Snow, Sleet and Freezing Rain !

February 2007 Ice Storm (Freezing Rain, Sleet accumulations) - Source: NWS

Last winter the Mid-Atlantic Region had one of its coldest and snowiest winters on record.  That was true despite the fact that last December was over 2.5 degrees warmer than average in the Nation’s Capital.  Washingtonians are on the borderline between snow, sleet, freezing rain and rain when winter storms take shape. 

Depending on the storm track, we could see all snow, all rain or a wintry mix. This is because of the unique geography that exists in the Mid-Atlantic Region.  Being a relatively short distance from two bodies of water, the Atlantic Ocean and Chesapeake Bay, temperatures are often slightly warmer than they would be away from a large body of water.  A difference of a few degrees can make a huge difference when forecasting winter precipitation.  Areas south and east of the Nation’s Capital tend to see much less snowfall than their neighbors to the north and west of town because of this. 

All precipitation falls from the clouds as ice crystals since the temperatures at that altitude are cold enough to support ice. If the column of air that extends from the cloud to the ground remains below freezing the entire distance, then the ice crystals will fall as snow.  Snow is the easiest type of winter precipitation to deal with since it can be plowed and shoveled.

But, if a portion of the column of air between the cloud and the ground is above freezing, then that presents a new host of problems. For example, if there is a column of above freezing air just above the ground then the ice crystals – or snowflakes – melt into raindrops.  If the surface temperature is below freezing, then these raindrops will refreeze upon reaching the ground creating “freezing rain.”

On the other hand, if the column of air with above freezing temperatures is higher in the sky with a greater amount of below freezing air above the ground, then the water droplets have a chance to refreeze into “sleet” pellets before hitting the ground. Sleet can accumulate like snow, but tends to be heavier than snow because the water content is higher which suppresses accumulations.  That’s because sleet compacts more than snow does keeping accumulations lower than if it were pure snow.