The cold, hard facts about the snow that lingers into spring

OK, Mother Nature, you clobbered us but good this winter. Sure, we whined and cursed as we vacillated between feeling sorry for ourselves and getting mad at you. But we got over it, and now we're ready to forgive and forget.

So why aren't you?

You keep finding ways to rub our weather-weary noses in our winter-for-the-record-books. Sure, the ground snow disappeared quickly once the temperatures started to climb, but the snowbanks that formed in shaded areas and, worse, some of the massive piles of snow that were left by plows are hanging tough.

Even when we flirted with a balmy 70 degrees last week, people enjoying the fresh air with a stroll most likely saw dirty piles of snow lurking here and there. It smacked of cruel and usual punishment.

"It's just a reminder that winter wasn't too long ago," said Pete Boulay, the assistant state climatologist.

Big piles of snow take longer to melt, he said. If you want to get all technical, there are factors beyond just the temperature that affect how quickly — or slowly — a snowbank melts, said John Abraham, an expert in heat transfer and a professor of thermal sciences at the University of St. Thomas.

"Snow has a lot of air in it," he said. "Even though the snowflakes are touching, there still are a lot of air gaps. As the snow starts to melt, it refreezes as solid ice, which is much denser. And the denser it is, the slower it will melt."

Unless the temperature skyrockets to ridiculously unseasonable levels — we're talking high 80s or even 90 — the sun is more potent than the air in melting ice. That's why snowbanks that are shielded from direct sunlight can last well past the point where we've put away our winter gear.

The dirt that caps the piles of snow left by plows might be ugly, but it helps with the thaw, Abraham said. White snow reflects the sunlight, negating some of its power, but the dirt absorbs sunshine and warms up.

That dirt isn't blown in by the wind, either; it's hidden inside the drift and surfaces as the snow melts around it.

"When it snows, we sand," Abraham said. "Then it snows again, so we plow, taking the sand with the snow. And then we put down more sand. And then it snows again, so we plow. And so on. Eventually we end up with a snowbank that's covering up layers of sand. As the snow melts, the pile gets dirtier and dirtier because all those layers of sand are being exposed."

Wind and rain also affect the melt rate. Wind causes evaporation and sublimation (the term for when a solid turns directly to a gas, skipping the liquid state). Rain eats through snow and ice, although the speed at which that happens depends on how hard it's raining and the temperature of the rain. Running water beneath a snowbank will erode the ice, too.

If there's a lesson in the enduring snowbanks, Abraham said, it's this: Don't get cocky. It's still only April.

"Remember last year, when we had snow in May?" he said. It's OK to get out your golf clubs, he said, as long as you keep your snow shovel handy, too.

Jeff Strickler • 612-673-7392