Fog over ocean water. The low-lying fog is within the atmospheric boundary layer.
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Image Courtesy of Amy Hatheway
Boundary Layer (Earth's Atmosphere)
The lowest part of the troposphere, closest to Earth's surface, is called the "boundary layer" (or planetary boundary layer or atmospheric boundary layer). Near the surface, the texture of the ground has a strong influence on the movement of winds. Higher up, above the boundary layer, wind speed is much less affected by the details of the surface below. As you might expect, different surfaces have more or less influence on wind flow, so the thickness of this boundary layer varies. The boundary layer is quite thin over smooth water or ice, and much thicker over hilly, tree-covered, or urban terrains with many large buildings. The boundary layer typically extends upward about 200 to 500 meters (650 to 1,640 feet), but can be as thin as 50 meters (164 feet) or as deep as 2 km (6,562 feet). The depth of the boundary layer also tends to vary with latitude. Like the thickness of the troposphere as a whole, the depth of the boundary layer is usually greatest in the tropics and least near the poles.
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