By now, if you live in the northeastern U.S. you have heard or even said the following: “This summer has been so cool. I love it.” Or: “This summer has been so cool. I hate it.” Yet if you live in Oregon or Washington, you’ve heard the opposite: “It’s been so hot this year!”

Maybe people just like to complain? Nope. The data is in and headlines are everywhere the past few days: temperatures across the Northeast have been unusually cool, and they have been unusually hot across the northwest. Boston typically hits 90 degrees Fahrenheit on about 15 different days of the year, but so far it has only reached that mark four times. Washington, D.C. , had hit the century mark 16 times by August 15, which might sound like a lot until you learn that the average by that date is 29. Meanwhile, Portland, Ore., has sweated 90 degrees F 12 times already; that usually happens just a few times a year.

A strange jet stream is behind the flip-flop between summer conditions in the two northern corners of the country. The polar jet stream is the prevailing band of wind that blows west to east across the upper half of the U.S. in summer. (There is a subtropical jet, also, that typically crosses northern Mexico). We often see the jet stream depicted on TV weather reports—that big, wavy line across the U.S. and Canada that bends south then north then south again. Low-pressure weather systems, sometimes called cold fronts, ride along the jet stream, bringing us much of our daily weather. But this summer the polar jet steam seems to be somewhat flattened out, and it’s been in that position more than usual. Meteorologist Dylan Dreyer of NBC’s Today show summed it up nicely in a short segment I happened to see this morning. Watch the video below, if you like.

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Data from the National Weather Service, and headlines from numerous states, reflect the flattened jet stream Dreyer shows in the video. In addition to the Northeast, middle-American cities from Chicago to Dallas and even further west to Denver have all had fewer hot days. So what could make the jet stream flat? A combination of forces in the atmosphere and the Pacific Ocean come into play day to day, but those factors themselves may be changing due to climate change. Several studies published in the last two years in leading journals have indicated that the jet stream’s waviness may be becoming more extreme, and it may be getting stuck in those patterns for longer periods of time.

In the winter, “extreme” often means an even larger, sharper curve than usual—bringing the infamous polar vortex down from the Arctic, for example. But this summer, extreme seems to mean flatter. A study led by S.-Y. Simon Wang at Utah State University, published in April in the journal Geophysical Research Letters, noted that the extremes have been made more likely because of climate change. As I’ve written before, our changing climate is not so much “global warming” as it is “global weirding.”

We will see, of course, whether this weather pattern holds up for the rest of the summer. But it’s already August 18, and the 10-day forecast for my home town in Masachusetts shows nothing but high temperatures in the 70s.

Jet stream image courtesy of NOAA

Video courtesy of NBC