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As you look at the scene and check it against the visibility data, you may wonder if the scene is as clear as it could be or if haze or fog is affecting the view.Here are some tips to help you interpret what you are seeing.When this happens, relative humidity will be near 100 percent and precipitation may be evident from the photo or indicated in the visibility information panel. It occurs naturally in the upper atmosphere, where it absorbs harmful ultraviolet rays.But at ground level, it forms as result of air pollution from cars, trucks, busses, power plants, fuel and paint vapors, and other sources. Concentrations may reach unhealthy levels on warm, sunny days.They can be emitted directly into the air during any process involving burning or combustion, including activities around the home (e.g.cooking, smoking, space heating, and open burning) and those involving motor vehicles, various engines, power plants, and other such sources.
However, most of the poor visibility will be due to the fog itself.If fine particle concentrations are low, then any visibility impairment is not likely due to pollution, but to natural causes.To learn more about fine particles and visibility, see the Causes of Poor Visibility.It is especially prevalent in diesel exhaust and smoke from the burning of wood and wastes.Black carbon absorbs light and contributes substantially to the low-altitude, brown clouds sometimes seen during the winter over urban areas and valleys.