What makes a droplet start sliding down the glass?

Each droplet grows by condensation over time. Once its radius crosses the gravity-threshold slider's value, gravity overcomes the surface tension holding it in place and it begins to slide.

Why do droplets merge into bigger ones?

When two droplets get close enough to touch, they combine into one, conserving volume (radius scales with the cube root of the summed volumes) rather than simply averaging their sizes.

How does wind affect the droplets?

The wind slider adds a constant horizontal drift to sliding droplets, on top of a small wobble, so they trace diagonal paths instead of falling straight down.

Home▸Fluid Dynamics & Aerodynamics▸Rain on Window (2D)

🧪 Rain on Window (2D)

A 2D canvas companion to the 3D glass-rain scene: droplets condense and grow, merge by conserved volume, and slide once they cross a gravity threshold — with live intensity, threshold and wind controls.

Fluid Dynamics & Aerodynamics2DModerate60 FPS📱 Mobile-adapted⇄ 3D version

This 2D companion drives the same droplet lifecycle as the 3D glass-rain scene — spawn, condense, threshold-triggered slide, volume-conserving merge, wind drift — through a flat canvas view instead of a shader-refracted pane, so the mechanics of why a drop starts moving and how two drops combine into one are easy to watch directly against a simple night, countryside or overcast backdrop.

⚙ Under the hood

2D canvas rain-on-glass lab: droplets condense and grow, merge by conserved volume (radius scales with cube root of summed volume), and slide once they cross an adjustable gravity threshold, drifting with wind.

fluid dynamicscondensationsurface tensiondroplet merging

2D · HTML5 Canvas 2D · 60 FPS target · runs fully client-side, no install