feeling that cold wind?

Feeling That Cold Wind? Here’s Why. - NYTimes.com Feeling That Cold Wind? Here?s Why.
By KENNETH CHANG
Published: January 9, 2010


What?s going on? Global cooling?

Nope. A mass of high pressure is sitting over Greenland like a rock in a river, deflecting the cold air of the jet stream farther to the south than usual.

This situation is caused by Arctic oscillation, in which opposing atmospheric pressure patterns at the top of the planet occasionally shift back and forth, affecting weather across much of the Northern Hemisphere.

What?s notable this year is that the pattern of high pressure over the Arctic is more pronounced than at any time since 1950.

In most years over the past few decades, the opposite has been true: there has been lower-than-average pressure over the Arctic, and higher-than-average pressure over the mid-latitudes ? the middle of which cuts through Maine, across the Great Lakes and on to Oregon.

That pattern allows the jet stream to blow unimpeded from west to east and keeps the cold Arctic air largely north of the United States. The result tends to be warmer temperatures across much of the United States east of the Rocky Mountains. ?Pretty much all of the Arctic is above normal,? said Dr. Walter Meier of the National Snow and Ice Data Center in Boulder, Colo. In some areas, the temperatures are as much as 15 degrees Fahrenheit above normal.

In terms of global average temperature, this winter?s arctic oscillation ?probably roughly cancels out,? Dr. Meier said. (In fact, last year ranked as the fifth-warmest year on record since 1850, the United Kingdom?s Met Office says.)

And it is certainly not the coldest air that has descended on the United States. In a great blizzard that swept across the East Coast in 1899, even parts of Florida dropped to below zero.

?We?re not close to those types of things,? said Michael Vojtesak of the National Weather Service.