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How big is the biggest possible raindrop?

鈥� Molecules inside a water drop are pulled equally in all directions by the neighbouring molecules. The attraction between molecules means the liquid squeezes itself until it has the lowest surface area possible: a sphere. The outer molecules are held by attractions within the drop and from the molecules beside them, providing the surface tension that allows insects to walk on water.

As a drop grows, this weak surface tension becomes increasingly unable to hold the spherical shape and the drop becomes distorted. This is why raindrops smaller than about 2 millimetres across remain spherical but, as they get bigger, they start to resemble a hamburger bun, with a flat bottom and a rounded top. Air resistance buffets raindrops as they fall, and water does not have sufficient viscosity to damp these disturbances so falling drops usually break up at a diameter of about 5 millimetres.

These facts were already known by 1904, courtesy of ingenious experiments designed by the Hungarian-born Nobel physics laureate and the American , best known for his early photographs of snowflakes. Lenard used a vertical wind tunnel to study raindrops as they fell, while Bentley measured the size of raindrops from the tiny pellets of dough they formed when they landed in a pan of wheat flour.

Experiments on the space shuttle showed that in microgravity 鈥� and with no wind to perturb them 鈥� drops of water can reach in excess of 3 centimetres in diameter, though footage shows the spheres quivering like jelly, since the surface tension is too weak to maintain the sphere and the viscosity is insufficient to damp out disturbances.

Mike Follows, Willenhall, West Midlands, UK

鈥� The average size of a raindrop that reaches the ground has a diameter between 1 and 2 millimetres. But raindrops between 8.8 and 10 mm were recorded twice a few years ago. The first time was by a research plane that flew over Brazil through cumulus congestus clouds which form over atmospheric regions that are undergoing convection 鈥� they are often created by strong updraughts. The drops are believed to have been formed by water condensing on smoke particles from burning forests below. Large drops were also spotted falling through clean, marine air over the Marshall Islands in Micronesia. They had formed within narrow regions of cloud where raindrops were able to collide and accumulate. It is unlikely that any of these large raindrops would have reached the ground intact because they readily break up as they fall, thanks to air resistance.

鈥淩aindrops usually have a diameter of 1 to 2 millimetres but drops of up to 10 millimetres have been recorded鈥�

Martin Dodds, By email, no address supplied

鈥� In the 1957 film , the protagonist, played by Grant Williams, is irradiated by a mysterious cloud which causes him gradually to dwindle in size. Near the end of the film he has shrunk to the size of an insect and is trapped in his own cellar, where a leaky tap threatens to drown him in what are, to him, enormous globules of dripping water.

Yet when the studio鈥檚 props department built their gigantic faucet and rigged it to drip water, the laws of physics kept all the water drops stubbornly normal sized. When I met the film鈥檚 director, Jack Arnold, years after the film had been made, he told me that he personally hit upon a method of creating the oversized water drops for this scene. He purchased 100 boxes of transparent latex condoms and the stagehands were given the job of filling them with water.

With the cameras rolling, Williams posed near the enormous faucet while a concealed stagehand pushed the condoms out of it, one at a time. The effect is quite convincing: each condom took a teardrop shape in mid-air as it plummeted to the ground, where it burst and released dozens of smaller, genuine droplets. Each discrete 鈥渨ater drop鈥� emerging from the giant faucet was slightly smaller than the shrinking man鈥檚 head.

Arnold told me that when the film鈥檚 producer demanded he justify his purchase of 100 boxes of condoms, Arnold replied: 鈥淲hen we finished shooting the movie, we had one hell of a wrap party鈥︹��

, New York, US