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temperature over the land cause the daily fluctuation in these winds. Ordinarily the wind from the ocean begins to blow inland during the forenoon and increases in force until late in the afternoon, when it gradually subsides. This is known as the sea breeze. During the night there is often a light breeze seaward, known as the land breeze. The sea and land breezes are but local modifications of the general drift of the atmosphere, and depend principally upon the difference in temperature between the land and the sea. In valleys that open toward the northwest and extend far inland between high mountain ranges, as the Salinas Valley, the sea winds are usually strong, and during the summer season often violent.
In the accompanying tables are given anemometer records of the total wind movement in miles per day at Salinas from January 1, 1902, to March 13, 1903, and at Kings City from January 1, 1902, to March 14, 1903. The greater part of this wind movement is up the Salinas Valley and takes place during the afternoon.
Total (laily urind morement, in miles, at Kings City-('ontinuel.
When the passage winds reach the coast they sometimes, under proper conditions, precipitate a portion of their moisture. One of these conditions is cooling of the air by contact with a cool land area or by being forced to a great altitude in passing over mountain ranges, for the cold summits of mountains are effective condensers of moisture. Such conditions do not, however, occur along the coast of Monterey County, for the general movement of the air currents there is from the northwest to the southeast, parallel with and not across the mountain ranges. In addition to this the air currents in their southeastward drift are continually passing into a warmer region, where the temperature of the air is raised and its capacity for absorbing moisture increased. Thus the passage winds, although moist and cool on the coast, become dry in the interior and abstract moisture by evaporation from every stream and lake, from the soil, and from all vegetation. These northwest winds bring no moisture to the land along the coast south of San Francisco.
The above-described climatic conditions are practically constant, recurring from day to day throughout the year unless modified by general atmospheric disturbances accompanying regional storms. As is well known, in cyclonic storms of vast extent the storm center is an area of low barometer toward which the winds blow from every direction. The path of these storms shifts with the seasons, moving southward during the autumn and reaching the coast of Washington and Oregon in October or November. Many pass eastward without bringing rain to central or southern California; others, however, are deflected southward by atmospheric conditions east of the Cascade Mountains and the Sierra Nevada and pass over California, Nevada, and Arizona. These storms are preceded by southeast, south, or southwest winds along the coast regions of California, or toward the storm center in the northern part of the State. The general direction of these storm winds is nearly at right angles to the general drift of the passage winds and across the trend of the mountain ranges of the State. This marked change in the direction of the wind is an important factor in the rainfall on the Pacific coast. These moist southerly winds pass from a warm to a cooler region and are cooled sufficiently to cause precipitation, thus giving rain over sea and land.
These moisture-laden storm winds from the Pacific, on encountering the mountain ranges along the coast and in the interior, are forced to rise to a considerable altitude, where they are still further cooled and precipitate a large part of their moisture, leaving the regions to the east of the mountain ranges dry and barren.
Aridity, or rather the unequal distribution of moisture, is largely the result of topography or inequalities of land surface. If the land were perfectly flat, it is probable that the winds, meeting with no obstructions, would distribute the rains with considerable uniformity in broad bands approximately parallel to the equator; but the relatively thin layer of dense atmosphere surrounding the globe is disturbed in its uniform flow by the lofty mountain masses which traverse the continents. The atmosphere surrounding the earth extends outward many miles, but it is the layer, a mile or two in thickness, resting immediately upon the surface, and relatively dense, within which occur the changes or disturbances that make up what we know as “weather.” The movements of the air above this thin layer concern us little, but the behavior of the clouds and winds near the surface of the ground brings success or failure to the farmer, and affects more or less directly other industries, and even health.a
The marked influence of topography on the distribution of rainfall is strikingly shown in fig. 1. This diagram show's graphically the increase in rainfall due to difference in altitude on the west side of the Sierra Nevada and the no less pronounced decrease on the east side, indicating how completely the moisture-laden winds have been robbed of their moisture by the cold mountain range. This diagram consists of a profile or section drawn along a line from Farallon Islands through San Francisco, across the Sacramento Valley and Sierra Nevada, and out on the Great Basin as far as Wadsworth. The various stations along this profile and their altitude above sea level are shown. The height of the small black columns represents the average annual rainfall in inches at each station.
a Newell, F. H., Irrigation, p. 16.