My husband and I are having a disagreement about whether it must be freezing or below before we can have a frost??? I say no!!! Thanks for your reply
Posted October 31, 2007
MIKE MOSS SAYS: Phyllis, I have to say it's kind of unique to to try and settle a disagreement amongst the Snows about Frost!
At any rate, there's a sense here in qhich you both are correct. From your point of view, you are right that the air temperature (typically measured about 4-5 feet above the ground) does not have to fall to the freezing mark for frost to form on the ground or other object (tops of cars, blades of grass, mailboxes, etc). This is because strong radiative cooling of those objects, which is most efficient on clear, calm nights with dry air in place through much of the depth of the atmosphere, can result in the surface of those objects becoming several degrees colder than the air a few feet above.
From the point of view of your husband on the other hand, yes, in order for frost to form something has to fall to 32 degrees or below, in this case the ground or other objects. If there happens to be enough water vapor in the air immediately adjacent to those objects, frost will form when they become cold enough.