National Weather ServiceIllinoisDense Fog AdvisoryArchive to October 1, 2026
Dense Fog Advisories in Illinois
The National Weather Service issued 93 dense fog advisories for Illinois in 2025 and 69 a year on average in 2008-2025; in 2021-2025 it had the 11th most of any state. 2026 through September: 50, 6% above the average of 47 for those months in 2008-2025. Kane was covered most often (207 times since 2008).
How many dense fog advisories does Illinois get?
Every dense fog advisory that covered part of Illinois. 2026 is an outline: the year is not over.
The numbers
| Year | Count |
|---|---|
| 2026 (through September) | 50 |
| 2025 | 93 |
| 2024 | 73 |
| 2023 | 77 |
| 2022 | 60 |
| 2021 | 75 |
| 2020 | 67 |
| 2019 | 80 |
| 2018 | 92 |
| 2017 | 93 |
| 2016 | 79 |
| 2015 | 71 |
| 2014 | 48 |
| 2013 | 55 |
| 2012 | 64 |
| 2011 | 36 |
| 2010 | 64 |
| 2009 | 60 |
| 2008 | 55 |
Source: Iowa Environmental Mesonet archive of National Weather Service watches, warnings and advisories (VTEC); National Weather Service. Data as of October 1, 2026.
When in the year does Illinois get them?
Sum of 2008-2025; the busiest month is labelled.
Source: Iowa Environmental Mesonet archive of National Weather Service watches, warnings and advisories (VTEC); National Weather Service. Data as of October 1, 2026.
Where in Illinois?
| County or forecast zone | Since 2008 | Since 2023 | Last issued |
|---|---|---|---|
| Kane | 207 | 51 | September 29, 2026 |
| McHenry | 204 | 45 | September 29, 2026 |
| Lee | 202 | 47 | September 10, 2026 |
| Ogle | 198 | 44 | September 10, 2026 |
| Boone | 196 | 42 | September 10, 2026 |
| Rock Island | 190 | 42 | September 11, 2026 |
| Winnebago | 189 | 40 | September 10, 2026 |
| Henry | 186 | 44 | September 11, 2026 |
| Jo Daviess | 186 | 42 | August 13, 2026 |
| McLean | 186 | 45 | August 13, 2026 |
| Piatt | 185 | 43 | September 28, 2026 |
| Champaign | 184 | 45 | September 28, 2026 |
The NWS issues these products for counties or forecast zones; a zone can be part of a county or a mountain range.
Source: Iowa Environmental Mesonet archive of National Weather Service watches, warnings and advisories (VTEC); National Weather Service. Data as of October 1, 2026.
What is a dense fog advisory?
A dense fog advisory means fog will cut visibility to a quarter mile or less over a wide area. An advisory is issued for a hazard less serious than a warning that can still cause inconvenience or danger if people are not careful. The national record · Every warning type in Illinois.