VISIBILITY AND OPTICAL DEPTH

Visual range is determined by the aerosol content of the atmosphere. When dust or smog is present the visual range drops. However, even when there is no dust or pollution present the visual range is finite, being determined by scattering of light by the molecular gases. This so-called Rayleigh-scattering limits the visual range to about 350 km. It is rare to find a condition when the air is this clean. However in Alaska, especially after a rainstorm or in early winter before Arctic Haze affects the airshed, the visual range often goes up to the Rayleigh limit.

Here, for example, is a photo of the foothills and mountains of the Alaska Range, approximately one hundred miles from my campus office window:

Often Mt McKinley, 350 km distant, shows against the twilight sky, frequently displaying interesting coloration due to the wavelength-dependent filtering of the atmosphere for the low sun of winter:

The wavelength-dependent filtering of the atmosphere, along with distortions introduced by strong thermocline structure in the winter atmosphere frequently causes interesting distortions of the low sun: