window cleaner has an ethanol like substance in it that evaporates to leave no trace of itself on the glass, in a cool environment the ethonal evaporates slowly, allowing the cleaning agent its also mixed with ample time to remove any grease, dust and other impuritys from the surface of the glass before evaporating to leave a perfectly clean finish. when the temprature of either the glass or the surrounding air is hotter the ethanol evaporates quicker so as you are wiping the impuritys away its leaving a streak of what looks like weak grease where the soap like substance in the cleaning solution is left behind, and this reflects the light on a slightly different angle than the glass, leaving a streak. you can polish it off with satin
the best time to clean your windows is during the short dry spell after a thunder storm as the air is cool and there is no static to attract dust to the cloth you are using, and any dust in the air has been caught by the rain
ps, i used to make double glazed units lol, not just useless pub quiz ammo