The effect of unequal distribution of precipitation on the windward and leeward slopes of a mountain range is in general to depress the snowline on the windward slopes where the greater amount falls, but this may be offset in high altitudes by temperature contrasts as in the westward trending Cordillera Vilcapampa, where north and south slopes are in opposition. If the Cordillera Vilcapampa ran north and south we should have the windward and leeward slopes equally exposed to the sun and the snowline would lie at a lower elevation on the eastern side. Among all the ranges the slopes have decreasing precipitation to the leeward, that is, westerly. The second and third passes, between Arma and Choquetira, are snow-free (though their elevations equal those of the first pass) because they are to leeward of the border range, hence receive less precipitation. The depressive effect of increased precipitation on the snowline is represented by A-B, [184] ; in an individual range the effect of heavier precipitation may be offset by temperature contrasts between shady and sunny slopes, as shown by the line a-b in the same figure.
The degree of canting of the snowline on opposite slopes of the Cordillera Vilcapampa varies between 5° and 12°, the higher value being represented four hours southwest of Arma on the Choquetira trail, looking northeast. A general view of the Cordillera looking east at this point ([Fig. 186]), shows the appearance of the snowline as one looks along the flanks of the range. In detail the snowline is further complicated by topography and varying insolation, each spur having a snow-clad and snow-free aspect as shown in the last figure. The degree of difference on these minor slopes may even exceed the difference between opposite aspects of the range in which they occur.
Fig. 184—To illustrate the canting of the snowline. A-B is the snowline depressed toward the north (right) in response to heavier precipitation. The line a-b represents a depression in the opposite direction due to the different degree of insolation on the northern (sunny) and southern (shady) slopes.
To these diversifying influences must be added the effect of warm up-valley winds that precede the regular afternoon snow squalls and that melt the latest fall of snow to exceptionally high elevations on both the valley floor and the spurs against which they impinge. The influence of the warmer air current is notably confined to the heads of those master valleys that run down the wind, as in the valley heading at the first pass, Cordillera Vilcapampa, and at the heads of the many valleys terminating at the passes of the Maritime Cordillera. Elsewhere the winds are dissipated in complex systems of minor valleys and their effect is too well distributed to be recognized.
It is clear from the conditions of the problem as outlined on preceding pages that the amount of canting may be expressed in feet of difference of the snowline on opposite sides of a range or in degrees. The former method has, heretofore, been employed. It is proposed that this method should be abolished and degrees substituted, on the following grounds: Let A and B, [190] , represent two mountain masses of unequal area and unequal elevation. Let the opposite ends of the snowlines of both figures lie 1,000 feet apart as between the windward and leeward sides of a broad cordillera (A), or as between the relatively sunnier and relatively shadier slopes of individual mountains or narrow ranges in high latitudes or high altitudes (B). With increasing elevation there is increasing contrast between temperatures in sunshine and in shade, hence a greater degree of canting (B). Tending toward a still greater degree of contrast is the effect of the differences in the amounts of snowy precipitation, which are always more marked on an isolated and lofty mountain summit than upon a broad mountain mass (1) because in the former there is a very restricted area where snow may accumulate, and (2) because with increase of elevation there is a rapid and differential decrease in both the rate of adiabatic cooling and the amount of water vapor; hence the snow-producing forces are more quickly dissipated.