News today that unexpected turbulence injured at least 26 people on Continental Flight 128, en route to Texas, is a stark reminder of why airlines ask you keep your seat belt on whenever seated, even if the flight is calm.

The plane dropped suddenly, throwing flight attendants to the roof, according to news reports.

Turbulence is the leading cause of non-fatal in-flight injuries, according to the U.S. Federal Aviation Administration, usually because passengers or crew aren't wearing seatbelts.

Turbulence is air movement that normally cannot be seen and often occurs unexpectedly. It can happen because of changes in air pressure, jet streams, waves created by the influence of mountains, cold or warm fronts, and thunderstorms. It can even occur when the sky is clear.

The rocky air can't always be predicted and radars can't spot it, though researchers are working on other ways to detect it by parsing the data on wind movement provided by radar.

While turbulence may cause you to knock your head hard, it isn't likely to be fatal. From 1980 through June 2004, U.S. air carriers had 198 turbulence accidents, resulting in 266 serious injuries and three fatalities, according to the FAA.

Despite today's event and other recent high-profile airline accidents, analysts say flying is safer than ever and remains far less dangerous than driving.