Color temperature changing with altitude?

Question

After 50+ years of traditional photography, I went 6 months ago for drone video&photography. Incredible what the change in perspective can do... Anyway, the question is now related with a dominant tint I'm getting when I shoot at very high altitudes. I just shot 5.5 hours of videoclips & photos in the high lands of Argentina and Bolivia, between 3,800 and 5,500 meters above sea level. When I watched them, back home, on a serious 4K computer screen, I realized that they were terribly tinted in brown/magenta and now I'm struggling in post-production to color-correct them. I used the Auto White Balance setting for all the takes, and it seems that the higher I went the more brown/magenta the videos are hued. Any idea to avoid the same problem in future? (Yes, the good old "adjust the white balance every time you shoot pointing at a white piece of paper" would also do...). Thanks, folks.

Answer 1

Yes, color temperature changes with altitude. It gets higher (bluer) with altitude, but it should not get a magenta tint. If anything, the white spectrum is purer at higher altitude, so it should be more neutral on the green-magenta axis.

The effect is pretty minor though, and should be negated by auto-WB on digital cameras. I would expect the problem you are seeing to be caused by something else.

For slide film, many people use an 81B warming filter at high altitudes, though I've always considered that too strong and used either an 81A or no filter at all.

Answer 2

Usually if an image has a strong magenta tint, it is because some kind of warming filter is being applied.

At higher altitudes you get higher-energy blues because there is less diffused light from the ground. So, to a camera sensor this seems like a more intense blue environment. Some cameras and/or software will react to a lot of intense blues in an image by trying to warm it by shifting the colors into the magenta/red part of the color space. This causes areas that should be gray, black or shadowed to turn purple.

Answer 3

As one moves higher in altitude the atmosphere is thinner and the filtering effect of the atmosphere on the sun's light is reduced. It's the opposite of what happens in "golden hour" at lower altitudes when the sun is low on the horizon and must pass through much more air due to the angle. This means that as one moves higher in altitude, the natural light from the sun will appear bluer.

I used the Auto White Balance setting for all the takes...

How each specific camera model might react to the the change in altitude and the resulting bluer light will vary. It all depends on the AWB routines programmed into the camera. Even with the same camera it will often vary greatly depending on the contents of the scene and what colors those things are. It seems your specific camera reacted to the higher color temperature of the sunlight at high altitude by overcompensating in the opposite direction.