"Only if there is some ambient light. NVGs take in ambient light and magnify it. An overcast night, no moon, no streetlights/houselights to reflect off the clouds equals no light, so nothing too magnify, so you can see nothing"
What is that ???
We see well at night every night. The intensity and clarity are different if it is cover or clear sky, but we still see all around us with NVG's.
There is no problem to fly with cover sky, it is just not so bright.