With temperatures below freezing and saturated air (high relative humidity) aircraft descending to land can cause snow. It’s a very localized event, not widespread snow, but snow can form and then fall as a result of planes descending under the right conditions.

This requires “supercooled atmosphere interacting with exhaust” and “water vapor [that is] below freezing but [that] couldn’t entirely transition into ice crystals because of a lack of particulates upon which to freeze.” Then the plane descending through the atmosphere causes snow to form.

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.

This phenomenon actually happened last week, and was noticed by the National Weather Service Chicago – localized snowfall from planes landing at both O’Hare and Midway airports.

Light snow near Chicago? We think this is being caused by airplanes descending into O'hare and Midway airports this evening. Be warned: there's lots of science ahead in the attached graphic! #science #snow #ILwx pic.twitter.com/L1SFioO631

— NWS Chicago (@NWSChicago) January 27, 2020

This is some pretty cool science, but no doubt will span chemtrail conspiracy theories and other theories of how human activity controls the weather.