UV Index Calculator

Higher elevations have thinner atmosphere above them, which absorbs and scatters less ultraviolet radiation before it reaches the ground. This calculator applies a 10% increase to effective UV index for every 1,000 feet of altitude entered -- a deliberately conservative rate rather than a direct read of published guidance, meaning a location at 5,000 feet sees roughly 50% more effective UV than sea level in this calculator's model (commonly cited guidance, expressed per 1,000 meters instead of feet, would put the real effect closer to 15% at that elevation). Either way, higher altitude is a common reason skiers and mountain hikers burn faster than they expect.

This calculator applies an 80% increase to effective UV index when Snow Cover is set to Yes, reflecting how strongly fresh snow bounces UV radiation back upward compared to bare ground or grass, which reflect only a small fraction. That reflected UV adds to the direct UV coming from the sky, which is why sunburn on ski slopes and snowfields is a well-known risk even on cool or overcast days.

This calculator's recommended SPF climbs from 15 at Low risk to 30 at Moderate and 50 at High risk, then holds at 50 for both Very High and Extreme -- SPF 50 blocks roughly 98% of UVB radiation, and going higher provides only marginal additional protection while giving a false sense of security about reapplication frequency, so public-health guidance generally caps the practical recommendation at 50 regardless of how much more extreme the UV gets.

Skin Type sets the baseline burn-time reference (lighter skin types have a much lower baseline number of minutes to burn than darker skin types), while altitude and snow cover both scale the effective UV index that baseline is divided by -- so a fair-skinned person and a very-dark-skinned person entering the same altitude and snow cover both see their burn time shortened by the same proportional amount, just starting from very different baseline minutes.