Tag Archives: sun

Why U.V. Intensity Increases with Elevation

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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

Weather and Health: Sun Exposure

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Summer is a season when many people spend time outdoors in the sun.  Overexposure to its UV rays, however, can cause a number of health problems, including sunburn, eye damage, and even skin cancer  This is why public health officials recommend taking precautions when the UV Index climbs above three.

The UV 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 UV light that reaches the ground. These include, the thickness of the ozone layer, latitude, season, elevation, and cloud cover. Developed by the NWS and EPA in the early 1990’s, it informs the public about the daily risk of unprotected exposure to the sun.

UV_Index

Chart Data: EPA

U.V. Index in Hawai’i

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Since returning from Hawai’i, everyone has been asking me, “Why aren’t you more tan?”  My response is, “SPF 70 and a hat.”  The UV index in Hawai’i ranges from high to extreme, so sun protection is necessary to avoid serious sunburn and other long-term skin problems.

The UV index is a scale that measures the intensity of the sun’s ultraviolet radiation reaching the Earth’s surface. Readings vary from place to place as local factors affect the amount of UV light that reaches the ground. These include, the thickness of the ozone layer, latitude, season, elevation, and cloud cover. Developed in the early 1990’s by the NWS and EPA to warn the public about the risk of overexposure to the sun, it is calculated on a daily basis for every city in the U.S.

Hawai’i is located in the tropics at approximately 20°N latitude.  As a result, the sun sits higher in the sky as compared to the mainland.  When the sun’s rays are more directly overhead, they filter through less of the atmosphere and are therefore more intense when they reach the ground. Consequently, the UV index in Hawai’i is higher than any other location in the U.S.  It averages around 7 in the winter and 11+ in the summer.

Chart Source: EPA