UV Index

What does UV Index mean?

The UV Index reports how strongly the ultraviolet reaching the ground can burn skin. It is an international scale weighted to the sunburn action spectrum, meaning each wavelength is scored by how efficiently it produces sunburn, then reported as a band rather than an exact figure. Higher bands mean unprotected skin takes on a burning dose faster, and the WHO protection categories begin at 3. Sun height, cloud, ozone, and other atmospheric conditions set the band at a given place and time. Read as a clock rather than a score, that makes the number genuinely useful: it climbs through the morning, tops out around the daily peak in sun height, and falls back through the afternoon, so the hours on either side of the peak are the gentler ones and the hours around it are where covering up matters most. Ultraviolet radiation is a carcinogen and the most preventable cause of skin cancer, which is what makes that timing worth knowing rather than merely interesting. The boundary worth knowing is what the scale leaves out. Sunburn-producing and vitamin D-producing ultraviolet overlap, but a sunburn-weighted dose is a poor predictor of how much vitamin D synthesis is possible, so the index is not a vitamin D readout and should never be used as one. That gap matters most when the sky looks mild, or when someone tries to read a vitamin D answer off the forecast. The full entry covers how to read the daily curve and where the protection bands sit. The sunlight and vitamin D entry covers why vitamin D follows a different question altogether.

Read the full UV Index entry

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