National Geographic : 1907 Apr
THE SALTON SEA annual hourly velocity of the wind at Yuma is nearly seven miles per hour, 3.1 meters per second, and the range is from an average velocity of three or four miles in the early morning hours to eight or ten miles in the afternoon. At Furnace Creek, in Death Valley, an independent north-south basin, an average wind veloc ity of 9.9 miles per hour, 4.5 meters per second, was recorded from May to Sep tember, inclusive, but here the force of the wind is doubtless augmented by the local topography, and the results are not of general application. In general, it seems reasonable to assume that while there is more or less interchange of air between different portions of the desert, there is no permanent flow of the surface air in any direction except in winter, when the Plateau region is occupied by an area of high pressure. Then the winds blow from the north with much steadi ness, so long as the Plateau high exists. THE VAPOR CONTENTS OE THE AIR The moisture contents of the winds, especially at Yuma, are surprisingly con stant. The north wind, since it descends from somewhat higher levels, is, in gen eral, a dry wind, yet in the winter season the greatest relative humidity of the month may be experienced with a north wind. The moisture contents of the dif ferent winds for a winter month (Feb ruary) and a summer month (August) are shown in the following table: Vapor Tension at Yuma, Arizona. (An average of ten years.) Direction. North . ............ Northeast.......... East............ Southeast. ...... .. South .... ....... . Southwest.......... W est .............. Northwest ......... February. Inches. o.16 0.20 0.20 0.25 0.21 0.22 0.21 0.20 August. Inches. 0.57 0.59 0.67 0.67 o.6o 0.55 0.56 0.54 AND RAINFALL 247 The amount of aqueous vapor actually present in the air may be expressed either by the expansive force or pressure that it exerts or by its weight in grains in a cubic foot of space. In the above ex ample it is stated in terms of its expan sive force, or barometric pressure, in inches of mercury. Whether expressed in terms of weight or pressure, the amount of vapor actually present is some times called the absolute humidity. It is very important to distinguish between the absolute humidity and the relative humid ity, sometimes referred to merely as the humidity. The relative humidity is the ratio of the amount of vapor actually present to that which might be present at the existing temperature if fully satu rated: Example from Death Valley, June, 1891, temperature of dry bulb, o108° F., wet bulb, 68° F., whence is obtained from hygrometric tables: dew-point, 39° F., relative humidity, io per cent. The observation quoted means, first, that in order to condense any of the moisture present into dew or rain the temperature would have to fall 69° (from 108° to 39° F.), or the amount of moisture then in the air would have to be increased ten fold. This point cannot be emphasized too strongly. At the temperatures which exist in the Colorado Desert, and under the general conditions of aridity which prevail, the atmosphere takes up vapor as a sponge absorbs water. It should be remembered, moreover, that the capacity of the air for vapor is vastly greater at high than at low temperatures; the prob lem in the Southwest, therefore, so far as the production of rain is concerned, is not essentially one of increasing the vapor contents of the air, but rather of dimin ishing the temperature to the point at which condensation takes place. There is sufficient moisture in the air to produce abundant precipitation if means of cool ing it were at hand. The absolute hu midity at Yuma is slightly greater than that of St. Louis, and only a little less than that of Vicksburg, both of which points have, in general, an abundance of rain and a so-called moist atmosphere.