Rain showers may be caused by vortices in the air

November 10, 1999

Clouds contain disproportionately large quantities of large and small water droplets, something which meteorologists were hitherto unable to explain. Dutch researchers now think they have discovered the reason: within areas of air turbulence there are spiral patterns a few centimetres in size, which may also explain why clouds produce rain. These are the results of calculations by a research team from Delft University of Technology (TUD) using the NWO's CRAY C90 supercomputer at the Academic Computation Centre in Amsterdam (SARA). The project was financed by the NWO's Council for Geosphere and Biosphere Sciences.

The Delft researchers calculated how hundreds of thousands of water droplets contained in about one litre of cloud move and grow. During this process, it would seem that tubular-shaped vortices a few centimetres in size are formed and that these force the droplets outwards by centrifugal force, so that they congregate at the edge. For rain to be precipitated, approximately one in every million droplets needs to acquire a diameter greater them 20 micrometers, which it does by colliding with other droplets. A chain reaction then takes place. Previous meteorological calculations had not considered the effect of small-scale areas of turbulence. In theory, this process should take more than three hours before clouds become "ripe" enough to release rain, whereas in actual fact it takes only half an hour or so. It would seem that the turbulence causes small droplets to collide more frequently than expected.

Moreover, hardly any droplets are present at the centre of each area of turbulence, so that the air there remains extremely supersaturated. This may mean that air which is higher than about one hundred metres above the bottom of a cloud also becomes so supersaturated with water vapour that droplets are created. Meteorologists had up to now considered this to be impossible.

A cloud is a rising bubble of moist air in which the temperature drops as a result of expansion. This causes the air to become supersaturated with water. The water vapour then condenses on particles, such as aerosols, with a radius of less than a micrometer. Such dust particles are present in abundance. Just how many small droplets develop depends on the level of supersaturation of the air.

Insight into how water droplets form is not only important in predicting the weather but also so as to understand the chemical reactions in the atmosphere and the absorption and reflection of solar radiation. These processes in fact frequently take place in or at the edge of water droplets.
Further information:
Bas van Haarlem (Delft University of Technology, TUD)
T +31 15 278 2991 or - 215 9744
F +31 15 278 2947
E-mail: B.A.vanHaarlem@wbmt.tudelft.nl

Netherlands Organization for Scientific Research

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