As of 5:38 PM PDT on August 18, 2026, out of 33 cities in San Mateo County, CA in our database, 27 cities (82%) have a UV Index reading of Moderate, while 6 cities (18%) have a UV Index reading of Low. The city with the highest UV Index reading right now is San Mateo, CA with a reading of 2.6.
Over the next 24 hours, the highest UV Index reading in San Mateo County, CA is expected to be 9.4 in Portola Valley between 1:00 PM - 2:00 PM PDT tomorrow.
| Range | 0 - 2 | 3 - 5 | 6 - 7 | 8 - 10 | 11+ |
|---|---|---|---|---|---|
| Color | |||||
| Risk | Low | Moderate | High | Very High | Extreme |