On a clear summer day a few years ago, with a newly acquired infrared thermometer prompting my inquisition, I pondered the difference between the air temperature and radiative temperatures around me outside. It was an 80 degree (Fahrenheit) day - by air temperature, and humidity was low - probably between 25% and 35%. Yes, it was dry.
To give my view some context, I was living in a place at the time with ceiling radiant heat. The ceiling warmed up - not to heat the air, but to warm the objects below it. Much like standing in the sun on crisp fall day, you could feel warm while the air temperature said you should feel cold. You could feel the warmth of the ceiling.
So I measured the grass below me, the roads, the trees, and most memorably, the clear sky (not including the sun). The roads were warmer than the air and the grass. The grass was notably cooler than the air , I forget the exact number , I believe it was in the low seventies. Not surprising. The trees were a little warmer, close to the "official" air temperature. And the sky, memorably, was 4 degrees Fahrenheit.
Four degrees Fahrenheit is pretty cold. Well below the freezing point of fresh water, and close to the freezing point of sea water.
This is how the night and the winter cool off from the withering heat of late summer afternoons. The cold, deep cold, of the clear dark sky.
And greenhouses work by blocking that. They let the bright sunlight in, where it becomes heat on the walls, tables, plants, and floor. But the heat doesn't go through the glass - it reflects back in before it goes to the cold clear sky. Polycarbonate roofing, acrylic, CO2, methane, and clouds also work this way.