Frost or no frost…that IS the question!

Have you ever wondered why frost forms on some mornings when temperatures are above freezing, yet may not form on other mornings when temps are deep into the teens or 20s? Read on to find out.
Frost is probably one of the trickiest weather parameters to forecast. The reason is that so many variables determine whether or not you will be greeted by an icy windshield when you walk out the door to work each morning.
The first and most obvious ingredient to get frost is cold weather. Yet, it can actually form when temperatures are well above freezing. How can that be? Frost is ice and ice can't exist if the temp is greater than 32. True, but the surface temperature we report to you on the news everyday is actually taken several feet above the ground making it possible, in fact likely, that there is a colder layer of air at the surface which may be 32. This causes low lying surfaces such as grass and car windshields to be cold enough for frost to form eventhough the official air temperature may be several degrees above freezing.
Another major factor is wind. Frost only occurs on nights with light, or better yet, calm winds. This is because wind keeps the atmosphere "stirred up," preventing very cold air aloft from sinking to the surface. It also keeps warm air near the surface from rising into the upper atmosphere. This is called radiational cooling. On a frosty night, this radiational cooling cools the air the surface to its dewpoint temperature (frostpoint temperature here I suppose). This results in moisture forming on your windshield in the form of ice crystals.
Other factors affect radiational cooling such as cloud cover. Clouds tend to act as a blanket which prevents air at the surface from cooling to its "frostpoint" temperature and also inhibits frost formation.
Moisture is another factor, although it usually only determines how thick the frost will be. If the air is dry, there won't be as much frost as there will be on a morning with higher humidity levels.
So there ya go! Now the next time a cold snap is in the forecast, you can decide for yourself whether or not to place that towel on your car windshield the night before. If the forecast calls for calm winds, clear skies, and temperatures anywhere in the 30s, heavy frost is on the way.

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