You could coat them with an ultraemissive material and point them at the sky, if its not cloudy it will get significantly below the ambient air temp. But the p

You could coat them with an ultraemissive material and point them at the sky, if its not cloudy it will get significantly below the ambient air temp. But the paper suggest that it will condense at ambient anyways, because it gets warmer so radiation to ambient is enough for it to work.