雪線の定義とその決定法

Cordillera Real を例として

抄録

A line on the earth surface intersected by a hypothetical surface on which ablation of snow and ice is balanced with snowfall is called a snowline. “The actual snowline” apears uneven and rugged, especially in an alpine zone. Irregularity of the snowline is caused by local variety of snowfall and its ablation due to topographic influences. “The orographic snowline” is drawn by connecting the lower limits of the actual snowline at which snow sheltered from wind and the sun may survive even at lower altitudes than snow exposed to them. “The regional or climatic snowline” is a hypothetical line excluded local irregularities caused by topography. An approximate regional snowline, therefore, is represented by an average altitude of the actual snowline, as far as the actual snowline is known in datail.
A number of methods for determining or estimating the regional and orographic snowlines are described briefly in this paper. The auther examined each method, refering to examples or the data obtained from the Cordillera Real, Bolivia. “Hugi's topographical method” cannot be generally accepted. The orographic snowline should not be datermined by the superficial forms of glaciers of which activities are remarkably controlled by the landforms formed in the last glacial epoch. Any of present cirque-glaciers are scarcely potent to form cirque landforam. “Brückner's hypsometric method” may be acceptable, but the height of snowlines determined in each glacier varies over a wide range. The hypothetical ratio of the ablation area to the nourishment area of 1:1 is preferred to the empirical ratio applied by Brückner, for the former gives the height of snowlines with a narrow range of variation. “Kurowski's mean altitude method”, which has been mathematically proved by Brückner, is better one. An advantage of this method is that it enables a further mathematical treatment of the problem. The author derived the following formula for calculating the change of snowline from the fluctuation of ice-tongue.
Δh=ΔS(h₀-n)/S+ΔS
Where h₀: the height of snowline
Δh: the change of h₀
S: the total area of glacier
ΔS: the area of advanced or retreated ice-tongue
n: the mean altitude of ΔS
By applying this formula to some recessional moraines, the change of snowlines can be estimated even where no distinct evidence of glacier actions at the snowlines is recognized.
The level of cirque excavation has not been found in the Cordillera Real. The cirque bottoms occupied by ice are found at the altitude from 5350 to 4850 meters, and the bottoms without ice from 5150 to 4550 meters. Ninety percent of them, whether they are occupied by ice or not, are distributed from 150 to 550 meters below summits. So the level of cirque excavation may be found only in case where the summit level is relatively even.
The regional snowline is estimated from the height at which the area covered with ice and snow is equal to the area uncovered. For the purpose to estimate the regional snowline, an accumulated temperature is preferable to an annual mean temperature, because ablation of snow is controlled by temperature and its duration.