Yesterday, a combination of moisture and cold caused snow to fall in a singularly odd pattern near Chicago:
Although no widespread weather systems were in the area to crank out snow, flurries were still falling across parts of the area.
These unusual phenomena were thanks to a supercooled atmosphere interacting with exhaust from a power plant and also the air flow around commercial aircraft.
Farther to the north, a bizarre radar signature in the shape of a loop showed up just northeast of the Windy City, out over Lake Michigan. It turns out this dash of winter was caused by aircraft landing at O'Hare International Airport.
Observations from the airport at the time reveal a temperature of 22 degrees and a dew point of 17 degrees, both well below freezing. Additionally, the closeness of the temperature to the dew point meant the air was near saturation. There was 82 percent relative humidity at the time.
It's likely that supercooled water droplets were present in this air mass. That means the water vapor was below freezing but couldn't entirely transition into ice crystals because of a lack of particulates upon which to freeze.
In this case, though, an aircraft - or many aircraft - passed through this layer.
Meanwhile, police and firefighters closed streets around tall buildings in downtown Chicago yesterday as chunks of ice came crashing down on them. (On the streets, not the firefighters.) You can imagine the commute.