Aircraft could reduce their impact on global warming just by making small changes in altitude. The streams of water vapour and ice particles that form behind an aircraft, called contrails, are known to create cirrus clouds. These clouds can trap heat radiating from the Earth's surface and thus add to global warming.

Until now the only strategy to avoid contrails was for planes to reduce their altitudes from about 33,000 feet to as low as 24,000 feet. At this height the air is not supersaturated with ice, so contrails cannot form. But it is not ideal. "If you lower the altitude substantially you place a heavy load on air traffic control, and the engines don't operate as efficiently," says Hermann Mannstein at the German Aerospace Center in Oberpfaffenhofen. This is because aircraft engines are optimised to fly at higher altitudes.

Now, Mannstein's team has used balloon measurements of relative humidity in the atmosphere to show that the regions of supersaturated air are only about 500 metres thick, and can easily be avoided. The ability to determine the exact altitude of these regions is limited at present, but such information could soon be relayed to pilots or obtained directly via sensors on the aircraft, says Mannstein (Transportation Research Part D, vol 10, p 421).