One of the undergraduate environmental labs at my university uses colored dye, water and salt to simulate smoke plumes and pollution. In a tank of distilled water a mix of salt water acts like an upside down version of smoke or steam pollution. After time these slightly more dense solutions will settle on the bottom of the tank much like a layer of smog can lay at a fixed altitude over a city.
In the photo below the red layer is a medium density salt water mixture that settled into the bottom of the tank like a layer of pollution. The blue mixture is even more dense and as it billows upwards into the atmosphere (or down into the water tank) it being slightly more dense than the red layer it will settle as a more dense layer below the red layer. Never expected my lab photos to turn out looking so artistic.