The morning after a clear winter night is colder than a cloudy one because the surface heat radiates out to space. Whether clouds have a net warming or cooling effect has more to do with their thickness than their altitude. Clouds are Earth’s iris and are the most significant factor in how much solar energy originally enters the oceans and climate. Human activity already influences them.
The heat radiates out because the clouds aren’t retaining the surface heat. Clouds during the day don’t block all the radiation but do, just as during the night, retain the longer wavelengths such as infrared/heat. They also reflect a lot of radio frequencies for the same reason.