With an actual temperature of 6 degrees F and a wind chill of -16, it was too cold to walk The Six-Year-Old to school this morning. It’s a shame, because otherwise it’s lovely outside. The sun is shining, and the air is relatively still for Chicago.
As we trudged to the car this morning, The Six-Year-Old’s mind had a question:
“Mommyo, if we’re in the middle of a polar vortex, why isn’t the air spinning?”
Before we begin, I would like to remind you that I am not a meteorologist, nor have I ever studied meteorology, aside from a partially remembered Oceans class I took back in the day at Rice University. I have, however, done quite a bit of reading in the lay press on the subject of polar vortices recently and distilled it into the following explanation for The Six-Year-Old. (But oh my goodness, did I ever simplify things. I suggest if you’re looking for more, you start by checking out some of the links at the end of this post.)
We aren’t actually in the middle of a polar vortex
Turns out all this talk of a polar vortex blanketing Chicago is a bit sloppy meteorologically. Strictly speaking, the polar vortex is a circulating mass of frigid air anchored over Baffin Bay, north of Canada. The temperature differences between the extremely cold air over the Arctic and the normally milder air over the United States combine with the spinning of the Earth’s surface to create a strong west-to-east wind (the jet stream) that generally holds the polar vortex in place over the Arctic during the winter months.
Why is it so cold?
As WGN-TV meteorologist Tom Skilling cheerfully says, cold air has been abundant in Chicago this winter. We’ve had 22 days with lows below zero this season. (Only three days to go to set a new record!) It’s been so cold here lately that even Anana, the polar bear at the Lincoln Park Zoo, has had to spend a few days indoors.
But the Arctic hasn’t been quite as cold as usual (remember all those stories about the rapidly melting Arctic ice?), which means the temperature differences between the Arctic and our part of the world haven’t been quite as stark. Without broad temperature differences to fuel it, the jet stream keeping the polar vortex in place has weakened a bit, allowing masses of frigid Arctic air to escape the polar vortex and move south to winter over the Midwestern and Northeastern portions of the United States.
If there’s so much wind, why isn’t the air in our parking lot spinning?
The jet stream — the river of wind that typically keeps the polar vortex in place — is very high up in the Earth’s atmosphere — seven miles up in what is known as the troposphere.
We can feel the effects of changes in the jet stream here on the ground — when the jet streams shifts south over Chicago in the winter, it brings Arctic air down with it, for example — but we typically don’t feel the jet stream winds themselves (unless we’re in an airplane traveling from Los Angeles to New York City that’s using the jet stream to boost its speed). Good thing too, as the winds in the jet stream generally pelt along at 110-250 mph.
So what makes the wind in the parking lot?
That, The Six-Year-Old, is an excellent question for another day.
- Polar Vortex, Part II? Nah, It’s Just Winter (livescience)
- Polar Vortex to Once Again Grip Midwest, Northeast (AccuWeather)
- Frigid arctic air strikes again, but only a little (CNN)
- Why we can blame a warm Arctic for this winter’s chill (Smithsonian)
- What is a polar vortex? The arctic winds that brought cold air and chaos to the US (The Independent)
- Wordless Wednesday: The View from my Window (Caterpickles)