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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 Waltham today, 3 October.
| Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec |
|---|---|---|---|---|---|---|---|---|---|---|---|
| 2.9 | 3.9 | 5.4 | 6.7 | 7.5 | 7.9 | 7.7 | 7.1 | 6.2 | 4.8 | 3.6 | 2.8 |
The table shows the calculated noon UV peak for latitude 42.3765° under clear sky. Real values vary with clouds, ozone and altitude — see the hourly chart for today's exact forecast.
Waltham sits at 42.3765° latitude (Northern hemisphere, temperate zone). The UV index here climbs from 2.8 in December to a peak of ≈ 7.9 in June; the annual average is ≈ 5.5. On a peak day, unprotected fair skin burns in about 15 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 Waltham's peaks.
Live hourly values come from Open-Meteo (ECMWF & NOAA models); the monthly table is a clear-sky calculation for latitude 42.3765° 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 ≈ 7.9 in June. 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 April, May, June, July, August and September: use SPF 30–50+ on sunny days, reapply every 2 hours.
The most comfortable tanning window (UV 2–5) is January, February, October, November and December — the tan develops slower but lasts longer.
8 and above: fair skin burns in 10–20 minutes; at 11+ in under 10. In Waltham such levels are practically impossible — the local maximum is ≈ 7.9.
The forecast peak for tomorrow is shown in the status block («tomorrow»); the hourly forecast for today is in the chart above.