I asked a question on biology SE as to the weird appearance of hippos in thermal imaging and in a comment @uhoh pointed out that in the image from this video that shows an example, the sky and trees are lighter than the ground, even though they should be darker in thermal emission (see, e.g., the answers here).

enter image description here

According to the video description, the crew were using a military thermal imager by Selex to capture the footage.

Why is the ground so dark compared to the sky in this image?


Some clarifications in response to comments: the footage was taken at night. It's hard to know if the sky was hazy/foggy at the time of the filming. Footage taken at daytime in the source video shows a hazy sky, but of course they might've cut things together.

  • 1
    $\begingroup$ @user6760 if the ground absorbs light, it should emit more thermal radiation, not less. $\endgroup$
    – Ruslan
    Feb 7, 2020 at 8:26
  • 1
    $\begingroup$ @OP was the sky hazy/foggy? I suppose this might be thermal radiation from the haze (water does absorb IR well, so emits also well, according to Kirchhoff's law of thermal radiation) $\endgroup$
    – Ruslan
    Feb 7, 2020 at 8:28
  • 2
    $\begingroup$ @Ruslan: this is taken at night, so the only source of heat are the animal(warm blooded) and the atmosphere(water vapor traps heat during the day) then the ground cools quicker due to heat capacity but I need to look into it more. $\endgroup$
    – user6760
    Feb 7, 2020 at 8:43
  • 1
    $\begingroup$ That would certainly be possible. Frost ground is a common sight. $\endgroup$
    – Alchimista
    Feb 7, 2020 at 9:02


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.