https://www.boulderweather.com/tags/sun-safety/Recent content in Sun Safety on BoulderWeather.comHugo -- gohugo.ioenBoulderweather.comWed, 24 Jun 2026 00:00:00 +0000https://www.boulderweather.com/post/boulder-uv-index-altitude/Wed, 24 Jun 2026 00:00:00 +0000https://www.boulderweather.com/post/boulder-uv-index-altitude/ <h2 id="the-uv-index-climbs-roughly-4-for-every-1000-feet-of-elevation">The UV Index Climbs Roughly 4% for Every 1,000 Feet of Elevation</h2> <p><script async src="https://pagead2.googlesyndication.com/pagead/js/adsbygoogle.js?client=ca-pub-1012089347386563" crossorigin="anonymous"></script> <ins class="adsbygoogle" style="display:block" data-ad-client="ca-pub-1012089347386563" data-ad-slot="1011306739" data-ad-format="auto" data-full-width-responsive="true"></ins> <script> (adsbygoogle = window.adsbygoogle || []).push({}); </script> The <a href="https://www.epa.gov/sunsafety/uv-index-1">UV Index</a> is a forecast of the strength of ultraviolet radiation reaching the ground, and it rises predictably with altitude: ultraviolet intensity climbs by roughly four percent for every 1,000 feet of elevation, because there is less atmosphere overhead to absorb and scatter it. Boulder sits at about 5,430 feet, which alone adds something on the order of 20 percent to the UV reaching the surface compared with a sea-level city at the same latitude on the same day. That is the core fact behind a piece of local advice that sounds like exaggeration until your skin proves it true: in Boulder, sun protection is not seasonal and it is not optional. On a clear summer day the index here routinely reaches the &quot;very high&quot; and &quot;extreme&quot; categories — 10, 11, or higher — values most of the country sees only rarely.</p>