The dry areas on your clothes or body after being outside in the rain. Primarily refers to the area underneath a rain coat, but a rain tan can also be achieved under an umbrella on a particularly windy day.
Similar to the phenomenon of a farmer's tan, but with rain rather than sun.
Jack: How come your pants and shoes are all wet, but your shirt is dry?
Rachel: I just came in from the rain and took off my rain coat. This is what you call a rain tan.