Not long since, in one of my rambles in Essex County, Massachusetts,
which is one of the most open and cultivated sections of the country, I
entered a little valley near the sea, comprising about fifty acres of
well-cultivated land, surrounded by a sort of amphitheatre of hills,
which were covered with a dense forest of pines and firs. It was
occupied by an intelligent farmer, whose careful observation had taken
note of many things which are overlooked by the generality of his class.
He remarked that his seed-time and harvest were several days earlier
than on the farms in the open country, and that he had crocuses and
tulips in his garden, on the south side of the surrounding wood, so
early as to astonish his neighbors in the outer world. In regard to the
relative temperature of the woods and of the open plain in summer, he
remarked that it varied according to the time of day or night. The woods
were cooler than the open country, in clear, calm weather, from about
nine o’clock in the morning until near noonday; after this time the heat
in them increased more rapidly than in the open country, and at the time
of dew-fall it was greater in the woods, and continued so during the
early part of the night. If the sky were cloudy, not much difference
could be perceived at any hour in the temperature of the two situations.
In cold and windy weather the woods afforded a comfortable shelter, and
this shelter made them apparently warmer, even when the thermometer
would indicate no difference.
The theory of my rustic friend contains the general results of all that
science has yet discovered in relation to the temperature of woods. But
the effects of clearing the forest are so different in different
situations as to have given origin to a multitude of theories. This
diversity of opinion, however, comes from a partial observation of
facts, without their qualifying circumstances. On a hot summer’s day we
sprinkle our floors with water, for the purpose of cooling the air of
the room. But how can it produce this effect, when by evaporation it
carries heat from the floor into the very air that is cooled by it? The
fact is easily explained. The greater coolness felt when the air of the
room is saturated with the moisture evaporated from the sprinkled floor
might not be exactly indicated by the thermometer. The sensation of
coolness is caused by the increased power of the air to conduct the heat
rapidly from our persons,—the effect of its greater humidity. By the
same law we may explain why, after a few clear cold days in the winter,
if a south-wind arises, we feel as if the cold were greater, because
this wind, while it raises the temperature, charges the air with
invisible moisture.
The coldness of the atmosphere over grassy meadows when the sky is
clear, after the decline of the sun in summer, is a matter of common
observation. As this phenomenon is most evident on the clearest nights,
it has given rise to the notion that the moon cools the night air. In
our rambles after sunset, we have all felt these constant changes of
temperature, which are remarkable when walking over an uneven road, the
degree of heat corresponding nearly with our altitude. When we occupy
high ground, the air is warm and dry; as soon as we descend into a
valley, we feel a sudden chill. These differences are not observed on a
cloudy night, or when a clear brisk wind is blowing. But in a calm state
of the atmosphere, as the lowest stratum of air contains the greatest
amount of moisture, its capacity for retaining heat is proportionally
diminished. Consequently the heat from the ground is radiated with great
rapidity through this damp stratum of air, while the higher strata
remain unchanged in their temperature. Indeed, it has been found by
experiment that while the greatest heat at noonday in calm summer
weather is very near the surface of the ground, yet after dew-fall the
highest temperature is several feet above this surface, increasing in
altitude for some hours after sunset.
The action of a wood checks this radiation in the early part of the
night. Like clouds in the evening, the trees form a canopy of foliage
over the ground, and thereby retain the heat many hours after it has
escaped by radiation in the open plain. According to these laws of the
radiation of heat, a longer time would be required to cool a tract of
forest than an equal area of open space, down to a given point. But, on
the other hand, a proportionally longer time is required to raise the
temperature in the woods to a given point. Hence it is still a question
among meteorologists whether the mean annual temperature of a large
tract of country is higher or lower when covered with forest than when
generally open and cleared. The sun acts with greater force upon an open
country; but the radiation of heat is greater in the same ratio during
the sun’s absence.
In considering the effects of clearing, travellers have often overlooked
the important advantages of protection afforded by woods to agricultural
crops. Even if the mean annual temperature of a country be the same
after it is cleared as when it was covered, it may at the same time be
too cold for certain plants which were formerly its common productions,
because there are no woods to protect them from the winds by day or from
the cold caused by excessive radiation at night. Palestine, two thousand
years ago, was a well-wooded country, and all the fruits of the
sub-tropical climates were raised there to perfection by its ancient
inhabitants. The date-palm, the fig-tree, and the olive grew there and
bore fruit abundantly. Palestine is now a treeless country, and the same
fruits are incapable of enduring its climate; yet recent observations
have demonstrated that its climate is not colder than it was in the days
of the kings of Israel. But as the country has been despoiled of its
forests, these sub-tropical fruits are deprived of their natural
conservatories, and cannot be raised without great labor and expense in
preparing artificial protection for them. Let the forests be restored to
the hills and mountains of Palestine, and, though the temperature of its
summers were not increased, the fields would be protected by these
forests from the winds, and the tender fruits, thriving under their
protection, would again become abundant.
The principles involved in these and similar facts form a distinct
branch of meteorological science, and would require a volume for their
illustration. I have only hinted at some of the general conclusions. It
is evident, indeed, that the same objects that serve to protect us from
cold may in an equal degree protect us from heat. The woodcutters will
continue their labor in a deep forest without discomfort on a winter’s
day, when they could not endure the intense cold of the open country.
The earliest flowers of spring, however, are found neither in a wood nor
in an open meadow, but under the protection of a wood on its southern
border, in little openings that are exposed to the beams of the sun.