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What is the UV index? The UV index (ultraviolet index) is the WHO scale 0–11+ of sunburn-risk intensity at the Earth's surface: 0–2 low, 3–5 moderate, 6–7 high, 8–10 very high, 11+ extreme. The higher the number — the faster skin burns. Below is the live value for San Cristobal de las Casas today, 2 October.
| Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec |
|---|---|---|---|---|---|---|---|---|---|---|---|
| 7.3 | 8.1 | 8.9 | 9.5 | 9.6 | 9.5 | 9.5 | 9.5 | 9.3 | 8.6 | 7.8 | 7.2 |
The table shows the calculated noon UV peak for latitude 16.7301° under clear sky. Real values vary with clouds, ozone and altitude — see the hourly chart for today's exact forecast.
San Cristobal de las Casas sits at 16.7301° latitude (Northern hemisphere, tropical zone). The UV index here climbs from 7.2 in December to a peak of ≈ 9.6 in May; the annual average is ≈ 8.7. On a peak day, unprotected fair skin burns in about 13 minutes.
| Value | WHO category | Time to burn (fair skin) |
|---|---|---|
| 8–10 | Very high | 15–25 minutes |
| 11+ | Extreme | 5–10 minutes |
| 12 | Maximum at surface | under 10 min; tropics & mountains only |
The highest UV index on Earth reaches 12–14 at sea level (equator, deserts, Australia) and above 20 in the Andes. In mid-latitudes summer peaks are 7–8, in the south 9–11. See the monthly table above for San Cristobal de las Casas's peaks.
Live hourly values come from Open-Meteo (ECMWF & NOAA models); the monthly table is a clear-sky calculation for latitude 16.7301° on the WHO 0–11+ scale. Real values vary with clouds, ozone and altitude.
The live value is at the top of this page; the hourly chart shows today's full forecast, and the status block shows tomorrow's peak. The UV index (WHO scale 0–11+) measures sunburn intensity: the higher, the faster skin burns.
The calculated clear-sky maximum is ≈ 9.6 in May. Values of 11–12 do not occur here — they are only possible near the equator, in deserts and high mountains (world records above 20 were set in the Andes).
The UV index reaches 6+ in January, February, March, April, May, June, July, August, September, October, November and December: use SPF 30–50+ on sunny days, reapply every 2 hours.
Comfortable tanning days (UV 2–5) are rare here. Avoid midday sun in February, March, April, May, June, July, August, September and October (UV 8+): you will burn before you tan.
8 and above: fair skin burns in 10–20 minutes; at 11+ in under 10. In San Cristobal de las Casas such levels occur in February, March, April, May, June, July, August, September and October.
The forecast peak for tomorrow is shown in the status block («tomorrow»); the hourly forecast for today is in the chart above.