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The wind blows water vapor from the ocean to the land. If the land is hotter, it evaporates; if cooler, it falls as precipitation. As the air ascends and cools, it loses its capacity to hold water (adiabatic cooling). Thus in hot weather thermals rise, and water condenses and falls as showers; also precipitation increases up mountainsides. Mountains serve as barriers to water vapor, causing rain shadows on their leeward sides. The tilting of the earth, with accompanying migration of the thermal equator, causes seasonal changes of wind direction responsible for the seasonality of precipitation patterns in the tropics; precipitation is also affected by the seasons in temperate latitudes. .
SOILS. Parent rock erodes physically and chemically into smaller and smaller particles, which become soils as they collect on both coarse and fine scales; the chemical nature of soil reflects the nature of its parent rock. Particles are further sorted by size when deposited by water and wind. Soils are classified by the size of the particles that make them up, from the finest (clay) through increasingly coarse (silt, sand) to the coarsest (gravel), and by organic content (humus). Loam is soil that is well mixed with sands, silts and clays. Soils form horizons, layers that are distinct from one another chemically and physically because of their distance from the surface (which in turn controls the amount of water and organic material present). The overall direction of water movement (up or down) through soil is of some importance in determining its structure. The water-holding capacity of soil is related to the soil type, with the finest soils holding water and coarser ones allowing it to percolate through. The value of soil to plants is related to its water-holding capacity, physical nature (what mixture of sand, silt and clay particles), and chemical content; both inorganic minerals and organic humus are important.
