241 



east side of the Rocky Mountains, when tlie temperature at Denver sometimes sinks 

 more than 20° below zero, there prevails in the Salt Lake basin an average tempera- 

 ture of about 30° ; and when by changes of atmospheric pressure this air is carried 

 over the mountains it may reach Denver with a temperature of 50°, resulting from a 

 precipitation of vapor on the mountains. We thus find a mass of air having a tem- 

 perature of -f 50° in close proximity to a mass having a temperature of —20°, and 

 by the movements of the atmosphere attending the progress of a great storm these 

 different masses of air may be brought successively over the same station, causing a 

 change of temperature of 50° in a single hour.' 



'^The following tables present numerical accounts of these changes from the same 

 authority ) 



Temperature and relative humidity, January, 1875. 







Salt Lake. 



Cbeyenue. 



Denver. 



Pike's 

 Peak. 



North 

 Platte. 





Hour 



(Washing- 













1 



Date. 



k <a>> 





<s >. 









!0 >-. 



cS 



1 





ton time). 



u 



t'.'S 



u 



1^ 



Put 





as 



m 



S.2 



43 'O 









n 



i' 



ll 



a s 





as 



ii 



a^ 



H 



a 







o 



Pr. ct. 



o 



Fr. ct. 



o 



Pr. ct. 



o 



Pr. ct. 



o 



Pr. ct. 



January 14 



7.35 



34 



61 



-11 



76 



-14 



100 



1 



100 



-13 



74 





4.35 



43 



50 



- 1 



68 



- 4 



64 



8 



100 



- 6 



61 





11.00 



43 



50 



24 



60 



1 



71 



5 



100 



-11 



76 



January 15 



7.35 



32 i 84 



28 



67 



43 



21 



6 



100 



- 8 



79 





4.35 



26 52 



5 



75 



10 



17 



12 



100 



- 1 



68 





11.00 



23 73 



-6 



61 



13 



. 44 



10 



100 



- 4 



64 



''Salt Lake, at the western foot of the Wahsatch range, in the Great Basin, is con- 

 stantly rather warm, and of ordinary humidity. Pike's Peak is cold and constantly 

 at its dew point. North Platte, on the plains east of the mountains, is much colder 

 and distinctly drier. Denver and Cheyenne are much alike in changes of tempera- 

 ture, as these stations are about equally distant from the eastern foot of the mountains ; 

 but Denver shows much the greater decrease in relative humidity, perhaps because 

 the range west of this station is higher than farther north. The second table shows 

 in greater detail how violent the changes of temperature were at Denver and how 

 completely they accorded with changes in the wind : 



Temperature and ivind, Denver, January 14, 15, 1875. 



Date. 



Hour. 



Tempera- 

 ture. 



Wind. 



NE, 

 NE. 

 SW. 

 SW. 



sw. 

 sw. 

 sw. 

 sw. 



NE. 



January 14 



2.43 p.m. 



9.00 p.m. 



9.15 p.m. 



9.20 p.m. 



9.30 p.m. 



9.35 p.m. 



5.43 a. m. 

 10.30 a.m. 

 12.30 p.m. 



2.43 I), m. 



o 



+i 



27 

 36 

 40 

 43 

 52 

 4 

 + 10 







"An observer who was considered perfectly reliable says that between 11 a. m. and 

 noon the thermometer fell from 58 to 22 (that is, 36°) m five minutes." (Amer. Journ. 

 Science, 1875, x, 12; 1880, xxii, 13, 14.) 



Any one who wishes to understand the climate of the Rocky Mountains will also 

 do well to read an excellent paper on the Chinook winds by Prof. M. W. Harrington 

 24738— Bull 2 -16 



