2 
232 
show its effect, from the fact that the force of the wind, with any given 
velocity, is less in the light air of this altitude than at lower elevations. 
Another advantage here is that the strongest winds come in winter or 
early spring, when they have less effect on the leafless trees than they 
ould have in summer, the windy season in many regions. Itis very rare 
to find trees blown down, and seldom are branches torn off. The specially 
injurious effect of extreme winds upon trees in Colorado is from the 
gravel, or smali sharp stones, caught up by them from the dry, loose, 
gravelly soil, and hurled with such force against the ie and branches 
as to cut the bark, and give it the appearance of having been rubbed 
down by some rough substance. The bark of young ieee has been 
often found scraped entirely oi on the north and west sides after such 
a wind storm. 
CLEARNESS OF THE SKIES. 
Ciearness of the skies is a direct result of the low humidity of this 
climate, and is one of the peculiar features of high altitude. Rapid 
Feuneaian from the earth, the absence of moist currents in the air, the 
rarity of the WET OSDHERS, all these prevent the formation of haze, mist, 
or clouds. The table gives the average number of days clear, fair, and 
eloudy, in each month, at Denver. In these records the observer counts 
tlre thinnest cloud, and many days are classed as cloudy or fair which 
would be generaily termed fair or clear. The record of sunny days 
gives a better idea of the fine weather. Aw Denver the records also 
show that the average number of days in a year on which rain or snow 
fellis eighty-one, and the yearly average cloudiness in tenths of the 
sky is2.6. During thirteen years the average number of days during 
the year on which the sun was not visible was three, and from October 
30, 1879, to February 5, 1581, fifteen mouths, the sun was not obscured 
an entire day..- 
This clearness of the sky is one cause for the daily fluctuations of 
temperature already noted, and its effect on solar and terrestial radia- 
tion is very marked. Solar radiation at this high altitude is rapid 
soon after sunrise, because of the slight resistance which tae dry, 
rarified air offers to the rays of the sun. While after sunset terrestial 
radiation is also rapid because there is no moist envelope shrouding the 
face of the earth to prevent the natural cooling of the dry ground. 
This is one cause for the late frosts which are so hurtful to trees and 
which farmers and cultivators never expect without a clear sky and 
dry air. Sunlight in Colorado is a nearer approach to white light than 
at the sea-level, and many persons find it necessary to wear blue or 
smoked glasses to protect their eyes against its effects. 
Sunlight plays a very important part in the development of plants, 
and the various questions of the sun’s rays, their quantity and action, 
their variations in the different hours and for different states of the 
sky, and the relative effects of their different elements, should be the 






