Radiational Cooling

Clouds restrict cooling at night because they absorb longwave radiation emitted by the surface and lower atmosphere and re-emit a significant portion back.

The criterion selected for defining significant restriction of radiational cooling is whether there will be 3 or more hours of at least broken clouds (i.e., 75% cloud cover) in the 6 hours preceeding 12Z (i.e., for 7Z, 8Z, ........, 12Z).