Scientists have found that rapid air movement within clouds can speed production of raindrops.Weizmann Institute physicists have found that turbulence in clouds can accelerate rain formation, a discovery that may provide an effective tool for rain predictions.

In a study published in Nature magazine, the Weizmann team – Prof. Gregory Falkovich of the Physics of Complex Systems Department, graduate student Alexander Fouxon and visiting scientist Michael Stepanov – described a formula that makes it possible to calculate how fast tiny droplets within clouds cluster into heavy, rain-producing drops.

Clouds are formed by warm water vapors rising to the sky. When a cloud cools, the vapors condense into droplets that increase in size and are eventually pulled back to earth by gravity, causing rain. Simple as this cycle may sound, when and where exactly the rain will fall is extremely difficult to predict. Precipitation is influenced by myriad meteorological factors, such as wind, pressure, warm and cold fronts, and – as the new study reveals – the turbulent flow of air inside clouds.

The emergence of raindrops from the cloud occurs in two stages. First, tiny moisture droplets condense and grow gradually until they reach a diameter of 20 micrometers, or about 20 thousandths of a millimeter across. At this size, the droplets begin to crash into one another and cluster into larger drops about a millimeter in size. The collisions are mainly caused by a turbulent airflow creating vortices and eddies inside the cloud.

The Weizmann team derived a mathematical formula that predicts the collision rate of droplets in a turbulent cloud, which in turn makes it possible to forecast when the cloud will shed rain.

The study included the discovery of a mechanism called “the sling effect,” that explains the link between turbulence and rainfall. It reveals that turbulent vortices within a cloud act as small centrifuges that spin heavy droplets outward, much like a sling whirled around to discharge a stone by centrifugal force. The droplets “hurled” by a turbulent vortex are more likely to collide with one another than droplets floating peacefully about.

Calculations based on the formula have led the researchers to conclude in their report: “Air turbulence can substantially accelerate the appearance of large droplets that trigger rain.” In other words, rain will form faster in turbulent air than in still or smooth-flowing air.

Apparently, every cloud has a turbulent lining: turbulent flows of different magnitudes exist in all clouds. The Weizmann Institute formula, which includes such variables as temperature, humidity and wind speed, may therefore prove useful for improving the precision of numerous meteorological forecasts.