Why Clean Windows Look Dirty in Sunlight

If your windows looked fine in August and look filmy now, the glass has probably not changed much. The light has. By the end of October the midday sun in the Twin Cities sits a little over 30 degrees above the horizon, so it comes at a window sideways instead of from overhead, and light arriving at that angle shows every film, smear and water spot on the pane.

The sun is about half as high as it was in June

At midday in late June the sun here is close to 70 degrees up, almost straight overhead. By late October it is just past 30. By the solstice in December it is around 21 and it never gets any higher all day.

A window is a vertical sheet of glass. Light coming down from overhead hits it steeply, passes through, and mostly keeps going. Light coming in low and flat travels a much longer path through whatever is sitting on the surface, and it scatters off every particle on the way. Same window, same thin film, completely different look.

Raking light does to glass what it does to a wall

Painters already know this one. Hold a work light flat against a finished wall and every ridge, roller mark and bad patch jumps out, even though the wall looked fine under the ceiling fixture. Turn the light back toward the wall straight on and it all disappears.

Window glass behaves the same way, and in late fall the sun is the work light. Overhead summer sun forgives a haze. Low fall sun does not.

It is also why a window looks bad at one particular hour and fine the rest of the day. East-facing glass shows it at breakfast. West-facing glass over the kitchen sink shows it around four in the afternoon, which is when most people notice and decide the windows need doing.

The leaves coming down is the other half of it

The angle is only part of the change. Through the summer a boulevard maple or an elm puts most of those windows in dappled shade for the whole afternoon. Once the leaves are down in October the sun has a clear shot at glass that spent five months behind a screen of foliage.

So two things happen at once in the same few weeks. The light gets lower, and more of it actually reaches the window. On a south or west wall under a big tree, that combination can make a window look like it aged a year in a fortnight.

How to tell a light effect from actual grime

Three checks, none of which take more than a minute.

• Move. Step a few feet to one side and look at the same pane from a different angle. A film stays. A reflection or a glare pattern moves or vanishes.
• Wait for a gray day. Minnesota supplies plenty in November. If the window looks clean under flat overcast light and filmy only in direct sun, there is very little on it.
• Check which side it is on. Wipe a small patch on the inside with a dry microfiber. If nothing changes, whatever you are seeing is on the outside. Indoor film has its own causes, covered in why there is a film on the inside of your windows.

Direction tells you something too. Streaks that all run the same way are left over from the last cleaning. Real dirt lands in patterns: a splash line along the bottom of the glass from rain hitting the sill, hard rings from a sprinkler, a dusty band where the storm window overlaps.

When it is worth doing something, and when it is not

If the film is real, late October into early November is the right time for the last exterior cleaning of the year. Clean glass going into winter is worth more than clean glass in July, because you are about to spend four months with very little daylight and you want all of it. There is more on timing in how often to clean windows in Minnesota.

If the only thing wrong is the angle of the sun, a cleaning will not fix it. The windows will still catch the light at four o'clock in December, because that is just what the sun does here in winter. Clean glass looks better in raking light than dirty glass does, but nothing makes a pane invisible.

And if you do it yourself, pick an overcast day. The same low sun that exposes the film will dry your solution on the glass before you can pull it off, which leaves you with new streaks on top of the old ones. The method is in how to clean windows without streaks.

If you would rather have it done before the snow comes, we work across St. Paul and the metro. Get a quote or book a time.