Why are Bridges Icy?

With the winter season upon us, many in the U.S. are faced with difficult winter driving conditions.  One such sign you may see on the road is:

Why do bridges ice over before roads on solid ground?  Unlike a road on solid ground, a bridge is surrounded by air on both sides (top and bottom).  All roads lose heat (energy) during cold weather, but a bridge, being surrounded by cold air on both sides, can lose more heat more quickly.  As a result, bridges will develop ice before roads on solid ground.  Thus the importance of taking bridges seriously during cold weather.  Even if the road is wet and not icy, moving from a road to a bridge can be quite dangerous if one is not careful.  

My children are still a few years away from learning how to drive (scary!), but warning your children at a young age of the importance of driving carefully across bridges during cold weather is a god life lesson to learn early in life.