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The Effect of Fog on UV Light

Fog is a mass of condensed vapor that exists near the ground -- a low-lying cloud. According to the U.S. Environmental Protection Agency (EPA), ultraviolet (UV) levels decrease, but are not eliminated, when there is cloud cover.

Types
- There are three types of UV light, classified according to their wavelengths in nanometers (nm): UVA (320-400nm), UVB (290-320nm), and UVC (100-290nm). The ozone layer absorbs all UVC rays, most UVB rays and no UVA rays.
Energy
- UVA radiation has a longer wavelength than UVB rays, but UVB rays have more energy. Fog can completely block UVA rays, depending on the density of the cover, but UVB rays are only partially blocked. The suspended water particles in the fog act as deflectors that get in the way of the UV light path. In the instance of low to moderate fog coverage, low energy UVA rays quickly slow down and stop, while UVB rays merely slow down.
Transmission
- Clear skies allow for 100 percent of the ultraviolet light to pass through the air. To determine how much ultraviolet light passes through the various thicknesses of clouds, the U.S. National Weather Service has come up with some transmission estimations. Fog, most resembling overcast skies, only allows 31 percent of UV rays to pass through.

Nature