I think you’re talking about this thread:
Everything with a temperature above absolute zero is radiating heat at all times. Everything is also being hit by the heat being radiated by the stuff around it. The reason the Earth doesn’t cool off to near absolute zero is all that solar radiation, bathing it on one side like a turkey turning on a spit.
Additionally, when things of different temperatures are in contact, heat flows from the warm thing to the cold thing.
At night with a clear sky, everything is radiating heat every which way. Heat radiated out into the clear sky is lost, since a clear sky doesn’t radiate a whole lot back in return. But it takes a lot of heat loss for buildings, hills, the air and the ground to drop significantly in temperature. So objects left out at night under a clear sky do tend to get colder, but not to a great degree.
If you insulate your object from the ground, say with blankets or expanded polystyrene, you prevent heat from conducting to it from the ground. Now, if the only thing your object can “see” is the clear night sky, i.e. there isn’t a warm hill radiating down onto it, it can lose a lot of heat by radiation. Of course, some of this will be replaced by conduction from the air. One of my cites in the other thread calculated that you could still get to freezing with an air temperature of 60 deg. F. You may have to do it several nights in succession, keeping the cold object well insulated during the day to stop it warming up again too much.
Having said that, although the anecdote that this has been done is widespread, I’ve had difficulty finding a really solid cite. The one I gave in the other thread seems credible - it details the results of pointing an infra-red thermometer at the night sky, on a physics message board. There are a couple of comments at the end - one to the effect that someone had personally duplicated the ice-in-desert trick, and the other citing G. Gamow as having reported that desert Arabs make ice in this way. G. Gamow is the chap who coined the phrase “Big Bang” but I wasn’t able to track down his cite directly - I presume it’s in a book or paper, if it exists at all.
Here’s that cite again: