Tuesday, October 23, 2012

Vertical Distribution of Ozone

I noticed that the sky right above the horizon seemed too blue after I implemented ozone absorption. I realized that since the most Rayleigh scattering happens in the low, dense part of the atmosphere near the ground, scattered light would be passing through too much ozone if the concentration were too high at these altitudes. I also thought more generally about the geometry of the atmosphere and the differing amounts of ozone that light would pass through based on the distribution of the ozone. I had made the ozone concentration a constant value of 0.6 parts per million, which had made the total ozone amount correct, but it had also made the amount near the ground far too high, and the amount higher in the atmosphere far too low. That amount of ozone at ground level would be dangerous to your health. In reality, most of the ozone in the atmosphere is found in the ozone layer in the stratosphere. The ozone layer is responsible for absorbing most of the sun's harmful UV rays.

To improve my results, I implemented a realistic distribution of ozone. Average data for the Earth's atmosphere was difficult to find, and the exact distribution of ozone is pretty variable anyway, so I read data from the NASA graph below, and made a few tweaks based on other information I had found (namely, lowering the concentration in the troposphere, and weighting the data up to make the total concentration closer to 0.6 ppm). I might want to tweak or replace the data in the future, but it's already much more accurate than the constant value I was using before.

Ozone concentration in the atmosphere. Image from NASA.

Below is a new render, and some old ones for comparison. In the new render, compared to the old ozone render, the sky around the zenith is bluer, and the the color and brightness near the horizon is more accurate.

Ozone with realistic vertical concentration profile.

Ozone with uniform concentration throughout the atmosphere.

No ozone.

Here's another new render, this one taken during twilight in the direction of the sun, with the sun −3° below the horizon:

Twilight + ozone.

By the way, ozone absorption should make the color of the Earth shadow at twilight more accurate (darker and bluer), so I'm looking forward to rendering some shots that illustrate that.

No comments:

Post a Comment