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We've seen staggering weather extremes in the Northern Hemisphere this year. In Colorado, residents are in the early stages of recovery after flooding. Before that, there were flash floods in Europe, record cold temperatures in Bangladesh and Florida, also a heat wave in Alaska.

NPR's Jon Hamilton reports that all these events are tied to unusual behavior by the powerful air currents known as the jet stream.

JON HAMILTON, BYLINE: When people in the U.S. talk about the jet stream, they usually mean the polar jet stream. Jennifer Francis is a research scientist at Rutgers University.

JENNIFER FRANCIS: So it's this ribbon of very fast-moving winds that's high over our heads. And through all the dynamics of the atmosphere, it actually is what creates the weather that we feel here on the surface.

HAMILTON: That's partly because the jet stream contains a lot of energy in the form of heat. Also its winds - which can reach 200 miles an hour - help move weather systems from west to east across the country. And Francis says the jet stream acts as a sort of dividing line.

FRANCIS: If it's north of you, you're in the warm air. And when it's south of you, you're in generally much colder air.

HAMILTON: The jet stream does not follow a straight line - it's wavy. Francis says it usually enters the U.S. somewhere in the Northwest.

FRANCIS: Seattle is a very typical place. And then it might dip down southward along the Rocky Mountains. And then it might shoot northward again, say, up towards the Great Lakes.

HAMILTON: But this year the jet stream has been making more dramatic swings to the north and south. And John Nielsen-Gammon of Texas A&M University says it's done something else that's odd.

JOHN NIELSEN-GAMMON: This year we've had a strong tendency for what I call a double jet stream.

HAMILTON: The single ribbon of fast-moving winds has split into two ribbons: one farther north, one farther south. Nielsen-Gammon, who is the Texas state climatologist, says back in May and June that split led to flooding in the northern Alps and in Alberta, Canada.

NIELSEN-GAMMON: The flood conditions took place in between the two jet streams, where weather patterns tended to move relatively slowly because they weren't being carried along by the upper level winds.

HAMILTON: And Nielsen-Gammon says during the summer the double jet stream produced a very strange temperature pattern in areas near the Pacific coast. Southern California was unusually hot and over in Death Valley it even hit 129 degrees. But Nielsen-Gammon says up in British Columbia the weather stayed unseasonably cool.

NIELSEN-GAMMON: And then you go farther north, then again temperatures are warmer than normal. So that makes Alaska have relatively warm conditions during the summertime compared to what it normally does.

HAMILTON: And finally, way up in the Arctic, temperatures were relatively cold again. Nielsen-Gammon says the double jet stream also played a big role in the Colorado flooding of a couple of weeks ago.

High up in the atmosphere, the jet stream was carrying moist air from the Pacific to the Rockies. Then, lower down, it had created an unusual eddy that was pulling in more moist air from the Gulf of Mexico. Finally, Nielsen-Gammon says, an unusual bulge in the jet stream was causing all of this weather to stall right around Boulder.

NIELSEN-GAMMON: So when you get all three of those things coming together underneath a jet stream pattern that isn't moving very rapidly, you get lots of rain for lots of days. And on occasion that can produce the kind of flooding we saw here.

HAMILTON: It's not clear whether global warming contributed to the recent extreme weather in Colorado. But Jennifer Francis of Rutgers says it could have, because the Arctic is warming up quickly.

FRANCIS: We realize that this rapid warming of the Arctic - which is about two to three times faster than the pace of warming in the rest of the Northern Hemisphere - must have an effect on the jet stream.

HAMILTON: That's because it's the temperature difference between the equator and the Arctic that powers the jet stream in the first place. So if the temperature difference decreases, the jet stream should become weaker. Francis says her work suggests the result will be that weather systems move more slowly from west to east. Instead of moving on, they will linger.

FRANCIS: So it could be drought. It could be heat waves. It could be flooding due to prolonged rainfall. All of those kinds of patterns should be coming more likely.

HAMILTON: It's still a controversial idea. But it's one that's got other scientists taking a closer look at changes in the jet stream.

Jon Hamilton, NPR News.

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