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Weather or Not | This year's outlook: Little snow

And for a glimpse at winter in the U.S., October in Siberia could be the key.

If the consensus among forecasters is correct - obviously a major qualifier - this could be a relatively gentle and snowless winter for much of the nation.

And it would dovetail with an increasingly apparent trend: The snows of yesteryear were more impressive than those of today.

The Northern Hemisphere's total snow cover (defined as geographic extent as opposed to, say, depth or frequency) has dropped off precipitously in the last three years, according to the Rutgers University Global Snow Lab, one of the nation's prime sources of snow data.

David Robinson, the geography professor who runs the lab, has a comprehensive satellite-era dataset dating to 1966, and he also has looked at surface records going back a century.

One set of numbers jumps out: the recent, rapid and "unprecedented" retreat of spring snow cover, Robinson says. "The melt season has come earlier."

The most likely suspect? The global warming trend that has accelerated in the last 30 years.

But as the U.N. Climate Change confab gets under way in Bali, Indonesia, this week, don't expect anyone to identify the loss of snow cover as the smoking ice cube that seals the case for an impending, human-induced global meltdown.

First, and curiously, the snow cover in other seasons (summer, fall and winter) hasn't shown big changes, Robinson says.

Second, after falling off in the late 1980s, reaching a record low in 1990, the snow cover rebounded - until three years ago. So the decreases in complete calendar years 2004, 2005 and 2006 could constitute an anomaly.

Third, Robinson, who contributed to the blue-ribbon Intergovernmental Panel on Climate Change (IPCC) reports, says that one could make the interesting argument that the world is warmer because of less snow.

"It's a chicken-egg question," he says.

Snow has a profound effect on temperature. That's a big reason why so much concern is focused on the melting of arctic ice.

The Earth acts something like a radiator. Sunlight warms the surface, and the heat is radiated outward.

White snow on the ground blocks the sun, and also diverts some of that energy for melting, so surface heating is limited. During the day, snow retards the rise of temperature. At night, especially when it's clear and winds are calm, any heat energy is rapidly radiated into space.

At daybreak in a snow-covered suburb, the temperature might be 10 to 15 degrees lower than it is above bare ground at Philadelphia International Airport.

"Snow cover is a good thermal insulator," says Judah Cohen, an atmospheric scientist who works for AER Inc., a research company in Massachusetts. "If you put it down, it pretty much puts an end to the exchange of heat between the ground and the atmosphere."

Take snow away, and the surface can heat up in a hurry. What's more, the warming becomes contagious. Melting begets melting; ice and snow in Greenland and the arctic are disappearing faster and faster.

Within the season, snow cover has a profound impact on the course of a winter. It can influence the path of the jet streams, the upper-air boundaries between warm and cold air that govern the movement of storms and arctic air. It also helps drive the position of those frontal boundaries.

Snow-cover tracking - primarily by analysis of satellite images - could become a useful tool for seasonal forecasting, Cohen says. He and a colleague, Christopher Fletcher of the University of Toronto, recently published a paper on the connection between Siberian snow cover in October and American temperatures in winter.

The physical mechanism isn't fully understood, Cohen says, but fall snow coverage evidently affects air pressure patterns that influence other changes: An above-normal snowpack in Siberia in October correlates with below-normal temperatures in the United States in winter.

October is the key, Cohen believes, because when the sun is strongest (summer), the snow is scarce; when the snow cover peaks (winter), the sun is absent. October has both.

"We're familiar with it," Mike Halpert, a forecaster at the government's Climate Prediction Center, says of Cohen's hypothesis.

But Halpert says he and his colleagues haven't seen enough evidence to use it as a forecasting tool.

The Inquirer's own look at Siberian snow cover in October and Philadelphia temperatures in winter got decidedly mixed results.

If there is a connection, snow lovers will find no solace in Siberia: Snow cover was below normal in October 2007.

By spring, odds are there won't be much snow to melt. Be warned, however. Nature may have other ideas.