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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 Los Altos today, 3 October.
| Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec |
|---|---|---|---|---|---|---|---|---|---|---|---|
| 3.6 | 4.6 | 6 | 7.2 | 7.8 | 8.1 | 8 | 7.5 | 6.7 | 5.5 | 4.3 | 3.5 |
The table shows the calculated noon UV peak for latitude 37.3852° under clear sky. Real values vary with clouds, ozone and altitude — see the hourly chart for today's exact forecast.
Los Altos sits at 37.3852° latitude (Northern hemisphere, temperate zone). The UV index here climbs from 3.5 in December to a peak of ≈ 8.1 in June; the annual average is ≈ 6.1. 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 Los Altos's peaks.
Live hourly values come from Open-Meteo (ECMWF & NOAA models); the monthly table is a clear-sky calculation for latitude 37.3852° 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 ≈ 8.1 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 March, 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, November and December — the tan develops slower but lasts longer. Avoid midday sun in June and July (UV 8+): you will burn before you tan.
8 and above: fair skin burns in 10–20 minutes; at 11+ in under 10. In Los Altos such levels occur in June and July.
The forecast peak for tomorrow is shown in the status block («tomorrow»); the hourly forecast for today is in the chart above.