Why U.V. Intensity Increases with Elevation

One of the most important items on the packing list for my trip to Colorado this past week was sun-block. Averaging 300 days of sunshine per year at a mean altitude of 6,800 feet above sea level, the U.V. index in the Centennial state can range from high to extreme during the summer months.

The U.V. index is a scale that measures the intensity of the sun’s ultraviolet radiation. Readings vary from place to place as local factors affect the amount of U.V. light that reaches the ground. These include, the thickness of the ozone layer, latitude, season, cloud cover, and elevation. Developed by the NWS and EPA in the early 1990’s, it informs the public about the daily health risk of unprotected exposure to the sun.

At high elevations, the atmosphere thins and is less able to absorb U.V. radiation. With every 1000-foot increase in height, according to the National Institutes of Health, U.V. levels increase by about 4%.  So, in Denver, “the Mile High City”, U.V. radiation is about 20% stronger than a location at sea level at the same latitude. Heading up into the Rocky Mountains, where peaks can reach above 14,000 feet, the U.V. intensity soars even higher.

Credit: EPA

Credit: EPA

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About Melissa Fleming

Melissa Fleming is an environmental communicator and visual artist working at the intersection of art and science. She is passionate about exploring, learning, and sharing information about the natural world. She has presented her interdisciplinary work in a variety of mediums at venues and conferences around the world.