As of 9:21 AM PDT on September 18, 2025, all 33 cities in San Mateo County, CA in our database 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 1.1.
Over the next 24 hours, the highest UV Index reading in San Mateo County, CA is expected to be 6.7 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 |