Rain does not make the windows dirty, dirt does. Dust accumulates on the outside of the glass from lawn
mowers, leaf blowers, tree pollen, car exhaust and many other sources. This dust settles as a uniform
layer on the glass. When it rains, the rain drops push the dust into different patterns and after
the water dries the dust remains on the glass. This new “mud residue” is now highly visible because
it is no longer uniformly spread but is sporadic throughout the pane. Clean glass will stay clean
after a rain because once the rain water dries, there is nothing left behind to obscure the view.