Mathematical model could help to predict rainfall.
Turbulence in clouds whips up rainstorms. Eddies of air help droplets of water to coalesce until they become big enough to fall as rain, new mathematical models suggest1. The calculations yield rules of thumb that could help meteorologists to forecast rainfall.
For a cloud to shed rain, the droplets of its fine white mist must grow large enough to fall quickly through the air. At first this growth occurs slowly as the droplets collect water vapour from the surrounding air. Once they reach about twenty thousandths of a millimetre across, they tend to coalesce with one another.
Droplets of fine drizzle are 200 micrometres or so in diameter, and fall at about half a metre per second. Those with a diameter of about a millimetre plunge earthwards at about nine metres per second.
In turbulent air, droplets, even small ones, collide, coalesce and grow much faster than they do in still or smooth-flowing air, say Gregory Falkovich and colleagues at the Weizmann Institute of Science in Rehovot, Israel.
They conclude that this is partly because turbulence concentrates droplets into dense clusters and partly because a 'slingshot' effect increases collisions. Air flow accelerates a train of droplets and then releases them in a kind of jet.
Wet and hard
At the simplest level, rainfall is easy to understand. Water vapour evaporates from the land or sea, and the moist air, warmed by heat from the ground, rises up and cools. Eventually, the vapour condenses into tiny droplets, and these grow into raindrops that are pulled back to earth by gravity.
But to understand exactly how, where and when rain will fall, the devil is in the details. Meteorologists have to think about warm and cold fronts stretching over hundreds of miles, about individual clouds perhaps a mile or so across, and even, as the new models illustrate, about tiny eddies perhaps a few centimetres or so in size.
Falkovich, G., Fouxon, A. & Stepanov, M. G. Acceleration of rain initiation by cloud turbulence. Nature, 419, 151 - 54, (2002).