UV Index Explained: When You Actually Need Sunscreen

The UV index measures the strength of sunburn-causing ultraviolet radiation on a scale that starts at 0 and, in the tropics at altitude, can pass 14.

The threshold that matters Sunscreen becomes genuinely necessary at UV 3. Below that, unprotected skin will not burn in a normal outing. At UV 3-5 fair skin burns in about 30 minutes; at 6-7 in roughly 20; at 8-10 in 15; above 11 in under 10 minutes.

The shadow rule If your shadow is shorter than you are, UV is high — you're within the peak window, typically 10am to 4pm local solar time.

What raises it Altitude adds roughly 10% per 1,000 m. Snow reflects up to 80% of UV, sand about 15%, water 10%. Thin cloud cuts UV only slightly, which is why overcast days produce so many surprise burns.

How much sunscreen About 30 ml — a shot glass — for a full body, reapplied every two hours and immediately after swimming. SPF 30 blocks roughly 97% of UVB, SPF 50 about 98%; the jump is smaller than the label suggests, so reapplication matters more than the number.