Chicagoland residents awoke Wednesday morning not to a bright sunrise, but to spectacular shelf cloud that sent social media abuzz on the last full day of summer.
A line of thunderstorms sagged southward into northern Illinois from Wisconsin early Wednesday morning.
While not a severe weather producer - mainly some 30-50 mph gusts, pea-size hail - the squall line hit less humid air ahead of it, generating a gust front that surged ahead of the line of thunderstorms into southeast Wisconsin and northern Illinois.
By the time it arrived in Chicago, the gust front, marking the leading edge of the squall line's stronger wind gusts, was about five miles ahead of the line of thunderstorms.
It was this gust front that produced a spectacular shelf cloud visible throughout Chicagoland.
While menacing in appearance, shelf clouds are not tornadoes or wall clouds.
What you're seeing in a shelf cloud is the boundary between a downdraft and updraft of a thunderstorm or line of thunderstorms.
Rain-chilled air descends in a thunderstorm's downdraft, then spreads laterally when reaching Earth's surface. Warmer, more moist air is lifted at the leading edge, or gust front, of this rain-cooled air. When this warm, moist air condenses, you see the shelf cloud.
As the shelf cloud passes, you feel an abrupt shift in wind direction and increased wind speed, followed within minutes by heavy rain or hail. Wind gusts once the shelf cloud has passed may be quite strong, causing downed trees, tree limbs and power outages.
The dark, turbulent clouds you see after the shelf cloud passes are known colloquially by meteorologists as the "whale's mouth".
In this case, the dark cloak of the whale's mouth contrasting with the bright sky in the eastern horizon over the Chicago skyline provided an impressive, almost cinematic contrast.
Short video of a shelf cloud approaching downtown Chicago! pic.twitter.com/XUQzCbTmyU— Craig (@cshimala) 21 Σεπτεμβρίου 2016