Foot's Forecast | Weather Intelligence

A Freezing Rain Trick...That Is No Treat

Image source and article: The SkyBrary

http://www.skybrary.aero/index.php/Freezing_Rain

6:00 AM EST 12/7 - Ever wonder what causes "freezing rain" ? By daybreak Monday morning, those in the Mid-Atlantic I-95 corridor and metro areas of DC/Baltimore will have a fast life lesson on the tricky science of a phenomena that has caught many a school system by surprise. If the significant icing that occurred in the Southern Plains and Tennessee Valley is an indication, portions of the Mid-Atlantic will face major travel disruption Sunday afternoon to Monday afternoon.

IS IT RAIN? The current NWS winter storm watches from West Virginia to Maryland and Virgina indicates several key ingredients will be in place to create a super-slippery surprise for those who think "it just looks like rain." The trick behind what turns a light rain into a disaster is called the "latent release of heat." With ground surfaces below freezing, anything that falls tomorrow morning will freeze on contact.

HOW TO GET TRICKED: If on Sunday night, snow and sleet are replaced by "rain" -- a weird thing will happen: The temperature will RISE even as a light glazing of ice is forming on surfaces. In the past, schools and businesses made their closing decisions on the change in temperature, as in "Look, the temp is going up! That means the ice will melt and we'll be in the clear!" Not so fast... A quick check of the latest Eastern Regional NWS advisory map shows they aren't falling for it.

KNOW THE SCIENCE: Freezing rain creates a different effect, for as the ice accretes on surfaces, the ground temperature often rises to near 32 F. Then, annoyingly, the temp can hovers near or at freezing for a few hours-- until enough warm air aloft can mix down to the surface. Why? Because accretion of ice is similar to condensation, a heating effect. While your thermometer reads air temps above 32, the ground UNDER the ice is may be less than 32! If any surface water on the ice "evaporates" - that causes a brief cooling effect, chilling the surface just enough to keep temps right around freezing.

A TRUE STORY: In Winter 2007-- one Maryland school system called for a 2-hour delay, only to have the bus drivers return to the lot reporting that the ice situation had actually gotten worse at 7:30 AM than it was at 5:30 AM! The delay was hastily changed to closed, and mayhem ensued with teachers having to turn around and go home, and parents having to call off work to pick up kids from schools that were open just 30 minutes before! What's your tale of winter woe from back in the day?

We chalk all this up to "Weather Politics" -- and this time, we hope that effective understanding of a scientific process will augment decision-making at those critical hours Tuesday morning.

(The Foot's Forecast Advisors)