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Winter Weather: Why bridges freeze faster than roads

One of the places winter driving can be most dangerous is over bridges. But why?

Posted: Jan. 16, 2019 6:44 PM
Updated: Jan. 16, 2019 6:44 PM

TIPPECANOE COUNTY, Ind. (WLFI) -- Winter weather has come in full force this week, and it's contributed to many crashes and slide-offs. One of the places winter driving can be most dangerous is over bridges. 

As temperatures fall, bridges freeze faster than normal roads.

Robin Tanamachi is an Associate Professor in Earth, Atmospheric, and Planetary Sciences at Purdue. Shesays the reason is due to how the surfaces cool. 

"Road surfaces that are on the ground can only lose heat in one direction. And that is upward into the atmoshpere overhead," said Tanamachi.

Because there is usually no ground beneath a bridge, cold air can move more freely above and below it. As a result, the bridge will cool down much faster.

"A really good way to analogize this would be having your hand up just below the level of your mouth. If you were to blow over it, you can feel the top part of your hand cooling off but the bottom side of your hand isn't so much. If you bring your hand up to your mouth level and actually blow across both the top and bottom, you'll feel both sides of your hand top and bottom cooling off."

Bridges are usually constructed using metal or concrete. Both of these get cold very easily. When we factor this all together, icy bridges become a common occurance during winter.

So what should a driver do if they lose control during icy conditions?

Sergeant Kim Riley with Indiana State Police explains, "Follow through with wherever the car is going. Just turn into the slide and take your foot off the pedal. Maybe pump your break a little but don't push on them hard. That makes the wheels lock up and you lose complete control and that's when we have the crashes."

Drivers should also take these tips into consideration:

  1. Make sure your tires have adequate tread and air pressure to ensure your car will have as much grip as possible.
  2. Keep your windows, mirrors and headlights clear of snow. This will allow you to see better and for others to see you.
  3. Last, follow at a safe enough distance to either stop or avoid colliding with a car that spins out ahead of you.

Riley also says drivers should always slow down when there is winter precipitation and allow extra time for commuting.

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Clouds clearing and calmer winds this evening with patchy frost overnight.
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