Behind the Forecast: Why bridges get slick before roads

Listen to Science Behind the Forecast with Meteorologist Tawana Andrew every Friday on 89.3 WFPL at 7:45 a.m.

Behind the Forecast: Why bridges get slick before roads
Listen to Science Behind the Forecast with Meteorologist Tawana Andrew every Friday on 89.3 WFPL at 7:45 a.m (Source: Tawana Andrew)

LOUISVILLE, KY (WAVE) - When driving you see the signs all the time, "Bridge Freezes Before Road." But why does that happen?

It all has to do with the conductivity of certain materials. The air and soil are great insulators but terrible conductors. This means that they don't allow heat to transfer quickly or easily through them. Bridges are typically made of concrete and steel; both are fantastic heat conductors.

When cold winds blow against the exposed sides, tops and bottoms of bridges and overpasses, heat is lost quickly. Since it loosing heat from more surface area, the bridge cools rapidly in comparison to the road that only loses heat from its surface. The earth provides enough of a heat source to keep the road warm enough to prevent icing even if air temperatures drop below freezing.

A bridge loses heat from all of its exposed sides.
A bridge loses heat from all of its exposed sides. (Source: http://icyroadsafety.com)

Because of all the exposed surface area and the typically rapid cooling, a bridge’s temperature will be much more similar to that of the air temperature. Because of this, ice and snow are more likely to stick on elevated surfaces; meaning this also applies to things like decks and railings.

According to the Federal Highway Administration, more 1,235,000 were weather related between 2007 and 2016. During this time span, around 5,000 people were killed and more than 418,000 people injured in these weather crashes. While most weather-related crashes happened in wet weather. A smaller amount of weather related crashes happen during winter weather: 18% during snow or sleet, 13% occur on icy pavement and 16% of weather-related crashes take place on snowy or slushy pavement.

Icy pavement caused 156,164 crashes, injured 41,860 people and killed 521 people between 2007 and 2016.

Science Behind the Forecast: Why bridges get slick before roads

Copyright 2018 WAVE 3 News. All rights reserved.