Clear view of the clouds will bring better weather forecasts

Accurately forecasting rain will be easier thanks to new insights into clouds from the University of Leeds, UCL (University College London) and others. Details of a new model for predicting cloud and rain-formation are published today in the Proceedings of the Royal Society (10 August 2005).

Existing forecasting models – including ones used by the UK’s Meteorological Office – assume rain droplets fall through still air within a cloud. However, there is turbulence within clouds that can speed up droplet settling and increase the likelihood of rain.

The international team developed a new mathematical model and showed for the first time how pockets of whirling air (tiny eddies) encourage collisions between very small droplets (about 1/1000 of a cm) and slightly larger droplets within a cloud. The collisions lead to the rapid growth of the larger drops – larger than a critical size of 20 microns ( 1 micron is a millionth of a metre). This size is necessary for rain to form, fall out of the clouds and, when conditions are right, reach the ground.

The model’s results were checked against earlier measurements from aircraft flying through different types of clouds. The cloud measurements showed the model was more accurate than existing ones, which often underestimate rainfall.

Leeds earth and environment research fellow Dr Sat Ghosh: “When your plane comes in to land you can see patterns formed by whirling air and sometimes feel the turbulence as you descend through a cloud. As cloud droplets descend through the smallest whirls of turbulence they speed up, causing them to collide with each other and the drops to grow, eventually getting big enough to fall as rain.”

Lord Julian Hunt from the UCL department of space and climate physics (and ex-Chief Executive of the UK Met Office) said: ” With this theory it is possible to explain how dust in the atmosphere, for example over urban areas or over deserts, can cause the initiation of very small droplets so that big drops do not form. This can reduce the average rain fall, but can trigger exceptionally heavy rain in very deep clouds. This may have happened recently in Mumbai and Romania.’’

Further work which will help improve weather forecasting, including the way ice crystals, water droplets and particles interact is planned.