62 
GREELY. 
the valley of the lower Colorado river, which are represented 
best by the records of Yuma. This barometric depression 
naturally induces on its eastern side the southerly vapor¬ 
bearing winds characteristic of these territories from June to 
September. The resulting rain falls partly by the cold of 
elevation and partly through feeble offshoots of low pressure 
which drift eastward from the permanent depression. 
As is well known, the barometric gradient between San 
Diego, California, and Yuma, Arizona, is greatest from July 
to September, inclusive, being then nearly three times as 
steep as from November to January. To the eastward quite 
marked contrasts, the strongest of the year, likewise exist 
between the summer pressures of Yuma and Fort Grant, 
Arizona. 
The effect of this peculiar distribution of atmospheric 
pressure from July to September upon the weather conditions 
of the extreme western part of Texas is plainly evident from 
the observations of the prevailing direction of the wind at 
El Paso for three years, selected at hazard, which show that 
24 per centum of the recorded directions of the wind from 
July to September, inclusive, the rainy months of the section, 
were from the east and 19 per centum more from the south¬ 
east or southwest. An examination of wind and pressure 
charts for the summer months of several years leaves no doubt 
as to the southerly atmospheric indraft on the eastern side 
of the Yuma barometric depression. The system of southerly 
winds inaugurated in this manner are more or less charged 
with aqueous vapor from the Gulf of California, which is 
situated within a moderate distance of southern Arizona, 
say from one to three hundred miles to the south or south¬ 
west. 
In consequence of these conditions of pressure, wind, and 
humidity, there obtains over a great part of southeastern 
Arizona an abnormal rainfall during August, which, exceeds 
on an average six inches over very considerable areas, and 
in favorable mountainous localities of Arizona and New 
Mexico even attains eight inches or more. From this great 
