The counterintuitive part
Everywhere else, rain cleans windows. Here, a monsoon storm is usually the thing that ruins them, and homeowners new to Arizona are consistently surprised by it.
Arizona's monsoon season runs from roughly mid-June through the end of September. A storm arrives with a wall of wind ahead of it, carrying an enormous quantity of extremely fine dust. That dust coats everything. Then the rain arrives, often light, often brief, and turns the dust on your glass into a thin slurry.
Then the sun comes back out, the temperature climbs, and that slurry dries onto the window as a bonded film. That's the streaky, spotted mess you're looking at, and it's very different from ordinary dust sitting loosely on glass.
Why the hose doesn't work
The instinct is to spray the windows off. It backfires for two reasons.
First, rinsing doesn't release a bonded film. Dried-on dust needs to be re-wetted, given time to soften, and physically removed. Water pressure alone mostly redistributes it.
Second, and worse: you're rinsing with hard water. Tap water in this area is hard, and when it evaporates off hot glass in minutes it leaves that mineral behind. So the hose removes some dust and deposits a fresh layer of mineral. Do it repeatedly across a summer and you've built exactly the hard water staining that's expensive to reverse.
What to do instead
- Don't chase individual storms. Cleaning after every storm during monsoon season is a losing game. Another one is coming.
- Wait for the season to finish. The high-value cleaning is in October or November, once the last storms have passed.
- Make an exception for mineral. If you see spotting that won't wipe off, deal with it regardless of season.
- Don't forget the roof. The same storms coat solar panels, where the cost isn't appearance, it's lost production.
The one-cleaning-a-year answer
If you only clean your windows once a year, do it in the fall after monsoon season ends. You clear a full season of accumulated dust right before the months when you'll actually be looking out those windows, and before the winter months when many people have guests.
More on timing: how often windows need cleaning in this climate.