European scientists are hiring a Russian spy plane and its pilot to fly through high polar clouds and measure their contribution to the degradation of the ozone layer.

The pilot will fly the plane, which holds the world altitude record of 21.35 kilometres, to about 20 km. Up there he will have to cope with a tenuous atmosphere and with vertical winds inside the clouds.

The plane is ideally suited to these conditions, according to Robert MacKenzie, scientific coordinator at the Centre for Atmospheric Sciences at Cambridge University.

Polar stratospheric clouds are made up of droplets of water and nitric acid. Their particles catalyse the destruction of the ozone layer by chlorine. Once compounds such as chlorofluorocarbons (CFCs) have been broken down into other chemicals by intense sunlight at the top of the atmosphere, they waft down on atmospheric currents. If they reach the polar clouds, the cloud droplets cause them to break down further into highly active chlorine atoms, which destroy ozone.

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But there are many unanswered questions about the clouds, such as their precise composition, and how much damage they are doing.

Dr MacKenzie chairs the scientific organisational committee of the Arctic Polar Experiment, which is a European Science Foundation programme. He said: "There have been aircraft and balloon measurements of these clouds before. But this is the first to be done with an aircraft designed specifically to go right into the clouds."

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Dr MacKenzie said that the Russians had offered their plane, an M-55 Geophysika, to the atmospheric science community as a way of branching out of military applications. The other five M-55 planes are still in military service.