The Salton Sea and Rainfall 



247 



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 hotirs 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 OF 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) 

 arc shown in the following table: 



Vapor Tension at Yuma, Arizona. 



(An average of ten years.) 



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, 108° F., 

 wet bulb, 68° F., whence is obtained 

 from hygrometric tables : dew-point, 

 39° F., relative humidity, 10 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. 



