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Little Climates -- Part One
Nature Bulletin No. 478-A   January 27, 1973
Forest Preserve District of Cook County
George W. Dunne, President
Roland F. Eisenbeis, Supt. of Conservation

LITTLE CLIMATES
-- Part One: Weather in the Soi.

Climate vitally affects our lives. Wherever we live, climate has largely determined the plant and animal life in that region, the development of civilization there and what people do. The climate of any region represents its overall weather picture: the sum of its weather today, tomorrow, and during past centuries. We are accustomed to think of climate as a set of conditions occurring entirely in the atmosphere above the earth's surface, and it may sound silly when we say that there are climates underground -- little climates just as real as those above -- but it's true, There are special kinds of weather in the soil.

Land is not a solid wall. It is like a sponge. There are pores or channels in it. It is a complex mixture of soil particles and air spaces. The spaces, or pores and channels, vary with the type of soil. In sand they are tiny pockets between the grains. In clays they are narrow or threadlike spaces frequently occurring as deep twisting tubes that taper downward. Weather from aloft can invade such pores or real air currents, fogs, drizzles and snows.

Air currents in the soil move generally up and down. Air sinks into the soil because it has weight and is pulled downward by gravity. It rises underground when it is energized by heat and expands; it also rises when it is shoved upward by other air or by water. It may escape above the surface or, if the "push" is weak, into another part of the soil.

Weather conditions underground, such as fogs, drizzles and snows, are created by air currents there and three forms of water. We are all familiar with liquid water and with ice, the solid form, but there is a third: vapor, which is the gaseous form. The latter occurs when liquid water or ice are heated to the point where their tiny particles, filled with energy, are expelled and streak away on separate courses. Widely separated and invisible, they form vapor. If vapor is chilled -- its store of heat energy is taken away -- the particles are brought together and become visible as liquid water or ice.

Drizzles and moist fogs are two kinds of liquid water that result from the cooling of water vapor, but a drizzle contains much larger particles which we call "drops ll. Both occur and travel underground. They are shoved around by subsurface air currents and are valuable sources of moisture for plant roots. At night, sometimes, it is possible to shine a strong flashlight into the cracks or large pores in a soil and glimpse an underground fog or drizzle.

Ice fogs and snow also occur underground. Sometimes, during winter months, they develop from moist fogs and drizzles chilled to or beyond the freezing point by cold air currents in the soil, and may be moved about by such currents. Ice fogs are composed of minute particles; snow particles are much larger and more visible but both may be seen occasionally by exploring with a flashlight at night. Eventually they are melted by warmer air currents, or by contact with deeper warmer layers of soil, and provide moisture for plants.

Underground moisture, from whatever source, is never "pure" water. It always contains dissolved gases and minerals of various kinds: some gathered by rainwater as it falls from the sky; some absorbed by subsurface fogs, drizzles and snows. Rainwater, for example, gathers dust particles and carbon dioxide gas. Plant roots suck in such moisture and then the dissolved gases and minerals are changed into foods that sustain plant and animal life.

Little climates, underground, are real and vital.


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