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Snow: Depth depends on height

Up to 2 inches so far, and elevation could make all the difference in snow totals.

This has been a classic March snowfall, one of those days when you might see snow on Billy Penn's hat above City Hall, and nothing on the ground.

When they start coming in, snow totals are going to vary widely, and not just by location – but by elevation.

As often happens in March when temperatures are borderline, the region's irregular topography is going to be a huge factor in whom gets what.

This is one reason that snow-accumulation forecasts can be bedeviling, and something that the big outfits such as AccuWeather and The Weather Channel grapple with all along the East Coast.

Subtle changes in elevation along the so-called Fall Line, which stretches from near Atlanta on up to New England and marks a DMZ between the coastal plain and the Appalachians, can be significant factors.

That line passes right through Philly, and we can almost guarantee that Roxborough will have more measureable snow than Philadelphia International Airport.

Particularly when winds are from the east, those small changes in height can add lift to moist air, increasing snow intensity; snow falls when warm air rises over cold.

What's more, surface temperatures decrease with height, about 3.5 degrees Fahrenheit per every 1,000 feet, as Tom Niziol at the Weather Channel points out, so give or take a degree for every 300 feet.

Places such as Chestnut Hill and Lancaster Avenue in downtown Wayne are about 400 feet above sea level.

And one basic consideration: If snow lands on an area elevated 400 feet, a flake has less of an opportunity to liquefy on its journey to the surface than it would if it had to travel all the way to sea level.

The first accumulation reports are coming in, and the heftiest totals we've seen approach 2 inches. Predictably, one of those came from Wayne.

Unfortunately, the reports include only place names, not elevations. But as Niziol said, "If you want snow, elevation is your friend."