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Little Climates -- Weather Just Above The Ground
Nature Bulletin No. 481-A February 17, 1973
Forest Preserve District of Cook County
George W, Dunne, President
Roland F. Eisenbeis, Supt. of Conservation
LITTLE CLIMATES -- Weather Just Above the Ground
In a previous bulletin we talked about little climates, underground,
resulting from weather conditions in the soil. Just above the ground
there is another "little climate" equally important. We frequently see
evidences of it without realizing how and why they were produced.
Just above the earth, there lies a narrow layer of changeable weather
that is affected at both surfaces by its mighty neighbors: the land below
and the restless air in the atmosphere above it. Under the spell of
gravity, it clings to the ground in spite of all but the swiftest winds. In
this layer there are special weather conditions overlooked by nearly
everyone.
Hovering on the brow of the earth, this climatic cloak with shifting
contents is very changeable. Compared with weather in the soil, its air
currents are variable and swift. Near its roof, frequent trading of
ingredients with the winds above goes on unseen. At the bottom its
contents continually bump into clods, mounds or other obstructions on
the earth's surface and, there, another exchange of air takes place -- an
exchange with the air in the soil -- with visible results such as fogs,
dews and frosts .
In order to understand this peculiar stratum of special weather, let's
begin with the air movements in it. As you know, air rushes in to fill
any empty space -- hence the old saying: "Nature abhors a vacuum".
Thus, any heat-energized air in this belt will rise and leave room for
replacements from aloft. Similarly, cold-enfeebled air will trade places
with air underground, sinking downward into some soil pores and
displacing warmer air which is forced upward to rise out of others,
There is also a third type of air current, virtually unknown underground:
lateral movements which are roughly horizontal and can cause
turnovers or "flip-flops" in this layer of air just above the surface.
Fogs
frequently occur in this narrow canopy of weather and they may
result from either of two conditions, both depending upon air chilling.
The first occurs when warm moist air is moved in over a land area that
has been previously cooled. Chilled by the ground surface, this air loses
its moisture into tiny droplets which become visible as fog.
The second condition is more complicated. It occurs on a calm
cloudless night after a warm day. If there is plenty of moisture in the
narrow belt of air above the ground, a low lying fog will result before
dawn. At night the ground, being warmer than the atmosphere, radiates
its heat upwards and, when there are no clouds to impede or reflect it,
this warmth continues to soar. Without any winds, and therefore no
mixing of the air currents, the earth's surface loses much of its heat and
becomes chilled. Then a fog forms. This phenomenon occurs frequently
in summer, especially in river bottoms, and may create dangerous
driving conditions on the highways.
Heavy dews often form when this second condition occurs and the
ground gives up its heat energy at an exceptionally rapid rate. Then the
ground becomes so much cooler than the warm moisture-laden air just
above it that this air is chilled, condenses, and deposits a liquid film
upon the earth and the plants. In the morning, if you walk through a
patch of tall grass or weeds, your clothing will become as drenched as if
there had been a rain during the night. In colder weather, such as in
autumn or early spring, ice fogs and frosts may result when either of the
two weather conditions occur and the ground temperature is at or near
the freezing point.
There's more to climate than meets the eye.
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Update: June 2012
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