Raindrops

Perhaps the wind has something to do with it??

John.

It's because the windows are dirty, and not perfectly flat. Water adheres to the glass and travels in the direction of least resistance, which is a zigzag motion.

Also, water is a light weight liquid - compare the movement of paint which falls in straight(er) lines, and the behavior of tiny droplets of atomized water such as fog, which do not move at all.

Also of note, Glass is hydrophobic, but very minute deposits of hydrophilic materials like salt can create a preferential path where there is less resistance to move sideways a little, than straigth down.

There are certain reasons as explained by other friends:

1. Surface of pane is not smooth & inclined @ presise angle & may have some twist in it. Pnes are not precisely aligned for vertical & horizontal erection.

2. Due to deposite of dust not only loose but also deposited on surface.

3. Wind is varying in speed, direction & twists it is taking.