How can there be frost even if the air temperature remains above zero?

Question

Overnight low of +1, frost seen on roofs and grass and cars.

Answer 1

It depends on WHERE that +1 reading was taken. It could have been in an area that retained heat better than wherever you saw the frost, or maybe the frost formed in a shaded area that was just a bit cooler than the surrounding areas. These are called microclimates.

Answer 2

Frost is definitely water vapor that freezes on an object that is below freezing. .

The answer is that there is a difference in the air temperature and the surface "skin" temperature as I call it of surface objects.

All material that is above absolute zero gives off long wave radiation that is in the infra-red wavelength (IR). When the sky is clear, and the winds are calm or near calm, the surfaces will radiate IR radiation directly into space. Air is for all practical purposes transparent to IR radiation but being a gas does not give off IR radiation to the extent a solid object like the ground, cars, grass bushes...etc does. Thus the air cools more by direct contact with the colder, "skin" temperatures of the solid objects than by radiating into space. So the solid objects lose heat directly to space and can be as much as 15 to 20 degrees F colder than the air 5 to 6 feet above ground where air temperatures are measured.

Thus, good old water will freeze directly from water vapor in the air to the much colder solid object. This is why the "heaviest" frost is often with dew points at or just above freezing and temperatures in the 36 to 40 degree F range and the clear skies and near calm winds.

A neat little gadget that measures IR temperature of objects can be bought at most good electronic stores. Sort of fun to play with to see what cools the fastest at night. You would be amazed at the differences.

Answer 3

actually that is very common when the air tempreature is below 32 degrees F