UV index in San Francisco in December
In December, the UV index in San Francisco typically peaks around 2.7 at midday — low on the WHO scale — with individual days reaching 3.2 in recent years. Most people won’t need sun protection on a typical day.
How fast would you burn?
Estimated time to sunburn in unprotected midday sun at UV 2.7:
| Skin type | Time to burn |
|---|---|
| I — Very fair | ~49 min |
| II — Fair | ~1h |
| III — Medium | ~1h 25m |
| IV — Olive | ~1h 50m |
Estimates from typical minimal erythemal doses — see methodology. Darker types (V–VI) burn slower still. Reflection off water or sand shortens all of these.
What to do about it
- Sunscreen is optional for most people — UV stays below the protection threshold on a typical day.
- Useful exception: snow, water, or altitude can bounce UV above the forecast, so pack SPF if that’s your trip.
- This is also too little UV for meaningful vitamin D — worth knowing if you’re visiting for months.
Planning a trip? Check the live UV index in San Francisco closer to your dates — day-to-day cloud cover moves the real number around this average.
San Francisco's UV year at a glance
| Month | Typical midday peak | Level |
|---|---|---|
| January | 2.9 | Low |
| February | 3.9 | Moderate |
| March | 5.5 | Moderate |
| April | 6.9 | High |
| May | 7.9 | High |
| June | 8.2 | Very high |
| July | 8.1 | Very high |
| August | 7.5 | High |
| September | 6.4 | High |
| October | 5 | Moderate |
| November | 3.3 | Moderate |
| December | 2.7 | Low |
The strongest month is June (typical peak 8.2); the gentlest is December (typical peak 2.7).
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