eli5: Why does rainwater stain my eyeglasses?

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I’ve found a logical explanation for why it stains windows: the dirt on the windows gets “sweeped up” by the raindrops, and once they dry, you have little collections of dirt all over your window (stains). However, I clean my glasses several times a day, and the surface of them isn’t so large that raindrops could travel very far on my glasses. Still, every time it rains, my glasses will be stained afterwards. Why?

In: Other

Rain will catch dust and debris in the air as it comes down. There may also be some air pollutants that get absorbed into the cloud/raindrop that get deposited on your glasses.

Rainwater is pretty pure, but not 100% pure water.

Unless you’re cleaning them with soap there’s still tiny particles of things all over them. That’s why you have to clean them multiple times a day. The rain brings whatever particles it has trapped in it and mixed with whatever it hits and creates the “little collections” when it’s dry.