Why we’re watching for frosty roads this weekend

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CEDAR RAPIDS, Iowa (KCRG-TV9) - There's a technique to forecasting the potential occurrence of frosty roads. Looking back at several previous situations, as well as some discussions within some workshops our team has attended at the National Weather Service, there are two parameters to look at that favor frosty roads.

The first is road temperatures. Road temperature sensors are very sparse even on main highways, and rural roads go without any measurements at all. In any case, we look for pavement temperatures that are below freezing.

The second parameter to look at is the dew point of the air moving over the roadway. Due to something known as heat capacity, the temperature changes in pavement and ground more slowly than air, and a sudden dew point rise will cause frost formation on roadways and other surfaces. This is due to the dew point in the air being higher than the temperature of the road, which causes moisture to form. The pavement temperature eventually does come up, but it can lag several hours behind.

We expect temperatures Saturday morning to be well below zero. By Sunday morning, temperatures will have likely risen into the 20s with dew points following into the 20s, too. It stands to reason that this sudden dew point rise may cause road frost to develop Saturday night into Sunday! Slick roads are possible, especially on bridges and overpasses as well as untreated roads.