Yes, clouds. With a clear sky, you're radiating (based on your own temperature) out to the blackness of space, which is radiating back at 4 Kelvin. Which is to

Yes, clouds. With a clear sky, you're radiating (based on your own temperature) out to the blackness of space, which is radiating back at 4 Kelvin. Which is to say, it's contributing basically nothing. You're just giving up heat to the cosmos. With a cloudy sky, you're radiating (based on your own temperature) out to the bottoms of the clouds, which are reflecting your radiation back to someone else on the ground, and their own heat is reflecting back to you. And such. The cloud looks white in the daytime because it's very reflective, so it acts like a mylar blanket to hold the heat in.