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Saturday, March 14, 2009

It Graupeled and Snowed Today



Blog entry for March 2, 2009

Although the calendar has flipped to March, winter here in ENC is hanging on. A winter storm was forecast to hit parts of western and central NC overnight Sunday (March 1st). In my neighborhood, the weatherman said maybe a trace to 1” accumulation. As a prelude to the winter arrival, we’ve had 5” for rain over the past 48 hours.

At 7:00 a.m. Monday morning (March 2nd) the skies were dark gray, temps hovering near freezing, and the wind was cold out of northwest. It felt like it was going to snow, but nothing yet.

A few flurries started later in the morning, but no accumulation. Off and on throughout the day it would precip briefly then stop, then start again. As of 11 p.m. it was snowing again. A narrow band of snow showers formed near the NC/VA boarder near Kerr Lake trailing southward. Could this be lake effect snow?

What made this event different is the type of precipitation. Most folks don’t know that not all snow is created equal. For the first time in recent memory (at least at my house) what fell from the sky was a combination of the typical fluffy flakes of white, and what meteorologists call graupel.

So what the heck is graupel anyway?

Unlike snowflakes, grauple or snow pellets are small Styrofoam balls(see photo below) of white ice particles that fall as precipitation, and easily break apart when it lands on a surface. It forms when a snowflake high in the atmosphere encounters supercooled water, and ice crystals begin to form instantly on its outside edges. As the ice accumulates on the surface, the original snowflake no longer is distinguishable instead forms a ball or pellet. This building up of ice crystals is called a riming.

How can you tell the difference between graupel and hail? Graupel typically falls apart when touched or when it hits the ground. Hail is formed when layers of ice accumulate forming a very hard solid piece of ice.

Because graupel is powdery and white, it’s considered to be a form of snow. However, some meteorologists have argued it’s more correct to call it “soft hail” because the formation via layering of ice crystals - is known as accretion - is similar to hailstones, which often accompany strong thunderstorms.

Another common frozen precip of winter is sleet. It forms in an entirely different way than either hail or graupel. Sleet forms when liquid water, either rain or melted snowflakes, falls through a shallow layer of cold air, and freezes solid before hitting the ground. Freezing rain is rain that freezes on contact with a surface at or below 32 degrees. For neat graphic illustrations of winter weather check out Snow, sleet or freezing rain? at usatoday.com.

Here is a cool video demonstrating supercooled water:
http://www.youtube.com/watch?v=fSPzMva9_CE

Below is a photo of graupel and snow. Notice in the first image the round form or balls while in the second image you see fine pointed needles typical of snow that forms when temps are around 23 degrees F.


photo of graupel



photo of needle snowflakes

~KT

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