Outside in low humidity, what is the coldest thing you can point your infrared camera at?

• A. The sky
• B. The ground
• C. A building
• D. A cloud

Answer: (A)

In infrared thermography, what you see as a temperature is tied to the radiant energy a surface emits. On a clear night with low humidity, the sky acts like a very cold radiator to space because there’s little atmospheric moisture to trap heat, so its effective radiating temperature is extremely low. That makes the sky appear colder than most ground surfaces, buildings, or clouds, which are warmer or emit greater infrared energy for their actual temperatures. The ground has stored heat from the day and tends to emit more IR, while clouds are at or near the surrounding air temperature and don’t typically register as cold as the sky under these conditions. So the sky is the coldest thing you can point an infrared camera at outdoors.