by David Murphy
It depends on the temperature of the road surface. An important thing to remember is that road surfaces take more time to react to temperature changes than certain other surfaces like grass. Since roads are flat and built of solidly packed material like asphalt, cold air has a hard time penetrating this material and changing its temperature. At the same time, asphalt and other paving materials do a very good job of retaining heat stored up during sunny, mild days. So, when a snowstorm arrives in the Delaware Valley a day after we've experienced mild weather, the road material may not have enough time to react to the cooler air and may remain too warm to support snow. In these cases, the snow melts on contact and the road becomes wet but not icy.
As an example, let's say a snowstorm arrives the day after we've had a string of afternoon highs of 50 degrees. Even though the air temperature may drop below freezing as the snow begins to fall, we would expect much of the snow to melt because the road surface would maintain some of the heat stored up the day before.
Having explained this general rule, it's also important to remember that this is not always what happens. In the winter of 2005, for example, there was a morning snow that followed a mild stretch of weather and many roads did unexpectedly freeze up, causing several dozen spin-out accidents, especially in the northern and western suburbs.
What can cause unexpected icy roads?
Reasons include a heavier, longer than expected snow which draws down even colder air. Also, in the northern and western suburbs, the elevation is higher which causes these areas to stay cooler than other places. Areas with hilly terrain can experience extra cooling on valley roads as cool air (which is more dense and heavy) sinks into the valleys. Overpasses freeze more quickly because the cold air is not only cooling the top of the road surface but the bottom as well. Finally, certain road materials freeze better than others.
The bottom line: no matter what the circumstances, it's always a good idea to be cautious during winter weather because even though there are certain general rules as to how the weather behaves, there are ALWAYS EXCEPTIONS to those rules.