Clear moon, frost soon.
If the atmosphere is clear, the surface of the earth will cool rapidly as heat is radiated away at night. There is no "blanket" of clouds to keep the heat that the ground absorbed during the day from radiating back up into space. If the temperature is low enough on these clear nights and there's no wind, frost may form.

澄淨的月，覆霜的夜
若地球周圍的環境沒有屏障，那地球表面的熱量就會在夜晚很快的散失。這是因為沒有雲層可以防止地表在白天所吸收的熱量再次輻射回到外太空。因此如果夜晚的溫度夠低且在無風的情況下，就會結霜。

資料來源：
http://www.wxdude.com/proverb.html