Does Rain Clean Your Windows? Why Glass Looks Worse After a Storm

No. Rain does not clean your windows, and most of the time it leaves them looking worse than they did before the storm. Rain is not clean water. Every drop forms around a speck of dust, collects more on the way down, and dries in place on the glass, leaving a ring of everything it was carrying.

Rain is dirty before it reaches the house

A raindrop needs something to form around. Water vapor condenses onto a microscopic particle of dust, pollen, smoke or salt, and that particle rides down inside the drop, gathering more of the same out of the air it falls through. Around the Twin Cities that means road dust, tire and brake particles, soot, and whatever the wind lifted off a field that week.

When a drop lands on a vertical pane, part of it runs and part of it dries where it sits. The water leaves as vapor and the solids do not, so you end up with a field of small rings, each one the outline of a drop that evaporated. Clean glass shows them more plainly than dirty glass does, which is why a window washed last week can look spotted after a single storm.

It is also why exterior glass gets cleaned with purified water. Water with nothing dissolved in it has nothing to leave behind, so it dries off the pane rather than onto it.

The first rain after a dry stretch spots the worst

Air keeps collecting particles the whole time it is not raining. After a dry week in October, with leaves breaking down and furnaces coming back on, there is plenty up there for the next storm to scrub out, and the opening minutes do most of the scrubbing.

A long soaking rain often ends cleaner than it started, because by the second hour the air above the house has been washed and the water reaching the glass carries less. A ten minute shower after ten dry days is what marks up glass the worst.

Most of what you see came from above the window

Look at a spotted window after a storm. The heaviest marks are usually along the top of the pane and in vertical streaks running down from the frame. That is not rainfall. That is runoff.

Water sheets off the roof, across the soffit, over the head of the window frame and down the glass, carrying whatever sat on those surfaces with it. Asphalt shingles shed granules and grit. The top edge of a frame holds dust. A screen holds far more than people expect, and once it is wet it drips that dirt straight down the pane below it. Under oaks or maples the runoff also pulls tannin out of the leaf litter packed in the gutter, which leaves a brown cast instead of a gray one.

Which matters if you are deciding what to clean. Wash the glass and leave the screens, frames and gutters dirty, and the next rain puts a good share of it back on the window.

Wind, walls and the splash line

Storms here mostly arrive out of the west and northwest, so west and south walls take the driven rain while the east side of the same house comes through nearly dry. Driven rain also pushes dirt sideways out of the frame joints, so spotting on a windward wall looks smeared rather than dotted.

The bottom foot of a ground floor window is splash rather than rainfall: water hitting a sidewalk or a mulch bed and bouncing soil back onto the pane. It looks worse than the rest of the window and comes off more easily.

What to do about it, and what to skip

Rain spotting is loose material sitting on top of the glass, so an ordinary wash takes it off. It is nothing like hard water spots from a sprinkler, which are mineral bonded to the surface and can etch it permanently after years. Rain does not etch glass, so nothing is being damaged while you wait.

So do not chase storms. Expecting windows to stay spotless until the next cleaning is not realistic, and re-cleaning after every rain is money spent on the weather. Twice a year suits most houses here, and our guide on how often to clean windows in Minnesota goes through the timing. The one thing worth timing is the end of the season, since exterior glass has to be washed while water still behaves outdoors.

Ground floor windows you can reach safely are an afternoon with a bucket and a squeegee. Upper glass is the part people put off. I clean exterior glass with purified water from a water-fed pole that reaches up to 40 feet from the ground, so there are no ladders on the siding and nothing left on the pane when it dries, and the screens, frames, sills and tracks get done with it. Window cleaning starts at $99, backed by the Gleam Guarantee.

I work in St. Paul, Minneapolis and the suburbs out to about 25 miles. You can request a free quote online or book a free 10-minute consultation, and there is more about the window cleaning service here.