Why is there frost on the cars on mornings when the day is going to be warm but not on days its going to stay cold?

Answer 1

One scenario is this:

If there are no clouds in the sky the night will generally be more chilly since clouds tends to reduce the heat emission from the earth. Likewise, in the daytime the absence of clouds let's the sun heat up the earth and make the day relatively warmer.

Answer 2

Frost forms on car windows in the morning when the temperature drops below freezing during the night. If the day is going to warm up quickly, frost might melt before you notice it. On cold days, the temperature might not rise enough for frost to form in the morning.