Passive Adiabatic Effect: Cold sky reflection on IR imagery.

[TheCornishMong]

I am currently in training to become an analyst using thermal imagery. In the studies I have been introduced to cold sky reflection on thermal imagery, however the instructor explained it as a surface 'reflecting the cold from the sky' which, as far as I'm aware, doesn't fit with the second law of thermodynamics. Can someone provide me with an explanation of what causes this effect?

 

[Andrew Mason]

I am currently in training to become an analyst using thermal imagery. In the studies I have been introduced to cold sky reflection on thermal imagery, however the instructor explained it as a surface 'reflecting the cold from the sky' which, as far as I'm aware, doesn't fit with the second law of thermodynamics. Can someone provide me with an explanation of what causes this effect?

If I understand the question, you are wondering about the physics involved when an object appears to be colder on the side facing the sky than the side facing the ground.

It has to do with the radiation from a body's surface. If the rate at which a surface receives radiation and the rate at which it loses radiation are not equal, the temperature of the surface will adjust until the rates are equal.

So if the surface facing the ground is receiving warm radiant energy from the Earth and the top surface is not, the top temperature of the top surface will be lower (unless, of course, the entire body is a perfect conductor so that the entire body has the same temperature at all times). You will see this on a device that detects thermal radiation from surfaces.

So it is not that the sky is reflecting cold. It is just that it is not reflecting or radiating as much heat as the earth.

AM

 

[Andy Resnick]

I am currently in training to become an analyst using thermal imagery. In the studies I have been introduced to cold sky reflection on thermal imagery, however the instructor explained it as a surface 'reflecting the cold from the sky' which, as far as I'm aware, doesn't fit with the second law of thermodynamics. Can someone provide me with an explanation of what causes this effect?

I'm more familiar with this effect in millimeter-wave imaging, but the reasoning should be the same. IR and MMW imagers work in wavebands that are largely unabsorbed by the atmosphere, so if your sensor is imaging a piece of metal that reflects the sky, what you actually detect is the cold of deep space.