Clear view of the clouds

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 yesterday in the Proceedings of the Royal Society.

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.