Temperature, humidity, wind, sun, panel size, solution. The math tells you how long the film will hold before the squeegee has to be on it — and whether you should be working this panel right now or chasing the shade.
Methodology: Penman-Magnus evaporation, calibrated against the Pacific corridor reference panels. The math is in the methodology guide.
The math says you have only 8 sec of dwell window on a panel that cleans in roughly 28 seconds — a ratio of 0.3× the clean time. That is the condition Article 001 (why-windows-look-worse) opens with: the film breaks before the squeegee finishes, the surfactant concentrates into a visible residue, and the second cleaning either reproduces the same haze or requires a stronger solvent that the panel did not need. The cost of working the panel anyway is the redo. The adjustments below are the route around it.
The single most common cause of a residential window-cleaning redo is the panel that was cleaned in conditions the operator did not think about. Surfactant hysteresis — the streak pattern this tool exists to prevent — is not a chemistry failure or a technique failure in isolation. It is the failure that happens when chemistry and technique that would be correct in cooler air are applied in warmer air without adjustment. The math is the adjustment.