Why can we see frost on roofs and cars when the air temperature is above freezing?
Such situations usually occur during or just after clear, calm nights. The heat from the day is able to escape, or radiate, into the upper atmosphere leading to strong cooling near the ground. The important fact here is that different substances cool at different rates. Water cools the slowest, air a bit faster, but bare ground even faster and glass or metal will cool faster and more than any of these other substances. That's why the air temperature may be above freezing (possibly by quite a bit), but a metal car or glass windshield will drop to below freezing, allowing water vapor in the air to crystallize into frost.