Rain and Puddles: The Physics of Falling Water

Raindrops aren't teardrop-shaped

Cartoons draw raindrops as teardrops, but real falling raindrops are not shaped that way at all. A small raindrop, held together by surface tension, starts out roughly spherical. As it falls faster and grows larger, air resistance pushes up against its underside and flattens it into something closer to a hamburger bun — rounded on top, flat on the bottom — and drops much larger than about 4–5 millimetres across become unstable and break apart into smaller drops entirely, which is why truly enormous raindrops essentially don't exist in nature.

A falling raindrop quickly reaches terminal velocity — the speed at which the upward force of air resistance exactly balances the downward pull of gravity, so the drop stops accelerating and falls at a constant speed for the rest of its journey. Terminal velocity depends on drop size: a fine mist droplet might fall at well under 1 metre per second, while a large raindrop several millimetres across can reach around 9 metres per second (roughly 32 km/h) — which is also why heavier rain, made of larger drops, hits harder and sounds louder on a roof.

terminal velocity: the speed where drag force = gravitational force

drag force (turbulent regime)  ≈  ½ · ρ_air · v² · Cd · A
weight                          =  m · g

at terminal velocity: drag force = weight, so acceleration = 0

Ripples: waves spreading from one point

When a raindrop hits a puddle, it displaces water at the impact point, and that disturbance spreads outward as a circular wave — a ripple ring — carrying energy away from the impact without carrying the water itself very far outward (each bit of water mostly moves up and down as the wave passes through it, not sideways with it). The ring expands because the disturbance at its leading edge continuously excites the still water just ahead of it, passing the wave onward, while the water behind it settles back down. Multiple raindrops striking a puddle in quick succession create overlapping rings that pass through each other unchanged wherever they cross — a property called superposition, the same principle that lets sound waves and light waves pass through each other without disturbing one another.

Clouds: how the water gets up there in the first place

Rain begins as water vapour that has condensed into tiny liquid droplets inside a cloud, usually around microscopic airborne particles like dust, pollen or sea salt that act as condensation nuclei — surfaces that make it much easier for water molecules to cluster together than starting from nothing. Individual cloud droplets are far too small and light to fall as rain on their own; they only become rain once they collide and merge with enough other droplets (a process called coalescence) to grow heavy enough that gravity finally overcomes the updrafts and air resistance holding them aloft.

Four kinds of rain, one underlying physics

Drizzle consists of many small, slow-falling droplets (typically under half a millimetre) that barely disturb a puddle's surface. Ordinary rain has larger, faster drops producing clearly visible ripple rings. A storm combines large drop size with high volume and often strong wind, driving rain sideways as much as down. Sunny rain — rain falling while the sun is still shining, often from a passing shower at a distance — is meteorologically identical falling water physics; what makes it distinctive is simply that the rain cloud hasn't yet blocked the sun from the observer's position, and the combination can produce a rainbow if the sunlight refracts and reflects through the falling drops at the right angle relative to the viewer.

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