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Thursday, April 15, 1999 Published at 14:10 GMT 15:10 UK

Clear air turbulence: A menace at high altitude

The most certain thing about clear air turbulence is that any experience of it will constitute the longest and most terrifying few seconds of your life.

The freak combination of wind and temperature capable of plunging or rising an aircraft hundreds of feet is invisible to radar and, as a result, completely unpredictable.

The movement turns anything not securely fixed in the cabin into a missile.

The phenomenon, which struck a BA flight from Brisbane to Singapore this week on which the UK Prime Minister Tony Blair's 11-year-old daughter was a passenger, has in the past left hundreds of air travellers and crew shocked at best and, at worst, fatally injured.

Turbulence caused by cloud or heavy weather can usually be avoided but there is less chance of avoiding havoc lurking in a clear sky.

Even the most experienced pilots fly blindly into clear air turbulence.

A sudden dip and roll of the plane the first warning that they are in the front seat of a high-altitude roller coaster ride.

What is the cause?

It happens when aircraft move across the edges of the jet stream at high altitude, mixing slow-moving air with high-speed winds and hot and cold air to cause a complex series of conflicting weather patterns.

Experts draw parallels with fast-moving currents and changes in the depth of water.

Meteorological expert Martin Airey says the skies are equally ruled by sudden waves of air capable of exerting massive forces.

"The aircraft gets caught on a wave with updraughts and downdraughts of air causing sudden and violent movements," he says.

"The plane is moving through air of different densities and temperatures while also moving in different directions. The forces produced are huge and a jet flying through them can be thrown about like a toy."

Help on the way

Although clear air turbulence is invisible to the radar equipment which allows aircraft to avoid storms, US scientists are perfecting laser technology which will be able to detect changes in air density.

Known as Light Detection and Ranging or Lidar, the system emits energy pulses before using feedback to compare it with the circumstances in which clear air turbulence is know to occur.

It is hoped that Lidar could provide pilots with a vital 60-second warning to tell passengers and crew to put on their seatbelts.

That would at least save them perhaps from injury - if not complete terror.

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