Spotting

The National Glass Association's reasoning for its pure-water number describes spotting directly. In that technique the rinse water is generally allowed to evaporate from the glass, so NGA requires the water used in both wash and rinse to have a total dissolved solids content of 20 parts per million or less to prevent spotting and streaking, and says the use of tap water is not acceptable for it.

Dissolved solids are why the number exists. USGS defines water hardness as the amount of dissolved calcium and magnesium in the water, minerals it picks up as it moves through soil and rock. Water that carries more dissolved solids has more to leave on the glass when it dries, and NGA's 20 PPM limit belongs to its pure-water method alone, not to water in general. The minerals themselves are their own entry at hard water.

USGS puts the same mechanism in a kitchen: hard water is one of the most common causes of cloudy dishes and glassware, and the spots or film left on a glass taken out of the dishwasher are what USGS calls hard-water residue.

A window dries the way a glass does, by evaporation leaving whatever the water carried, though USGS's sentence is written about dishes and glassware, not windows. EPA's secondary drinking water standards sit further still from a pane: they list hardness, deposits and staining among the noticeable effects of total dissolved solids above 500 milligrams per liter, a number written for drinking water that says nothing about glass.

Streaking is the neighboring word, and NGA names both in one sentence. A spot is what water leaves after it evaporates where it sat; a streak is a line left when solution or rinse water is not fully removed from the glass. NGA's quick-reference rule not to allow water or cleaning residue to remain on the glass speaks to both.

Water from a sprinkler head that reaches a window is one way water gets left to dry on glass, answered at why sprinklers spot windows. Why glass can look worse when the sun is low is its own question, at why windows look dirty when the sun is low, and marks that gather below a frame rather than across the pane are answered at drip lines under window frames. NGA's specification itself, the water behind the 20 PPM limit, is the entry for pure water.