As of 3:59 PM CDT on May 18, 2026, all 10 cities in Jefferson County, IL in our database have a UV Index reading of High. The city with the highest UV Index reading right now is Mount Vernon, IL with a reading of 6.8.
Over the next 24 hours, the highest UV Index reading in Jefferson County, IL is expected to be 9.3 in Mount Vernon between 1:00 PM - 2:00 PM CDT tomorrow.
| Range | 0 - 2 | 3 - 5 | 6 - 7 | 8 - 10 | 11+ |
|---|---|---|---|---|---|
| Color | |||||
| Risk | Low | Moderate | High | Very High | Extreme |