Does rain make clean windows dirty?

USGS describes the first step of ordinary rain in plain terms: "For precipitation to happen, first tiny water droplets must condense on even tinier dust, salt, or smoke particles, which act as a nucleus, binding them together." A raindrop is not pure water falling from a clean sky. It is water that has already gathered around something else before it ever leaves the cloud.

So the question is not whether rain is clean water. It is what a drop brings with it, and what happens to that load once the drop dries on glass.

A raindrop that runs down a pane and dries leaves what it carried in from the cloud and whatever it picked up running down the glass on the way, which is the sense in which rain can dirty a window that was clean an hour before. No read source measures how much of a deposit a typical Naples storm leaves, only that the mechanism is real.

NGA's quick-reference list, written for cleaning rather than weather, names the same outcome: "DO NOT allow water or cleaning residue to remain on the glass or adjacent materials." Rain is not a cleaning, but water left on glass to dry is the condition that rule describes, and the mark it can leave is what spotting means.

Two opposite slogans get said about rain and glass, rain washes windows and rain is dirty, and no source read supports either one. The first leaves out what USGS says a raindrop forms on, a particle of dust, salt or smoke, and what that water leaves when it dries on a pane with nothing to wipe it away. The second goes past USGS too: a drop forming on a particle is not a finding that rain is dirty, and no source read measures how much any rain carries.

What the sources support sits between the two: rain can leave something behind on glass the way any water left to dry does. Cleaning glass while it is still raining is a different question, answered on can windows be cleaned in the rain.

The window makers' rinse instructions deal with the same drying problem for a cleaning solution rather than rain, and they are set out step by step in keeping windows clean between professional cleanings. NGA's own trigger for cleaning is the same condition-based rule either way: glass should be cleaned as dirt and residue appear, whatever put it there.

• A named storm's wind and spray, rather than an ordinary rain, is its own question, answered on why windows go spotty after a storm.
• Lines that begin right under a window frame have their own list of possible sources, set out on why some windows have drip lines under the frames.
• The pure-water rinse NGA records, whose water is held to a dissolved-solids limit to prevent spotting, is covered on pure water and the water-fed pole.
• The yellow coating some windows show in spring is its own question, answered on why windows turn yellow in spring.