic surroundings as a lookout.
| ANALYTICAL TABLE OF THE STATE OF THE SKY, MACHU PICCHU, 1912 | ||||||||||||
| Morning | Total | Noon | Total | Evening | Total | |||||||
| Aug.-Sept. | Oct.-Nov. | Days | % | Aug.-Sept. | Oct.-Nov. | Days | % | Aug.-Sept. | Oct.-Nov. | Days | % | |
| Foggy | 3.0 | 14.0 | 17.0 | 28.4 | 1.0 | — | 1.0 | 2.6 | 1.0 | 2.0 | 3.0 | 4.3 |
| Overcast | 12.0 | 3.0 | 15.0 | 25.0 | 6.0 | 8.0 | 14.0 | 36.8 | 13.0 | 11.0 | 24.0 | 34.8 |
| 50-100% cloudy | 4.0 | 10.0 | 14.0 | 23.3 | 0.0 | 7.0 | 16.0 | 42.2 | 8.0 | 15.0 | 23.0 | 33.3 |
| 0-50% cloudy | 6.0 | 4.0 | 10.0 | 16.7 | 5.0 | 2.0 | 7.0 | 18.4 | 9.0 | 4.0 | 13.0 | 18.8 |
| Clear | 3.0 | 1.0 | 4.0 | 6.6 | 0.0 | 0.0 | 0.0 | 0.0 | 3.0 | 3.0 | 6.0 | 8.8 |
Santa Lucia [29]
Santa Lucia is a mining center in the province of Puno (16° S.), at the head of a valley here running northeast towards Lake Titicaca. Its elevation, 15,500 feet above sea level, confers on it unusual interest as a meteorological station. A thermograph has been installed which enables a closer study of the temperature to be made than in the case of the other stations. It is unfortunate, however, that the observations upon clouds, wind directions, etc., should not have been taken at regular hours. The time ranges from 8.30 to 11.30 for morning hours and from 2.30 to 5.30 for afternoon. The observations cover portions of the years 1913 and 1914.
TEMPERATURE
Perhaps the most striking features of the weather of Santa Lucia are the highly regular changes of temperature from night to day or the uniformly great diurnal range and the small differences of temperature from day to day or the low diurnal variability. For the whole period of nearly a year the diurnal variability never exceeds 9.5° F. (5.3° C.) and for days at a time it does not exceed 2-3° F. (1.1°-1.7° C.). The most frequent variation, occurring on 71 per cent of the total number of days, is from 0-3° F., and the mean for the year gives the low variability of 1.9° F. (1.06° C.). These facts, illustrative of a type of weather comparable in uniformity with low stations on the Amazon plains, are shown in the table following as well as in the accompanying curves.