HOAR-FROST
All in this country are familiar with the beauty of hoar-frost. The
children are delighted with the funny figures on the glass of the bedroom
window on a cold winter morning. Frost is a wonderful artist; during the
night he has been dipping his brush into something like diluted schist,
and laying it gracefully on the smooth panes.
And, as you walk over the meadows, you observe the thin white films of ice
on the green pasture; and the clear, slender blades seem like crystal
spears, or the "lashes of light that trim the stars."
You all know what hoar-frost is, though most in the country give it the
expressive name of "rime." But you are not all aware of how it is formed.
Hoar-frost is just frozen dew. In a learned paper, written in 1784,
Professor Wilson of Glasgow made this significant remark: "This is a
subject which, besides its entire novelty, seems, upon other accounts, to
have a claim to some attention." He observed, in that exceptionally cold
winter, that, when sheets of paper and plates of metal were laid out, all
began to attract hoar-frost as soon as they had time to cool down to the
temperature of the air. He was struck with the fact that, while the
thermometer indicated 36 degrees of frost a few feet above the ground and
44 degrees of frost at the surface of the snow, there were only 8 degrees
of frost at a point 3 inches below the surface of the snow. If he had
only thought of placing the thermometer on the grass, under the snow, he
would have found it to register the freezing-point only. And had he
inserted the instrument below the ground, he would have found it
registering a still higher temperature. That fact would have suggested to
him the formation of hoar-frost; that the water-vapour from the warm soil
was trapped by a cold stratum of air and frozen when in the form of dew.
One of the most interesting experiments, without apparatus, which you can
make is in connection with the formation of hoar-frost, when there is no
snow on the ground, in very cold weather. If it has been a bright, clear,
sunny day in January, the effect can be better observed. Look over the
garden, grass, and walks on the morning after the intense cold of the
night; big plane-tree leaves may be found scattered over the place. You
see little or no hoar-frost on the _upper_ surface of the leaves. But turn
up the surface next the earth, or the road, or the grass, and what do you
see? You have only to handle the leaf in this way to be brightly
astonished. A thick white coating of hoar-frost, as thick as a layer of
snow, is on the _under_ surface. If a number of leaves have been
overlapping each other, there will be no coating of hoar-frost under the
top leaves; but when you reach the lowest layer, next the bare ground, you
will find the hoar-frost on the under surface of the leaves. Now that is
positive proof that the hoar-frost has not fallen from the air, but has
risen from the earth.
The sun's heat on the previous day warmed the earth. This heat the earth
retained till evening. As the air chilled, the water-vapour from the
warmer earth rose from its surface, and was arrested by the cold surface
of the leaves. So cold was that surface that it froze the water-vapour
when rising from the earth, and formed hoar-frost in very large
quantities. When this happens later on in the season, one may be almost
sure of having rain in the forenoon.
As hoar-frost is just frozen dew, I can even more surely convince you of
the formation of hoar-frost as rising from the ground by observations made
by me at my manse in Strathmore, in June 1892. I mention this particularly
because then was the most favourable testing-time that has _ever_ occurred
during meteorological observations. June 9th was the warmest June day
(with one exception) for twenty years. The thermometer reached 83 deg. Fahr.
in the shade. Next day was the coldest June day (with one exception) for
twenty years, when the thermometer was as low as 51 deg. in the shade. But
during the night my thermometer on the grass registered 32 deg.--the freezing
point. On the evening of the sultry day I examined the soil at 10 o'clock.
It was damp, and the grass round it was filmy moist. The leaves of the
trees were crackling dry, and all above was void of moisture. The air
became gradually chilly; and as gradually the moisture rose in height on
the shrubs and lower branches of small trees. The moon shone bright, and
the stars showed their clear, chilly eyes. The soil soon became quite wet,
the low grass was dripping with moisture, and the longer grass was
becoming dewed. This gave the best natural evidence of the rising of the
dew that I ever witnessed. But everything was favourable for the
observation--the cold air incumbent on the rising, warm, moist vapour from
the soil fixing the dew-point, when the projecting blades seized the
moisture greedily and formed dew. Had the temperature been a little below
the freezing-point, hoar-frost would have been beautifully formed.