As of 1:55 PM PDT on July 9, 2026, out of 33 cities in San Mateo County, CA in our database, 21 cities (64%) have a UV Index reading of Extreme, while 12 cities (36%) have a UV Index reading of Very High. The city with the highest UV Index reading right now is Portola Valley, CA with a reading of 10.7.
Over the next 24 hours, the highest UV Index reading in San Mateo County, CA is expected to be 10.7 in Portola Valley between 1:00 PM - 2:00 PM PDT today.
| Range | 0 - 2 | 3 - 5 | 6 - 7 | 8 - 10 | 11+ |
|---|---|---|---|---|---|
| Color | |||||
| Risk | Low | Moderate | High | Very High | Extreme |