232 



show its effect, from the fact that the force of the wind, \Yith 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 

 would have in summer, the windy season in many regions. It is 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 small sharp stones, caught up by them from the dry, loose, 

 gravelly soil, and hurled with^uch force against the trunk 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 trees has been 

 often found scraped entirely off on the north and west sides after such 

 a wind storm. 



cleae>:ess of the skies. 



Clearness of the skies is a direct result of the low humidity of this 

 climate, and is one of the peculiar features of high altitude. Eapid 

 radiation from the earth, the absence of moist currents in the air, the 

 rarity of the atmosphere, all these prevent the formation of haze, mist, 

 or clouds. The table gives the average number of dnys clear, fair, and 

 cloudy, in each month, at Denver. In these records the observer counts 

 the thinnest cloud, and many days are classed as cloudy or fair which 

 wotild be generally termed fair or clear. The record of sunny days 

 gives a better idea of the fine weather. Ar Denver the records also 

 show that the average number of days inji year on which rain or snow 

 fell is eighty-one, and the yearly average cloudiness in tenths of the 

 sky is 2.6. During thirteen years the average number of days during 

 the year on which the sun was not visible was three, and from October 

 30, 1870. to February 5, ISSl, fifteen months, 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 eft'ect 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 the dry, 

 rarified air offers to the rays of the sun. Vrhile 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 tban 

 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 



