Avoiding waves that go bump in the flight

New Scientist - 11 October 2008 - Magazine issue 2677 - page 23

IT STRIKES without warning and can jangle the nerves of even seasoned air travellers - but maybe not for much longer. Clear air turbulence just got a lot easier to predict and avoid.

Two types of turbulence affect aircraft. The first, caused by storms, high winds or the flow of air over mountains, is fairly predictable. Clear air turbulence (CAT) is a different matter: "The skies are clear and blue, everything looks fine, but there is invisible turbulence there and pilots fly through it," says Paul Williams at the University of Reading, UK. As a result, CAT causes hundreds of injuries a year to airline passengers.

The existing method of warning pilots of CAT, called Graphical Turbulence Guidance, relies on pilot reports and observations of the atmosphere, including lightning data, but is not particularly accurate. To improve on these predictions, Williams and his colleagues decided to focus on the cause of the turbulence: the gravity waves generated at the boundary of fast-moving high-altitude jet streams with slower-moving air. Their model uses wind speed measurements to predict where these boundaries lie, and so where the gravity waves are likely to be strongest.

When they applied the model to a 144-day period in 2005 and 2006, and compared the results against actual reports of CAT, it successfully predicted 83 per cent of the reported incidents (Journal of the Atmospheric Sciences, DOI: 10.1175/2008JAS2477.1). Results so far are raising hopes that the new forecasting approach may be more effective than the best methods used today, Williams says.