16 November 1999

What Causes Rain?

by Kate Melville

Until recently meteorologists were unable to explain why clouds contain disproportionately large quantities of large and small water droplets. Now Dutch researchers claim that areas of air turbulence that forms very small spiral patterns and that this then causes clouds to produce rain.

The research was carried out by Delft University of Technology and used the computational facilities at the Academic Computation Centre in Amsterdam. Researchers used a Cray C90 super computer to calculate how hundreds of thousands of water droplets contained in one litre of cloud move and grow.

During this process small tubular-shaped vortices are formed and that these force droplets outwards by centrifugal force, so that they congregate at the edge. For rain to happen one in every million droplets needs grow to a diameter greater them 20 micrometers and this happens as droplets collide - this then sets off a chain reaction within the cloud.

This new data turns on its head old meteorological calculations that did not include the effect of small-scale areas of turbulence. The process should take more than three hours before clouds become dense enough to release rain, but in actual fact this process only takes about thirty minutes.

What has also been shown is that there are very few droplets in the center of each area of turbulence and that as a consequence the air there remains supersaturated. Until recently meteorologists had considered this to be impossible, but the results also indicated that air more than one hundred meters above the base of a cloud becomes so supersaturated with water vapour that droplets are created. Water vapour then condenses on particles (like dust), with a radius of less than a micrometer. Just how many small droplets develop depends on the level of supersaturation of the air.

Meteorologists may now have to revise many of their long-held assumptions about clouds and rain. This may not sound like a major scientific feat, but it's right up there for many dry continents like Africa and Australia where water may be one of the key regional security issues in the 21st century.