What Time of Day Is Best?

What direct sun does

Water on a sunlit pane evaporates faster than it can be worked. Whatever was dissolved in it gets left behind exactly where it dried, in a pattern. That is the streaking people blame on technique or on the cleaner they bought, and it is really just physics. On a 95-degree August afternoon a west-facing window is effectively uncleanable.

The practical rule

Work whichever elevation is in shade. That usually means east-facing glass in the afternoon and west-facing glass in the morning, with north elevations workable most of the day. Professionals sequence a house this way as a matter of routine, which is part of why a crew does not simply start at the front door and work round.

Early morning has its own catch

Overnight dew and, in winter, frost mean very early starts can be working wet glass in cold air where nothing dries. Mid-morning onward is usually the sweet spot outside high summer, when earlier is better simply to beat the heat.

Why this matters less with pure water

Deionized water contains nothing to leave behind, so it is far more forgiving of imperfect conditions than soap and tap water. It still is not magic: on a scorching sunlit pane, water evaporating in seconds is a problem for any method, which is why scheduling around sun is standard practice rather than a preference.