Temperature tells you how to dress. The UV index tells you what the sun is doing to your skin — and the two have almost nothing to do with each other. A cool, breezy December morning in Bangkok can carry more UV than a sweltering afternoon in London in July.
Where the number comes from
The Global Solar UV Index was built jointly by the World Health Organization, the UN Environment Programme, the World Meteorological Organization and ICNIRP, so the scale means the same thing in Phuket as it does in Perth. It is an open-ended scale starting at 0, and it describes the strength of UV radiation reaching the ground at a given place and time.
Higher means faster. A UV index of 10 does not burn you twice as badly as a 5 — it burns you roughly twice as fast. The dose is intensity multiplied by time, which is why the clock is as much a tool as the bottle.
The scale, and what to do at each step
| Index | Band | What it means for you |
|---|---|---|
| 0–2 | Low (green) | Safe for most people. No protection needed for ordinary short exposure. |
| 3–5 | Moderate (yellow) | Broad-spectrum SPF 30, a hat, shade around midday. |
| 6–7 | High (orange) | SPF 50, sunglasses, reapply every two hours. Seek shade 10:00–16:00. |
| 8–10 | Very high (red) | Full SPF 50 · PA+++, cover up, avoid the middle of the day. |
| 11+ | Extreme (violet) | Unprotected skin burns quickly. Postpone the beach, or take the shade seriously. |
Four things that quietly raise your dose
- Water and sand. Both reflect UV back up at you, so you collect it from below as well as above. Time on a boat or a white beach is more exposure than the same hour on grass.
- Altitude. UV rises meaningfully as you climb — a hill walk carries more than the same hour at sea level.
- Cloud. Thin or broken cloud lets a large share of UV through. Overcast is not the same as protected, and cooler air removes the sensation that normally warns you.
- Time of day. The index peaks in the hours around solar noon. Same beach, same person, entirely different bill.
The shadow rule
You do not need an app to sanity-check the sky. Stand outside and look at your shadow: if your shadow is shorter than you are, the sun is high and UV is strong. Long shadow, weaker sun. It is a crude proxy for solar angle, and it works anywhere on earth without a signal.
For the real number, the WHO scale is reported by most weather apps and national meteorological services. We also publish a live reading for Bangkok on the Sun School page, pulled fresh each time you load it — so you can check the sky before you pack the bag.