26 
The Colorado Experiment Station 
to such plants as alfalfa. The wind movement in the winter bears 
a relation to plant growth. For example, there is ample snowfall 
in North Park, but the winds pile the snow into drifts, sweeping 
it from the level places. The exposure to the low temperatures 
thus brought about often makes conditions unfavorable for the 
wintering over of alfalfa, strawberries, etc. 
HUMIDITY 
The rate of water loss from the surfaces of plants is largely 
determined by the relative humidity of the atmosphere. The higher 
the relative humidity, that is, the nearer the air is to the point of 
saturation, the less rapid the rate of water loss (transpiration) ; 
the lower the relative humidity, the more rapid is the loss of water. 
For example, a plant growing in an atmosphere with a relative 
humidity of 50% will lose water at a much greater rate than one 
in the same kind of atmosphere having a relative humidity of 60% 
or 70%, providing other environmental conditions are approxi¬ 
mately the same. 
The relative humidity of the atmosphere is affected by the 
following conditions: i. 7 'emperature .—In two regions, each 
with the same precipitation, the warmer has the lower relative hu¬ 
midity, that is, is drier. High temperatures increase the capacity 
of the air for moisture, and hence decrease the relative humidity. 
The relative humidity of the air falls during the day, and rises dur¬ 
ing the night. It is nearly always higher at 6 a. m. than at 6 
p. m. For example, a ten-year average of the relative humidity at 
Denver for the year at 6 a. m. varies from 61% to 70%, and at 
6 p. m. from 37% to 48%. 2. Wind .—The relative humidity is 
less in a region where high winds prevail. Winds have a pro¬ 
nounced drying effect. Winds are particularly injurious to plants 
that have parts of them exposed during the winter season, for 
then the cold soil retards the intake of water by the roots, and the 
rate of intake does not keep pace with the rate of outgo. It is well 
to remember that the bare stems of fruit trees, the bare canes of 
raspberries, currants, blackberries and gooseberries, the crowns of 
alfalfa, and of strawberries, are losing some water throughout the 
winter period. Winter killing of fruit trees, cane fruits, and al¬ 
falfa in Colorado is not the result of low temperatures alone, but 
of the combined effect of low temperatures and a dry atmosphere. 
Plants are particularly sensitive to winter cold if the air is dry. 
3. Air Pressure .—Other factors remaining constant, a decrease in 
the air pressure lowers the relative humidity. 4. Exposure .— 
South exposures, with their greater heat, have a lower relative hu¬ 
midity than north exposures. Moreover, slopes exposed to the 
