The extremely cold temperatures from last winter and the current stifling heat waves blanketing the U.S. and U.K. are caused by what may be an increasingly common type of weather formation called a "blocking high." According to New Scientist, these systems of high pressure form and then remain stubbornly in place, causing extended periods of extreme weather.

Blocking highs have been blamed not only for this year's intense heat, but also for Russia's devastating wildfires in 2010, as well as the catastrophic floods in Pakistan that same year. These bubbles of high pressure form in the atmosphere when the jet stream -- one of the major weather-defining weather system of the northern hemisphere -- meanders off course, disrupting weather systems around the world.

Rutgers University climatologist Jennifer Francis told New Scientist that weather is getting "more stuck" because of blocking highs. She said that the jet stream is flowing about 14 percent slower than it was 30 years ago. As a result, it drifts north toward the warming Arctic and south toward the tropics more frequently, setting up perfect conditions for blocking highs.

Other scientists disagree, including Ian Simmonds of the University of Exeter, UK, who published a report earlier this year arguing that the jet stream is not, in fact, meandering any more than it ever has, so now, said New Scientist, "everyone is confused."

This week, the intensely high temperatures in the northeastern U.S. were caused by what CBS News called a "wrong-way heatwave," where wind patterns that usually blow from west-to-east have been flowing east-to-west, bringing sweltering temperatures and high humidity to Boston, New York City, Philadelphia and other northeastern cities.

Climatologists blame a high pressure system that was born over the western Atlantic Ocean and is still pushing westward. A cold front is finally expected to move through the area Saturday night, bringing lower temperatures and relief to the area's millions of residents.

Jon Gottschalck of the National Weather Service told CBS that the unusual weather pattern cropped up so suddenly that weather forecasters were caught flat-footed.

"It's definitely unusual and going the wrong way," he said. "This is pretty rare."

The jet stream, which is currently parked over Canada, is expected to return to a more normal course next week.

[image of man cooling off his dog under a spigot via ChameleonsEye /]