CHAPTER X IY. 
Meteorology of the Field Explored. 
The meteorology of the field covered by the exploration is one of the most interesting, as it 
is one of the most important subjects of inquiry. It is not obstructed by deep snow, nor is 
the temperature as low as has been generally imagined. There is a great depression in the 
whole mountain chain of the Rocky mountains, the higher plateaus being nearly three.thousand, 
and the lower two thousand feet above the sea; whereas, at the 41st parallel, the higher 
plateaus are six thousand, and the lower and more general ones are four thousand five hundred 
feet above the sea. The greatest elevation of the Rocky mountains is south of the South Pass, 
in latitude 39° to 40°, where the Platte, the Rio Grande, the Arkansas, and the Colorado of the 
Gulf of California have their rise. The mountain chain then rapidly declines to near the 48th 
parallel. The temperature of the Rocky mountains at this parallel is as mild as any part 
down to the 35th parallel of latitude. Moreover, on the western coast, the prevailing westerly 
winds and the currents of the Pacific ocean, similar to, though less known than the Gulf 
Stream, have modified the climate to such a degree that the isothermal lines run nearly parallel 
to the coast, making the climate of Puget sound nearly if not quite as mild as that of San 
Francisco, and causing it to correspond with that of the western coast of Europe in the same 
latitude. Puget sound and Vancouver’s island are strikingly like Ireland and West Shetland in 
temperature; the first locality having a mean temperature for July of 65°, while Dublin has but 
60°, and the British islands range from 57° to 63°. At Sitka, in Russian America, the mean of 
winter at 35°, and the summer mean of 56°, correspond to the north of Ireland and Scotland. 
The effect of this amelioration of temperature not only extends to the Rocky mountains, but is 
felt on the eastern slope and for some distance on the plains, where, in turn, it is met by the 
temperatures from the Atlantic sweeping over the vast interior continental areas, growing some¬ 
what colder till the western end of Lake Superior and the Red river settlement of the north is 
reached, and then growing milder till it meets the temperatures from the Pacific at an equilibrium. 
As regards the distribution of rain and snow, much of the moisture is deposited before reaching 
this high latitude, except on the coast, where there is a large local precipitation; and the Cas¬ 
cades mountains of Oregon and Washington arrest much of the rain that would be distributed 
farther in the interior, especially in winter; and, as a consequence, the Rocky mountains in that 
latitude have little winter precipitation, and the plains eastward have still less. The general 
plateau from the head of the Mississippi westward, to and including the Rocky mountains, has 
indeed the least winter precipitation of any portion of the continent, and can furnish no accu¬ 
mulation of snow from the two or three inches of water falling in a frozen state in the winter 
months. 
The latitude is too high up for a large amount of precipitation, except near the coast. The 
great summer precipitation of the upper portion of the Mississippi valley shows the line of pro¬ 
fuse rains to be at its farthest point northward there at that season of the year. On the plains 
it extends farther north into British America, and on the coast of the Pacific it stretches from 
Sitka northward nearly over the whole line of the coast. 
From these general facts of distribution of the water falling in rain and snow in the extreme 
seasons, the observed facts of the winter climate of the interior are seen to have merely their 
natural place. Little accumulation of snows can exist in the interior of these latitudes, at what- 
