If you happened to glance at the Chicago radar Tuesday morning, Nov. 27, you probably saw something a bit wacky. 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.
The first aberration showed up on the radar of the National Weather Service in Chicago at 6:36 a.m. A tendril of light radar returns streamed southeastward, emanating from a point on the shore of Lake Michigan in Gary, Ind. What else is there? Gary Works, a major steel mill employing 5,000.
Steam from the plant's smokestacks combined with chilly air in the 20s to deposit a thin strip of snow roughly 18 miles long and pointed southeast from the city. It stretched to Porter County, Indiana, where meteorology students at Valparaiso University first noticed the phenomenon and tweeted about it.
The day's magic wasn't done.
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.
Because NWS Chicago's radar dome is about 25 miles from the airport, it scans low in the sky - between 1,800 and 2,300 feet. That's roughly the height of airplanes as they enter landing patterns during their final descent.
With a bit of sleuthing, we can figure out what was going on that caused airplanes to trigger snowfall. 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.
A weather balloon launched around then from Davenport, Ill., suggests that this layer of moisture extended upward to about 4,500 feet.
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.
There are a couple potential causes. One is that the aerosols, soot, sulfates and hydrocarbons engines spew act as tiny nuclei for snowflakes to form. The other is that lift around the wing causes ice crystals to form, seeding the air in similar fashion.
No matter the cause, there was one result: a narrow ring of snow flurry activity, with flakes and overcast skies reported at O'Hare during the morning. A day earlier, a similar process resulted in swirls of clouds appearing on satellite 75 miles west of O'Hare, where planes were maintaining a holding pattern.
This is not the first time planes and power plants or other industrial practices have contributed to snowfall. In fact, man-made snow is much more common than one might think.
On Jan. 22, 2013, nearly an inch came down in Shippingport, Pennsylvania, downwind of Beaver Valley Nuclear Power Plant. Earlier this year, Boone County, Kentucky, picked up 4 inches in a similar manner. Tulsa saw power-plant-induced snow the same week.
It's not only power plants whose steam can do the trick. On Jan. 19, 2011, two slaughterhouses contributed to wintry ambiance northwest of Dodge City, Kansas.
This article was written by Matthew Cappucci, a reporter for The Washington Post.