Everyone's been there. The flight attendants have dimmed the lights, you've fussed with your pillow and are just about to nod off. Then the whole plane judders, seems to drop in mid-air, and the Fasten Seatbelts sign bongs on.
For some, it barely registers; for others, it means white knuckles for hours. But is there any need to be fearful of turbulence?
"Turbulence is the plane's reaction to air that's changing its state," says Cameron Tribe, senior industry fellow at RMIT's Aviation Academy.
"Wind changes with altitude - it gets stronger as you get higher - and temperature. It can be caused by weather, such as cold fronts and storms, and when air flows over mountain ranges."
The good news is that commercial pilots begin learning about turbulence as soon as training begins. Then there are special weather charts showing weather and areas of clear-air turbulence, and pilots broadcasting unexpected turbulence they encounter during a flight as a courtesy to other aircraft.
Certainly, pilots try to make every flight as boring as possible.
Plus, the newest aircraft have systems that dampen turbulence and make flights smoother.
"There is some unpredictability, which is why we always say to wear your seatbelt even when the sign is off," says Tribe. "Certainly, pilots try to make every flight as boring as possible.
"If you're moving around when a plane hits turbulence, hold onto surrounding seats or the luggage bins and make your way back to your seat. If it's really rough, sit in any vacant seat and wait until it passes."
As for choosing a holiday destination based on whether you'll avoid turbulence, it's not really possible.
"Like the weather, it's seasonal," explains Tribe. "In summer, hot air rising and thunderstorms affect the northern states like Queensland. In winter, though, it's often the high-altitude jet-stream winds from cold fronts that cause turbulence, in which case you're looking at routes from the east coast to Perth.
"Internationally, around the equator, there's a phenomenon called the Inter Tropical Convergence Zone." This is where trade winds from the northern and southern hemispheres converge, making flight conditions less predictable.
"If you're flying from here to Manila, for example," says Tribe, "you'll often get thunderstorms and some bumps."
Enter a website called Turbli that allows passengers to input departure and arrival airports up to 36 hours before flying. Then they choose the flight they're on for a projected turbulence reading and conditions at take-off and landing.
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For nervous flyers, the question is whether turbulence can bring a plane down.
"Historically, there have been accidents caused by turbulence," says Tribe. "But not many, and only then by very severe turbulence caused by severe wind shear near the ground, or by aircraft that have flown into severe thunderstorms or flown near very tall mountains in high winds.
"But the technology available these days is phenomenal, so passengers don't need to worry."
