Short answer
Hard water spots are mineral deposits left behind when sprinkler, irrigation, or well water dries on glass. Fresh spotting usually comes off in a normal professional cleaning. Spots that have baked on for a season or longer need a separate hard-water treatment — and some very old etching can't be fully reversed.
Why the Treasure Valley is so hard on glass
Water here carries a lot of dissolved minerals, and many subdivisions water lawns with pressurized irrigation drawn from canals, which adds silt. When that water hits a window and evaporates in our dry summer sun, the minerals stay behind as white or cloudy spots. Every watering adds another layer.
Fresh spots vs. baked-on minerals
We look at this before we quote, because it changes the job:
Fresh spots (weeks old): usually lift off with a normal cleaning
Built-up spots (a season or more): need a dedicated mineral treatment, quoted separately before we start
Etched glass (years of buildup): the minerals can chemically damage the surface; treatment improves it, but may not make it perfect
How to keep it from coming back
The best fix is upstream: adjust sprinkler heads so they don't hit the house, and water early in the morning. Then keep the glass on a regular cleaning schedule so spots never get a chance to bake on. See how to stop sprinklers from spotting your windows.
Rather have a pro handle it?
Pro Solutions does window cleaning across Boise and the Treasure Valley. Quotes are free, itemized, and usually texted the same day.