This latter phase of the Observatory’s work has developed into a most novel and valuable contribution to practical forestry and conservation of water, under Dr. Church’s clear and logical direction. At Contact Pass, 9000 feet elevation, and at the base of the mountain, supplementary stations have been established, where measurements of snow depth and density, the evaporation of snow, and temperatures within the snow have been taken. Lake Tahoe, with its seventy miles of coast line also affords ready access throughout the winter, by means of motor boat, snow-shoes and explorer’s camp, to forests of various types and densities where snow measurements of the highest importance have been made.
Delicate instruments of measurement and weight, etc., have been invented by Dr. Church and his associates to meet the needs as they have arisen, and continuous observations for several years seem to justify the following general conclusions. These are quoted from a bulletin by Dr. Church, issued by the International Irrigation Congress.
The conservation of snow is dependent on mountains and forests and is most complete where these two factors are combined. The mountain range is not only the recipient of more snow than the plain or the valley at its base, but in consequence of the lower temperature prevailing on its slopes the snow there melts more slowly.
However, mountains, because of their elevation, are exposed to the sweep of violent winds which not only blow the snow in considerable quantities to lower levels, where the temperature is higher, but also dissipate and evaporate the snow to a wasteful degree. The southern slopes, also, are so tilted as to be more completely exposed to the direct rays of the sun, and in the Sierra Nevada and probably elsewhere are subjected to the persistent action of the prevailing southwest wind.
On the other hand, the mountain mass, by breaking the force of the wind, causes much of the drifting snow to pile up on its lee slope and at the base of its cliffs, where it finds comparative shelter from the wind and sun.
Forests, also, conserve the snow. In wind-swept regions, they break the force of the wind, catching the snow and holding it in position even on the windward slopes of the mountains. On the lower slopes, where the wind is less violent, the forests catch the falling snow directly in proportion to their openness, but conserve it after it has fallen directly in proportion to their density. This phenomenon is due to the crowns of the trees, which catch the falling snow and expose it to rapid evaporation in the open air but likewise shut out the sun and wind from the snow that has succeeded in passing through the forest crowns to the ground. Both mountains and forests, therefore, are to a certain extent wasters of snow—the mountains because they are partially exposed to sun and wind; the trees, because they catch a portion of the falling