This summer's wacky weather explained

A mother and daughter cover themselves from the rain in Los Angeles.

If you live in certain parts of the U.S., you haven't had to use your air conditioner much. Whatever happened to global warming?

More than 3,000 record low temperatures were set across the United States in July. Some skeptics are asking: What happened to global warming?

On July 3, in Waco the high topped out at 58 degrees — the coldest temperature ever in July in this usually parched, scorching hot Texas town. Twelve days later, the high in Norman, Okla., reached 73 —that's 22 degrees below normal. And at the end of the month, temperatures nuzzled the low 40s in Sioux City, Iowa, and 50 degrees in Cincinnati.

In what is usually one of the hottest month of the year, unseasonably cool spells lingered over many spots east of the Rockies.

Many meteorologists believe that climate change — especially rapid warming in the Arctic — is actually behind the summer chill as well as some unusually severe heat waves in the Northeast and the West. Here's how it works -- the jet stream (the zone of upper-atmosphere winds encircling the Northern Hemisphere) is like a big belt that normally tightly holds the polar air in check. But now it's not so cold up in the northern pole regions, so the jet has less work to do.

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"If you loosen a belt around a big stomach, the fat kind of spills down over it," says meteorologist Jeff Masters, co-founder at the popular website Weather Underground. That's why the Arctic air sags southward, he says, since nothing is stopping it. Rather than blowing strongly west to east as it normally does, the jet stream has gotten lazier, often contorting itself into big snake-like twists and turns and staying kinked like that for longer stretches.

When this happens, huge dips, or troughs, of cool air plunge far to the south and, many hundreds of miles to the east or west, corresponding humps in the jet stream form sharp ridges that push warm air incredibly far north. And very dry or very rainy weather systems get stuck in the jet stream's kinks and folds for days or weeks. As Masters recently blogged:

"When the jet stream goes into one of these extreme configurations, it freezes in its tracks for weeks, resulting in an extended period of extreme heat or flooding."

That's why this summer has featured temperatures as high as 98 degrees in Alaska, massive forest fires in Quebec, record rainstorms in Toronto, Calgary, and Philadelphia, and a week of oppressive 100-degree days in New York City. Averaged over all land and ocean surfaces, temperatures have warmed roughly 1.33 degrees F over the last century, according to the Intergovernmental Panel on Climate Change.

A month ago, with weather patterns stagnant, a massive dome of intense heat built over the Southwest, producing all-time highs in Utah and Nevada. The day before Waco was chilling, on July 2, Death Valley, California, reached an incredible 129 degrees, the highest temperature on Earth in a century — since 1913, when some 130-plus readings were recorded there.

Related: Experts and CIA look at manipulating weather

For the same reason, Europe has seen extremes this summer, too — with record heat (87 above the Arctic Circle in Finland) and historic floods along the Danube.
Abnormal weather is the new normal. Scientists at the Potsdam Institute for Climate Impact Research in Germany say extreme summertime jet stream patterns were twice as common so far this century than in the period from 1979 to 2000.

What to expect for August? According to the National Weather Center, it's going to be much hotter than usual in — can you believe it? Idaho, Alaska, and Maine — and mild most everywhere else. Hit the beach around Bangor or Anchorage? Maybe, but pack a change of clothing, in case the jet stream suddenly wakes up from its nap and starts to wobble.

And Masters expects extreme weather of many kinds to continue in summers to come.

"I would expect to see some crazy stuff, even some unexpected things," he says, noting "our models don't handle chaos very well."


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