Well, smog is kept at a certain thickness due to an "inversion layer" of the atmosphere where it actually gets warmer with altitude. This is a cap on the smog.
So, on a rainy day when the air is unstable, the inversion disappears, and the smog disperses. Then, as you walk out after the rain has stopped, and the clouds are breaking up, you wonder, "My, the air is quite clear and crisp today!"
Anyway, from the air, at about the altitude of the inversion, you may see clear blue sky above you, and a "horizon", below which is just brown. Just a thick brown blanket over everything. From the ground, you barely notice it, just a grey pallor to the sky. But from the air the smog layer as stark as day. It's a comforting thought, knowing that you are breathing in that muck.
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