Why do my windows get cloudy spots after rain?

Cloudy spots after rain are mostly what the drops leave behind as they dry. Rain lands on glass that already has dust and pollen on it, picks up dirt as it runs off screens, sills and gutters, and dries in place. Where a sprinkler or hose reaches the windows, hard water adds mineral spots that look much the same. Most of this washes off; the spots that do not are a sign of something else.

Where the spots come from

What the rain carries. Rain is not perfectly clean water. The National Weather Service explains that water vapor needs something to condense onto, and that the air carries millions of minute salt, dust and smoke particles, called condensation nuclei, that let it happen.^[1] Even the cleanest air on Earth holds about 1,000 dust particles per cubic meter.^[1] That is a small contribution next to the dirt already on the glass and the dirt picked up on the way down, which the next sections cover.

What is already on the glass. Dust, pollen and grime build up between cleanings. Rain loosens them, and when drops dry they pull that film into rings and spots. Glass that was cleaned recently usually spots much less after the same rain.

Water that ran over something dirty first. A lot of the water hitting a window has run over a screen, a sill, a ledge or the wall above it, or spilled from a gutter that overflows. It carries that dirt down onto the glass. For gutters that spill over, see why gutters overflow.

Sprinklers and hoses. This is the most common cause of spots that will not wipe off with plain water. Hard water contains high levels of calcium and magnesium,^[3] and Virginia Cooperative Extension notes it can leave mineral deposits, or scale, on dishes and fixtures.^[3] Glass is no different. Andersen's window cleaning advice is that hosing windows down leaves mineral spots even with soft water, and to be wary of spraying the windows with a sprinkler.^[2] If your house has a softener, the outdoor water may not be softened at all: VCE notes that water for outdoor use does not need to be softened.^[3]

To know how hard your water is, test it. A qualified laboratory can test your water for hardness,^[3] and on VCE's scale, water hardness below 7.0 grains per gallon, or 120 mg/L, generally does not require treatment.^[3] For more on that choice, see whether you need a water softener in Virginia.

Spots that are not dirt

Morning dew on the outside. Exterior condensation forms when moist air meets glass that is cooler than the dew point,^[5] and it is most likely when a cool night follows a warm day, typically in spring and fall.^[5][7] MI Windows and Doors says it is a sign that the insulating glass is doing its job.^[5] Dew that dries on dusty glass can leave spots just like rain, and the fix is the same: clean the glass.

Haze between the panes. If the cloudiness is inside the window, where no cloth can reach, it is not a cleaning problem. Andersen says condensation between the two panes of a double-pane window is a sign that the seal has failed.^[7] See how to know if windows need to be replaced.

Getting mineral spots off

Wash the glass normally first, out of direct sun, with mild soapy water and a squeegee; see how to clean windows without streaks. Then treat what is left. Vinegar is a mild acid that removes rust or lime deposits on the outside of windows,^[4] and one simple mix is 1 cup of distilled vinegar to 1 cup of water in a spray bottle.^[4] Give it a minute on the spots, rub gently with a soft cloth, then rinse and squeegee so no vinegar is left on the glass or frame.

Be careful with stronger products. MI lists hydrochloric and phosphoric acid among products not to use on its windows, screens, doors or hardware,^[5] and Harvey warns against rust removal products and abrasive cleaning agents with cerium oxide on its glass.^[6] Check your window maker's care guide before you try anything stronger than vinegar.

Keeping the spots from coming back

Adjust sprinkler heads so the spray stops short of the windows and the wall around them. Keep screens, sills and gutters clean, since dirty water from them ends up on the glass. And clean the windows on a regular schedule; see how often to clean windows.

Stop and call a pro if

• The haze is between the panes and wipes off from neither side.
• The glass stays cloudy after cleaning and feels rough, or shows scratches or etching in raking light.
• The spotted windows are upstairs glass that only a tall ladder reaches.
• Water is coming in around the window frame, not just running down the outside of the glass.

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