WATER 



181 



tance appears when the snow crust is strong 

 enough to bear the Ughter predators, like 

 lynx and wolf, but will not support a cari- 

 bou or deer, placing the herbivorous prey 

 at an extreme disadvantage. 



Other means of meeting the snow hazard 

 include trail making and the formation of 

 restricted winter "yards" in which the snow 

 can be kept packed down wholly or in part. 

 Canadian moose may be restricted to a yard 

 less than 300 feet in radius (Seton, 1909). 

 Ungulates frequently paw through the snow 

 to underlying vegetation and are often ac- 

 companied by feeding symbiotes that are 

 able to obtain otherwise unavailable food 

 from the partially cleared areas. The willow 

 ptarmigan (Lagopiis albus) may keep near 

 a reindeer herd during the winter months 

 and so get food that is often deeply buried 

 under the snow (Sdobnikov, 1935). 



A series of polar birds and mammals have 

 a predominately white color. Others, even in 

 cold-temperate areas, show seasonal color 

 changes well exemplified by the varying 

 hare and by the common weasel of Canada; 

 the summer brown upper parts of the latter 

 turn pure white in wdnter, only the black 

 tail tip remaining unchanged. The color 

 changes of both the varying hare and wea- 

 sels have been experimentally controlled by 

 manipulating the length of day (p. 122). 

 Theories about protective or cryptic colora- 

 tion of such changes need not be aban- 

 doned in all cases. The ptarmigan tends to 

 keep to lingering snow patches until the 

 birds lose their white winter plumage. 

 Although white coloration is proportionately 

 more frequent towards both poles, the ex- 

 planation of inconspicuous coloration cannot 

 well be the only operating causation, since 

 the importance of color concealment in the 

 short dull days of the polar winter appears 

 to be small (Hesse, Allee, and Schmidt, 

 1937). 



MOISTURE IN THE ATMOSPHERE 



Water exists in air in three forms : ( 1 ) as 

 solid hail, sleet, or snow; (2) as Uquid 

 droplets suspended in fog or cloud or as 

 rain drops; and (3) as invisible water 

 vapor. We shall neglect the first two for the 

 present and focus attention on water vapor. 

 This acts as do other gases in the atmos- 

 phere; it exerts pressure, called vapor 

 pressure, and has definite heat relations. Un- 

 like the associated atmospheric gases, it 

 varies in partial pressure both with time 



and with location, whereas each of the other 

 gases of the troposphere makes up a re- 

 markably constant part of the whole. The 

 amount of water vapor the air can hold 

 changes with the temperature, and the 

 amount of possible variation is greater than 

 that for any other vapor (Henderson, 1922, 

 p. 413). 



Some definitions are needed. The absolute 

 humidity of the air is the amount of water 

 vapor in a given amount of air; it may be 

 expressed as grams of water vapor per 

 kilogram of air. Specific humidity is the 

 ratio of the weight of water vapor to the 

 weight of humid air containing it. Relative 

 humidity is the amount of water vapor 

 present in comparison with the amount 

 required to produce saturation at the same 

 temperature and atmospheric pressure; it is 

 expressed as the percentage of saturation. 

 Vapor pressure is the partial pressure of the 

 water vapor measured in millimeters of mer- 

 cury or by other appropriate standards. 

 The vapor pressure deficit or saturation defi- 

 cit is the converse of relative humidity; it 

 measures the difference between the vapor 

 pressure at saturation at a given location 

 (El) and the actual vapor pressure (ei) at 

 the same spot; the saturation deficit is the 

 difference Ei — ei and should not be used, 

 as it has been at times, to represent the dif- 

 ference between saturation of an evaporat- 

 ing surface (Eo) and the observed vapor 

 pressure in some other location (ci) (Leigh- 

 ly, 1937; Thornthwaite, 1940). The dew 

 point is the temperature of saturation of 

 the air by water vapor; with falling tem- 

 perature, condensation begins at the dew 

 point. 



Evaporation is a dynamic physical proc- 

 ess; practically speaking, it occurs when 

 the number of molecules of water leaving a 

 surface in a unit of time is greater than the 

 number entering it. The converse process 

 is called condensation. Evaporation is de- 

 termined more by energy at a water surface 

 than by the humidity above the surface. 

 BoiUng water sends off vapor into saturated 

 air, and dew may form from air that is not 

 saturated except in the micro-niche formed 

 by the cooler air next to the surface where 

 condensation actually takes place. The 

 vapor pressure at the surface is lowered by 

 dissolved salts. This is one of the associated 

 coUigative properties of solutions that in- 

 clude also osmotic pressure, freezing point 

 depression, and boiling point elevation- If 



