If you happened to glance at the radar image on the front page of the NWS Chicago/Romeoville website Tuesday afternoon, you may have noticed what looked like rain showers over northeast Illinois. How could that be? It was sunny, with barely a cloud in the sky! What you saw, was the radar reflectivity image of a smoke plume from a controlled burn being conducted at Moraine Hills State Park, near Mc Henry, Illinois. The WSR-88D, especially when operating in one of the "clear air" mode volume coverage patterns, can detect the particulate matter in smoke plumes, given that they are dense enough and fill a sufficiently large percentage of the radar beam-width. The plume was produced between approximately 1:30 pm and 3:30 pm CDT, though residual weaker returns were still visible over Lake Michigan at 5 pm as the plume continued to drift off to the southeast.
The image below shows the radar reflectivity data at 3:35 pm CDT. The smoke plume can be seen as an area of colorized radar returns, some as high as 40 dBZ, trailing southeast across northeast Cook County and out over Lake Michigan. The orange square, near the start of the plume, is the approximate location of Moraine Hills State Park. The height of the radar beam over this location is approximately 3500 feet above ground level (AGL).
An aircraft departing O'Hare International Airport at approximately the same time as the radar image shown above, provided the following ascent sounding of temperature and wind: Note that winds in the 3500' AGL range were northwest at around 25 knots, which is why the plume propagated to the southeast, across the Buffalo Grove, Northbrook and Evanston areas.
So, even on a sunny day, with few clouds in the sky, the radar can show fascinating detail of non-weather related phenomena!