34 BOTANY OF THE DEATH VALLEY EXPEDITION. 



fall, soon shows its effect in the springing up of the annual vegetation 

 Little rain falls in April, and during that month the annuals mature' 

 their fruit and die, while the shrubs prepare themselves for the coming 

 drought of the hot mouths. 



The source of the moisture which gives rise to the rainfall of the des- 

 ert region, has been made the subject of an interesting contribution to 

 meterological knowledge by Lieut. J. P. Finley. The scantiness of its 

 rain supply is due to its form and situation — a broad, depressed basin far 

 removed from oceanic waters and nearly surrounded by high mountains. 

 Whatever moisture is carried toward this basin by the east winds from 

 the Gulf of Mexico, is precipitated upon the high plateaus and mountains 

 of Texas and New Mexico; while the west winds from the Pacific de- 

 posit their surplus moisture largely upon the coast ranges of California 

 and more completely in the cold : high altitudes of the Sierra Nevada, 

 the San Bernardino, and the San Jacinto mountains. The only channel, 

 therefore, through which the wind can bring oceanic moisture into the 

 desert, is the broad valley of the lower Colorado River, connecting with 

 the Gulf of California; and this is the source of the winter rain storms, 

 characterized by their slow formation, wide extent, and duration usually 

 of two to four days. 



The summer rains are quite different in character and origin. They 

 are showers or thunderstorms varying in length from a few minutes to 

 several hours, and confined within very narrow limits. Occasionally they 

 are intensified into furious cloud-bursts. In Death Valley these rains- 

 are of comparatively frequent occurrence, twenty-two being recorded 

 during the five months' stay of the weather observer. In several cases 

 only a trace of rain fell, and the greatest fall in any single instance 

 amounted to only .54 inch, the total for the five months being, as already 

 stated, only 1.4 inches. 



The moisture that forms these rains is believed by Lieut. Finley to 

 be derived immediately from the snow upon the higher peaks of the 

 desert, deposited there during the long winter storms. The amount of 

 each fall of rain in summer is so slight that the dry, hot air quickly reab- 

 sorbs it. Its effect upon the growth of vegetation is therefore in almost 

 all cases inappreciable. 



Perhaps the most significant of the meteorological observations taken 

 in Death Valley, so far as they relate to the growth of vegetation, were 

 those on the humidity of the air. The amount of moisture in any at- 

 mosphere is usually expressed in terms of relative humidity, or the pro- 

 portion of the actual amount of moisture present to the amount which 

 would render the air, at the same temperature, saturated. The average 

 relative humidity, at 5 o'clock in the afternoon, for the five months 

 covered by the observations, was 15.G per cent; for July, 12.6" per cent; 

 - and for August, 13 per cent; while on the 4th and 5th of August a 

 minimum of only 5 per cent was reached. These records of the rela- 

 tive humidity in Death Valley are decidedly lower than those of any 



