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Forecasting where and when the rain will fall

November 24, 2003

Leeds researchers are aiming to unlock the secrets of the British weather, bringing forecasters one step closer to that elusive holy grail: the ability to predict exactly where, when and how much rain is going to fall.

Dr Alan Blyth from the school of the environment explains: "There's still a lot we don't know about exactly how rain is formed, so it's no surprise that the forecasting models don't always get it right, particularly in relation to the quantity of rain that's going to fall."

Most rain in the UK is caused by ice crystals growing to such a size that they fall to earth, melting as they enter warmer air. But how and when these crystals form still isn't clear. Some water droplets in clouds will freeze and begin the process at -10oC; others need temperatures closer to -40oC.

Using the latest equipment developed in Britain and the USA, the research team will measure the size and shape of ice crystals in the clouds from a specially equipped NERC aircraft. Although the plane travels faster than 100 metres a second, the 'cloud particle imager' can capture pictures of ice crystals just 0.001mm in size.

"We thought that the droplets which froze first were those which had a tiny mineral grain in them, known as an 'ice nucleus'." said Dr Blyth. "We think there's approximately one ice nucleus per hundred thousand water droplets, but earlier tests I carried out in New Mexico showed that at -15oC, far more ice crystals are forming than there could be ice nuclei, so there must be another process at work."

The ability to take pictures of such small ice crystals means the researchers can see what's happening at a very early stage of the process, while the crystals are just starting to form. Their findings can then be fed into the forecasting models to provide more accurate predictions.

The research is a collaboration with the Met Office, UMIST, the University of Hertfordshire and the national centre for atmospheric research in the USA.

Leeds, University of