The strongest window changes with the sun, cloud cover and wind. This page calculates the current best daylight window instead of repeating a generic morning-or-afternoon rule.
Combining sun angle, cloud cover, mist and visibility.
The falls already provide the droplets. The model is trying to find when direct sunlight reaches that mist at favorable rainbow geometry. Small cloud changes can still turn a strong forecast off quickly.
No. The correct window changes seasonally and with cloud cover, wind and the viewpoint. The live model uses the actual sun position every 10 minutes.
No. Niagara's mist supplies the droplets. Direct sunlight reaching the mist at the right angle can produce a rainbow on a dry day.