June 15………………….| 64 | 51 | 13 " 27………………….| 62 | 51 | 11 July 16………………….| 62 | 51 | 11 " 30………………….| 65 1/2 | 55 1/2 | 10 Aug. 15………………….| 68 | 58 | 10 " 31………………….| 59 1/2 | 55 | 4 1/2 Sept.15………………….| 59 1/2 | 55 | 4 1/2 Oct. 1………………….| 59 1/2 | 55 | 4 1/2 " 15………………….| 49 | 49 | 0 Nov. 1………………….| 43 | 43 | 0 " 16………………….| 43 1/2 | 43 1/2 | 0
On the 14th of January, 1791, in a winter remarkable for its extreme severity, he found the ground, on a plain open field where the snow had been blown away, frozen to the depth of three feet and five inches; in the woods where the snow was three feet deep, and where the soil had frozen to the depth of six inches before the snow fell, the thermometer, at six inches below the surface of the ground, stood at 39 degrees. In consequence of the covering of the snow, therefore, the previously frozen ground had been thawed and raised to seven degrees above the freezing-point.—William's Vermont, i., p. 74.
Boussingault's observations are important. Employing three thermometers, one with the bulb an inch below the surface of powdery snow; one on the surface of the ground beneath the snow, then four inches deep; and one in the open air, forty feet above the ground, on the north side of a building, he found, at 5 P.M., the FIRST thermometer at -1.5 degrees Centigrade, the second at 0 degrees, and the THIRD at + 2.5 degrees; at 7 A.M. the next morning, the first stood at -12 degrees, the second at -3.5 degrees and the third at -3 degrees; at 5.30 the same evening No. 1 stood at -1.4 degrees, No. 2 at 0 degrees, and No. 3 at + 3 degrees. Other experiments were tried, and though the temperature was affected by the radiation, which varied with the hour of the day and the state of the sky, the upper surface of the snow was uniformly colder than the lower, or than the open air.
According to the Report of the Department of Agriculture for May and June, 1872, Mr. C. G. Prindle, of Vermont, in the preceding winter, found, for four successive days, the temperature immediately above the snow at 13 degrees below zero; beneath the snow, which was but four inches deep, at 19 degrees above zero; and under a drift two feet deep, at 27 degrees above.
On the borders and in the glades of the American forest, violets and other small plants begin to vegetate as soon as the snow has thawed the soil around their roots, and they are not unfrequently found in full flower under two or three feet of snow.—American Naturalist, May, 1869, pp. 155, 156.
In very cold weather, when the ground is covered with light snow, flocks of the grouse of the Eastern States often plunge into the snow about sunset, and pass the night in this warm shelter. If the weather moderates before morning, a frozen crust is sometimes formed on the surface too strong to be broken by the birds, which consequently perish.] The snow in contact with the earth now begins to melt, with greater or less rapidity, according to the relative temperature of the earth and the air, while the water resulting from its dissolution is imbibed by the vegetable mould, and carried off by infiltration so fast that both the snow and the layers of leaves in contact with it often seem comparatively dry, when, in fact, the under-surface of the former is in a state of perpetual thaw. No doubt a certain proportion of the snow is given off to the atmosphere by direct evaporation, but in the woods, the protection against the sun by even leafless trees prevents much loss in this way, and besides, the snow receives much moisture from the air by absorption and condensation. Very little water runs off in the winter by superficial water-courses, except in rare cases of sudden thaw, and there can be no question that much the greater part of the snow deposited in the forest is slowly melted and absorbed by the earth.
The immense importance of the forest, as a reservoir of this stock of moisture, becomes apparent, when we consider that a large proportion of the summer rain either flows into the valleys and the rivers, because it falls faster than the ground can imbibe it; or, if absorbed by the warm superficial strata, is evaporated from them without sinking deep enough to reach wells and springs, which, of course, depend much on winter rains and snows for their entire supply. This observation, though specially true of cleared and cultivated grounds, is not wholly inapplicable to the forest, particularly when, as is too often the case in Europe, the underwood and the decaying leaves are removed.
The quantity of snow that falls in extensive forests, far from the open country, has seldom been ascertained by direct observation, because there are few meteorological stations in or near the forest. According to Thompson, [Footnote: Thompson's Vermont, Appendix, p. 8.] the proportion of water which falls in snow in the Northern States does not exceed one-fifth of the total precipitation, but the moisture derived from it is doubtless considerably increased by the atmospheric vapor absorbed by it, or condensed and frozen on its surface. I think I can say from experience—and I am confirmed in this opinion by the testimony of competent observers whose attention has been directed specially to the point—that though much snow is intercepted by the trees, and the quantity on the ground in the woods is consequently less than in open land in the first part of the winter, yet most of what reaches the ground at that season remains under the protection of the wood until melted, and as it occasionally receives new supplies the depth of snow in the forest in the latter half of winter is considerably greater than in the cleared fields. Careful measurements in a snowy region in New England, in the month of February, gave a mean of 38 inches in the open ground and 44 inches in the woods. [Footnote: As the loss of snow by evaporation has been probably exaggerated by popular opinion, an observation or two on the subject may not be amiss in this place. It is true that in the open grounds, in clear weather and with a dry atmosphere, snow and ice are evaporated with great rapidity even when the thermometer is much below the freezing-point; and Darwin informs us that the snow on the summit of Aconcagua, 23,000 feet high, and of course in a temperature of perpetual frost, is sometimes carried off by evaporation. The surface of the snow in our woods, however, does not indicate much loss in this way. Very small deposits of snow-flakes remain unevaporated in the forest, for many days after snow which fell at the same time in the cleared field has disappeared without either a thaw to melt it or a wind powerful enough to drift it away. Even when bared of their leaven, the trees of a wood obstruct, in an important degree, both the direct action of the sun's rays on the snow and the movement of drying and thawing winds.
Dr. Piper (Trees of America, p. 48) records the following observations: "A body of snow, one foot in depth and sixteen feet square, was protected from the wind by a tight board fence about five feet high, while another body of snow, much more sheltered from the sun than the first, six feet in depth, and about sixteen feet square, was fully exposed to the wind. When the thaw came on, which lasted about a fortnight, the larger body of snow was entirely dissolved in less than a week, while the smaller body was not wholly gone at the end of the second week. "Equal quantities of snow were placed in vessels of the samekind and capacity, the temperature of the air being seventy degrees. In the one case, a constant current of air was kept passing over the open vessel, while the other was protected by a cover. The snow in the first was dissolved in sixteen minutes, while the latter had a small unthawed proportion remaining at the end of eighty-five minutes." The snow in the woods is protected in the same way, though not literally to the same extent, as by the fence in one of these cases and the cover in the other.]
The general effect of the forest in cold climates is to assimilate the winter state of the ground to that of wooded regions under softer skies; and it is a circumstance well worth noting, that in Southern Europe, where Nature has denied to the earth a warm winter-garment of flocculent snow, she has, by one of those compensations in which her empire is so rich, clothed the hillsides with umbrella and other pines, ilexes, cork-oaks, bays and other trees of persistent foliage, whose evergreen leaves afford to the soil a protection analogous to that which it derives from snow in more northern climates.