It is primarily caused by diesel engines in urban areas.

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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.

We have two options for you to view mountain conditions. This camera is located at the base of the Sunbowl Express Quad and gives a view of the lower portions of the trails Wingding, Kartwheel and Cataract.

Click here for the Sunbowl Express webcam The Spruce Lodge webcam will show you what's happening in part of the base area as well as any races on Eggbeater, the 603 Terrain Park and Airbag (when open) and North Peak moguls on Upper Flying Goose.

Fine particle levels are highest on warm, sunny days and on clear, calm winter mornings.

Fine particles affect visibility in two ways -- by absorbing light and by scattering light.

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.

Most haze is caused by coal-fired emissions from power plants.