Fig. 79—Wind roses for Mollendo. The figures are drawn from data in Peruvian Meteorology (1892-1895), Annals of the Astronomical Observatory of Harvard College, Vol. 30, Pt. 2, Cambridge, Mass., 1906. Observations for an earlier period, Feb. 1889-March 1890, (Id. Vol. 39, Pt. 1, Cambridge, Mass. 1890) record S. E. wind at 2 p. m. 97 per cent of the observation time.
Fig. 80—Wind roses for the summer and winter seasons of the years 1911-1913. The diameter of the circle in each case shows the proportion of calm. Figures are drawn from data in the Anuario Meteorológico de Chile, Publications No. 3, (1911), 6 (1912) and 13 (1913), Santiago, 1912, 1914, 1914.
Turning now to local factors we find on the west coast a regional topography that favors a diurnal periodicity of air movement. The strong slopes of the Cordillera and the Coast Range create up-slope or eastward air gradients by day and opposite gradients by night. To this circumstance, in combination with the low temperature of the ocean water and the direction of the prevailing winds, is due the remarkable development of the sea-breeze, without exception the most important meteorological feature of the Peruvian Coast. Several graphic representations are appended to show the dominance of the sea-breeze (see wind roses for Callao, Mollendo, Arica, and Iquique), but interest in the phenomenon is far from being confined to the theoretical. Everywhere along the coast the virazon, as the sea-breeze is called in contradistinction to the terral or land-breeze, enters deeply into the affairs of human life. According to its strength it aids or hinders shipping; sailing boats may enter port on it or it may be so violent, as, for example, it commonly is at Pisco, that cargo cannot be loaded or unloaded during the afternoon. On the nitrate pampa of northern Chile (20° to 25° S.) it not infrequently breaks with a roar that heralds its coming an hour in advance. In the Majes Valley (12° S.) it blows gustily for a half-hour and about noon (often by eleven o’clock) it settles down to an uncomfortable gale. For an hour or two before the sea-breeze begins the air is hot and stifling, and dust clouds hover about the traveler. The maximum temperature is attained at this time and not around 2.00 P. M. as is normally the case. Yet so boisterous is the noon wind that the laborers time their siesta by it, and not by the high temperatures of earlier hours. In the afternoon it settles down to a steady, comfortable, and dustless wind, and by nightfall the air is once more calm.