Science says "clear-air turbulence," which evidently jolted an Air Canada flight Thursday over the Pacific Ocean, strikes almost literally out of the blue, with no visible warning in the sky ahead.
An aircraft's radar can't spot it coming either.
But passengers can certainly feel it.
Some on the Air Canada flight were slammed against the ceiling, and more than two dozen were taken to hospitals after it made an emergency landing in Honolulu.
Clear-air turbulence happens most often in or near the high-altitude rivers of air called jet streams.
The culprit is wind shear, which is when two huge air masses close to each other are moving at different speeds.
If the difference in speed is big enough, the atmosphere can't handle the strain, and it breaks into turbulent patterns like eddies in water.
Another source of turbulence is masses of air that bob up and down in the atmosphere, somewhat like waves in the ocean.
They can arise spontaneously or form as air flowing over mountains is forced upward, starting the up-and-down cycle.
Weather forecasters can't be much help in warning pilots about where they'll encounter clear-air turbulence, says Thomas Guinn, a meteorology professor at the Embry-Riddle Aeronautical University in Daytona Beach, Florida.
"It's probably one of the most challenging forecast problems we have right now for aviation meteorology," he said.
Paul Williams of the University of Reading in England, who is working on forecasting clear-air turbulence, said some tests suggest that specialized radar-like devices could make the atmospheric disruptions visible to pilots.
But the devices are expensive and very heavy, a drawback for airplanes, so they are not widely used.
Once pilots hit a patch of turbulence, they can try to fly out of it, possibly by changing altitude, said Clint Balog, also of Embry-Riddle.
Passengers violently ejected from seats on turbulent flight
Shocked passengers described being tossed around an Air Canada passenger jet as it hit a pocket of turbulence on its way to Australia on Thursday and was forced to make an emergency landing.
Flight AC33 from Vancouver to Sydney hit trouble around two hours beyond Hawaii and diverted back to the US island chain's capital Honolulu, the company said in a statement.
The Boeing 777-200 landed "normally" at 6.45 am (1645 GMT), the statement went on, adding that "approximately 35 people appear to have sustained minor injuries."
Passenger Jess Smith, told the local television station KHON that "we all hit the roof, and everything fell down... people went flying."
The aircraft suddenly dropped, and "some people that weren't strapped in, you saw them rise in the air and hit their heads on the roof... it was quite intense," another traveler, Fais Asad, said to KHON.
A spokesperson from the Federal Aviation Administration quoted by Canadian broadcaster CBC said the incident took place at 36,000 feet (10,973 meters), about 600 miles (966 km) southwest of Honolulu.
The aircraft was carrying 269 passengers and 15 crew members.
Air Canada was arranging hotels and food for passengers in Honolulu and looking at resuming the flight, the statement said.