Rain Storm Over City: Terminal Velocity & Puddle Accumulation (2D)

This 2D companion strips the 3D city rainstorm down to the numbers that actually govern it: each droplet falls at the terminal velocity given by the Atlas (1977) empirical formula v(d) = 9.65 − 10.3·e^−0.6d for its diameter d in millimetres, wind tilts every streak by the arctangent of wind speed over that fall speed, and a puddle at street level rises in real time as the rainfall-intensity slider (in millimetres per hour) is integrated over elapsed time — so cranking intensity up visibly floods the street instead of just adding more particles for show, and thinning or thickening droplets changes how fast they fall independently of how much rain is falling.