Colorado Climatology 5 
coming over the mountains, the air being warmed by compression 
during the descent to the foothills and plains, the mean temperature 
is raised considerably, and the capacity of the air for moisture is in¬ 
creased, so that there prevails in the eastern half of the State a long 
succession of relatively dry, warm and bright sunshiny days. Dur¬ 
ing the summer months a low pressure is prevalent over the Salt 
Lake region, causing little precipitation, but this area of low pres¬ 
sure causes easterly winds east of the mountains and the air which 
is drawn up the slopes of the mountains becomes chilled by the eleva¬ 
tion and causes precipitation during the warmer half of the year. 
Though distant, the influence of the Gulf of Mexico is felt to a 
varying extent. During the summer months when there is a gen¬ 
eral stagnation in the movement of the northern low pressure areas, 
sufficient time is afforded for moisture to be brought to the eastern 
slope, which causes increased precipitation east of the mountains 
during the warmer half of the year. The difference in the tempera¬ 
ture of the two slopes is quite marked in the winter months when 
cold waves from north of Montana sweep southward along the 
eastern slope of the Rocky Mountains, but do not pass to the west¬ 
ern slope, as the continental divide is an effective barrier. In the 
areas of high pressure or anti-cyclones, the greatest cold is generally 
confined to the lower stratum of air whose upper limit does not 
always reach as high as the mountain tops. As a matter of fact 
during these periods of low temperatures the adjacent high altitude 
stations experience relatively moderate temperatures, which condn 
tion, if not already in existence in the western valleys, is soon in 
evidence. Cold waves are the result of the transferring of cold air 
from the north, often increased by the radiation in the clear dry air. 
The continental divide is also effective in moderating the win¬ 
ter temperatures of the eastern slope. When the distribution of 
the pressure is favorable to westerly winds, remarkable rises of 
temperature occur. These are called “chinook” winds. That the 
“chinook,” or warm winds, blowing from the snow-covered moun¬ 
tains should be so warm and dry is explained by the fact that the 
air as it is forced up the western slope, owing to the high elevation, 
is unable to hold the moisture it contains and precipitation occurs. 
Its latent heat is liberated, so that the air reaches the top of the 
mountains colder but relatively warmer than when it began its 
ascent, and when in descending it is compressed, it reaches us as a 
warm, dry wind. Its effect in evaporating the snow on the plains 
has been the salvation of many herds of stock that must otherwise 
have perished. Locally they are accompanied by a low barometer, 
and soon a long wind cloud is seen like a banner lying close to the 
mountains, and later the wind springs up and the chinook is at hand. 
Chinooks are liable to occur at all seasons of the year, but the 
