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Understanding the 'Jet Stream'

The mercury is falling as it always does this time of year, but what determines the difference between a “mild” 32-degree winter day and a bone-chilling, single-digit nightmare?

The simple answer is the position of the northern polar jet stream referred to, simply, as “the jet stream” in local weather reports.

“It’s a river of air that serpentines around the globe,” explains National Weather Service Observer William Gregway.

The river in question flows in a westerly direction at an altitude of over 23,000 ft., at a speed of over 100 mph. It’s relatively thin, about 100 miles in width and only a couple miles deep. This river meanders, changing shape while always flowing in a westerly direction.

“Ben Franklin discovered that weather flowed from west to east by correlating letters with a friend on the coast,” says Gregway. It wasn’t until much later, during the II World War, that pilots made a remarkable discovery—it often took far less time to travel from the West to the East Coast than vice versa. The strong head wind they encountered flying westward was the jet stream. Alternately, the jet stream can be utilized to shorten an eastward trip.

The jet stream represents the border between frigid arctic air and warmer, sub-tropical air.

“If we’re on the right side [of the jet stream], the temperature will be warmer,” says Gregway. “If we’re on the left side, it will be colder.”

The shape of the jet stream is primarily defined by the Pacific Ocean, says Gregway. Periodic oceanic phenomena like El Nino can dramatically alter the course of the jet stream.

“[El Nino] affects the West Coast more than us. It’s actually strengthening right now,” says Gregway. “It usually gives the West Coast a wet winter. We usually have a wet, mild winter during an El Nino.” Gregway notes, however, that current predictions for the 2009-2010 season predict an “average” winter for the region.

During February, typically the most frigid month in Central New York, the jet stream tends to be sweeping frosty air southward from Hudson Bay according to Gregway.