You can feel this “cold radiation” by looking up at the stars on a summer night. Your face will feel cold since your body heat is pouring away into space. If you hold up an umbrella to block your view of the sky, you’ll feel warmer—almost as if the umbrella is “blocking the cold” from the sky. This “cold sky” effect can cool things down to below the ambient air temperature. If you leave out a tray of water under a clear sky, it can turn to ice overnight even if the air temperature stays well above freezing.
