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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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