From the ABC 7 Weather team

Sleet in D.C. Wednesday... or is it graupel?

March 30, 2011 - 05:07 PM
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The frozen stuff bouncing off of car hoods and bald heads is probably not sleet, but a much rarer precipitation type. Say hello to graupel.

A handful of graupel from a winter storm last year in Westwood, Mass. (Parkerjh)

There are scattered reports of a harder kind of rain falling this afternoon. The word people are throwing up on Twitter is sleet:

• There is SLEET in Bethesda. This is not okay.

• Snow & Sleet falling in Arlington...Cherry blossoms are out, temps were in the 60's last week...WTH?

• On my walk through the parking lot to my car found out that bald men and sleet don't mix! OUCH!

Sure, it’s icy and coming down like rain. But our ABC7 weather team is a stickler for the details, and according to meteorologist Alex Liggitt, what is bouncing off of sidewalks, car hoods and bald heads is more likely graupel.

So what's the difference?

Sleet is usually a see-through, solid ice grain. Sleet forms when raindrops or partially melted snowflakes refreeze as they fall through a layer of sub-zero air near the earth’s surface.

But graupel, a German word for what’s also known as “soft hail,” is slightly different. (It's also my favorite kind of precipitation.) This wintry precipitation is created as supercooled droplets collide with snowflakes and cover them with an accretion of rime. As the droplets keep sticking to the flake, the original crystal is obscured and something that looks like this takes its place:

gra

Beltsville Agricultural Research Center (USDA)

Graupel is normally cloudy or white, not transparent, and is not often seen in the D.C. region. If it's not out of here already, it will be very soon. Here is Liggitt’s definitive word on our brief, exciting Wednesday graupelling:

“You can really get it anytime from fall through spring. For true sleet, there needs to be snow completely melting into rain, then refreezing before hitting the ground. That means there must be some kind of a layer aloft where there is warm air, then cold air. For graupel, it must depend where the cloud layer is located and how cold it is as supercooled droplets interact and freeze on contact with snowflakes. This is also a good time for it as the profile above us must have a shallow warm layer so it doesn't melt on the way to the surface. Nice and confusing, but also not a common occurrence around here.”

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