Winter Weather Handbook: Why to Bridges Freeze First

Severe Weather Handbook

(WBRE/WYOU-TV) — As we head into the winter season, driving on the roadways can be especially dangerous during the onset of a storm, especially on bridges.

Meteorologist Logan Westrope explains the science to how bridges freeze first before pavement.

It’s that time of year again when we have to deal with winter weather and driving out on the roads. Usually, before any type of frozen precipitation nears, roadways are treated with either salt or brine.

I’m sure when you have been driving, you pass signs that say: Bridge Ices Before Road pretty frequently. But what exactly is the “science” to why bridges and underpasses freeze first?

Logan spoke with Dr. Jon Nese, a Penn State Meteorology professor, who explained why exactly this happens.

“Think about maybe putting a half-gallon of ice cream into the middle shelf of your freezer. You’ve got chill below, chill above, so you get this nice, even cooling to keep the ice cream fresh and frozen. In a similar fashion, a bridge has cold air below and cold air above, so simply any water on that bridge will freeze faster because it is being chilled from both sides,” said Dr. Nese.

It is important to not let a warm, sunny day fool you, however. Especially when nighttime temperatures drop at or below freezing and water is present on the roads, this poses the risk of black ice.

“Certainly, if you have had a nice sunny day, and the top of the bridge has been illuminated all day, it will be a little warmer than the underside of the bridge. But still, once that sun sets, the ground cools quickly and the cool air begins to mix. And so, by later in the evening, the chilling effect will be about equal on the top and bottom,” added Dr. Nese.

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