Midwest Fog Provides Challenges for Aviation
The morning of 5 March 2026 saw widespread fog over the Midwestern United States. The following animation shows the Day Cloud Phase Distinction RGB product from GOES-19 with some surface observations overlaid on top. The surface observations show just how widespread the fog was. The standard meteorological chart code uses two horizontal parallel lines to denote mist and three such lines for fog. The visibility at a station (in miles) is shown by the number in the lower-right of each plot. In northwestern Illinois, for example, visibilities were between 2 and 3 miles, while in Chicago it was 1.5 miles and in Milwaukee it was as low as a quarter of a mile. Many of the lcoations across the map are also showing conditions at or very close to saturation, with numerous places have dew points at the same value as the temperature or just a degree or two apart.
The day cloud phase distinction product does well in discriminating between liquid (cyan) and ice (orange-yellow) clouds. A similar product, the Day Snow/Fog RGB, is another tool for identifying low clouds and fog from satellite. Here, teh frozen surface areas (lake ice or snow) show up as red with the flog as various shades of gray or yellow-gray.
An additional data source worth monitoring are the aircraft profiles. Many commercial aircraft provide observations of temperature and winds as they take off and land, while a small subset also provide water vapor. This smaller selection of temperature + water vapor observations (less than 10% of the total) is publicly available in real time, although there’s not a lot of places to easily access the data. Fortunately, a weather enthusiast has made a page to produce Skew-T plots of recent aircraft profiles, which can be found here. Here’s a plot of the observations from a flight into Chicago’s Midway Airport at 9:58 AM (1558 UTC) this morning. Note how the layer between the surface and the nocturnal inversion is fully saturated. Unlike radiosondes, where the disposable sensors often struggle to record full saturation, the aircraft-based sensors can be engineered to greater precision since they’ll be reused for thousands of profiles over their lifespan.
The low visibilities over the past day did have some adverse impacts on aviation. A plane scheduled to fly from Chicago O’Hare to Bloomington-Normal in central Illinois made it as far as the vicinity of its destination before having to divert all the way to Madison, Wisconsin, due to the low visibilities. The flight track, from FlightAware, is seen here:
