* Clouds are made of water droplets or ice crystals: These are generally considered clean in the sense that they are just water.

* Clouds can contain other things: Clouds can carry dust, pollen, smoke, and even pollutants. These things are not "clean" in the traditional sense.

* The impact of pollution: Pollution can affect the formation and appearance of clouds, sometimes making them darker or less reflective.

So, are clouds "clean?" It depends on what you mean by "clean."

* In terms of their basic composition (water): Yes, they are "clean."

* In terms of what they can carry: They can be "dirty" if they contain pollutants.

Ultimately, whether a cloud is considered "clean" is a matter of perspective and what you consider to be contaminants.