Q: What causes frost to form?
A: Frost forms when the temperature at the ground reaches freezing, says Brent McRoberts of Texas A&M University. “Very often on calm, clear nights, the temperature near the ground can be several degrees cooler than temperatures above the ground,” he explains. “The thermometer might read 34 degrees on top of a roof, but at the ground, it might read 32 degrees. This is because the ground loses heat quickly. So water vapor in the air, because it is so cool, condenses as supercooled dew and then turns to frost and maybe ice. Frost tends to form on glass, such as car windshields or windows, metal or rock surfaces first because these tend to lose more heat quicker. So usually, a car windshield will frost over before vegetation does.”
Q: Does frost form quicker in some areas than others?
A: Yes, McRoberts says. “Cold air is dense and it tends to sink,” he explains. “On a calm night, cold air will tend to sink into low spots. That’s why valleys can be much cooler than the surrounding area because the cold air from the hills sinks into the lower areas. So when the cold air starts sinking, frost will form quicker in the low spots than the high ones. It’s not uncommon for some valleys to be cooler by 10 degrees or more, thus more frost tends to form in these low areas. Also, some soil types, such as sand, retain less heat than others, and frost tends to form quicker on these.”
Weather Whys is a service of the Department of Atmospheric Sciences at Texas A&M University.