Answers
Can windows be cleaned in the summer sun?
Yes. No glass standard sets a temperature limit. NGA avoids direct sun on tinted and reflective glass, and the makers prefer cooler glass out of the sun.
Read itAnswers
Why do my windows look dirty when the sun is low?
A pane that looks fine at midday and hazy near sunset leaves a homeowner wondering whether the low sun is revealing dirt or something the daylight had been hiding.
Short answer
Low sun can change what a pane shows. The National Glass Association says scratches not easily seen under a dark or gray sky may be very noticeable when the sun is low, and that daylight is needed to evaluate a glass surface. A mark that shows only in low sun may be dirt, a scratch or a mark on the other face, and the light alone cannot say which.
Under cloud
Under a gray sky, a scratch can hide
The National Glass Association writes that light is part of what a person can see on a pane: “The ability to detect certain surface damage, such as light scratches, may vary greatly with the lighting conditions. Daylight conditions are needed to properly evaluate a glass surface for damage.” Under a dark or gray sky, by that account, a light scratch can be in the glass and not show.
A pane with a light scratch can pass for clean on a gray day for that reason. Whether a given pane carries a scratch at all, as opposed to a streak or a film, is not something a description settles; whether window cleaning can scratch glass takes up where scratches come from.
Late in the day
When the sun drops, the same scratch can turn obvious
NGA finishes the thought for the other end of the day: “Scratches that are not easily seen with a dark or gray sky may be very noticeable when the sun is at a certain angle in the sky or when the sun is low in the sky.” For a scratch, nothing in the glass changes between a gray sky and a low sun; the light does.
Those lines sit beside NGA's instruction to test clean a small area of one window and examine the surface for damage, quoted in the test-clean step of the window cleaning method. Direct sun raises a separate question about the cleaning itself, covered under cleaning windows in the summer sun.
On the glass
What the light cannot tell you, and who can
No source read explains why a low sun would make dirt, streaking or spotting more visible; NGA's lines are about scratches and surface damage, and this page prints them that way. NGA's trigger for cleaning is the condition of the glass, not the calendar: “clean glass when dirt and residue appear.”
A mark a low sun shows may be dirt a wash can reach, a scratch in the glass, or a mark on the room-side face. What the film on windows is and why some spots stay after regular window cleaning are separate questions; which one a given pane has is judged in daylight, on the glass, by a glass professional, not from how it looked at sunset.
Does low sun make a window dirtier, or only easier to see?
No source read measures a change in how dirty a pane is between a gray sky and a low sun. NGA's lines describe visibility: a scratch not easily seen with a dark or gray sky may be very noticeable when the sun is low. For a scratch, the glass has not changed; the light has. For dirt, no source read offers an explanation.
Is a gray day a good time to check a window for scratches?
Not by NGA's account. It says the ability to detect light scratches may vary greatly with the lighting conditions, that daylight conditions are needed to properly evaluate a glass surface for damage, and that scratches not easily seen with a dark or gray sky may be very noticeable when the sun is low or at a certain angle.
Do NGA's lighting lines apply to streaks and dirt as well as scratches?
Not as written. NGA's lines are about surface damage, such as light scratches, and how much of it shows under different light. No source read explains why a low sun would show a streak, a spot or dirt, so which mark a pane carries is judged on the glass, in daylight, by a glass professional, rather than inferred from the hour it was noticed.
Sources
Where this page's facts come from.
Every technical statement above is drawn from one of these documents. Nothing on this page about 239 Exteriors goes beyond what the company itself states.
- National Glass Association (NGA). Glass Technical Paper FB01-00 (2023), Proper Procedures for Cleaning Architectural Glass Products
Page reviewed September 14, 2026. Florida statutes are quoted from the 2026 Florida Statutes; every image on this page is a labeled concept illustration, not a photograph of a completed job.
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