How big is the biggest possible raindrop?
• 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 ...
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