(Image: Climate Reanalyzer/Climate Change Institute, University of Maine)
What you're looking at is a map of today's temperature "anomaly" — the difference between today's mercury reading and the baseline set between 1979 and 2000. Where it's colder than usual at this time of year, the map skews purple, and where it's warmer, it gets redder.
The image is a dramatic depiction of the effect of the polar vortex that has been wreaking havoc on the eastern half of Canada and the United States. As climate and geophysics researcher Stefan Rahmstorf put it succinctly on Twitter, "cold eastern US, hot Europe, warm Arctic."
Here's another look at the effects of the polar vortex across the Great Lakes region, this time from NASA's Earth Observatory:
(Image: Jesse Allen and Robert Simmon/NASA)
This one shows what happens when the cold Arctic air passes over the (relatively) warm waters of Lakes Michigan and Superior: a cloud of "steam fog." Snow here is shown in bright orange, with water clouds in white and mixed clouds, containing both water and ice crystals, in peach. Here's what the steam fog looks like up close:
If you're not entirely clear on what this whole "polar vortex" thing is, here's a quick primer: The polar vortex is a semi-permanent low-pressure system that circles around the Arctic from west to east, according to Scientific American. When that vortex of air weakens — as it does from time to time — it can push extremely cold air down toward the rest of Canada and the U.S., or to regions of Eastern Europe. It can even mess up the jet stream, the band of wind that flows from west to east across the northern hemisphere, pushing it further south and resulting in heavy snow.
But what causes the polar vortex to weaken and migrate southward?
According to some climate researchers, it's believed the phenomenon could be related to the loss of Arctic sea ice over the last few years. As the ice melts in the summer, it can push warmer air into the atmosphere, which may weaken the vortex over the subsequent winter — sending cold air south.
"I do think that what has happened in North America, including the U.S. this winter, so far fits under the paradigm of 'warm Arctic, cold continents,'" Judah Cohen, a climate forecaster at Atmospheric and Environmental Research in Massachusetts, told Climate Central. Still, the jury's out on the strength of the connection between a warming Arctic and the chilly temperatures that have fallen over so much of North America this week.