What is clear-air turbulence? What to know about the "very violent" phenomenon

Video and passenger accounts have painted a picture of chaos aboard Singapore Airlines Flight SQ321 after the passenger aircraft encountered what the airline called "sudden extreme turbulence." 

One person died during the incident. Authorities believe the passenger, identified as a 73-year-old British man, had a heart attack. Dozens more passengers were injured. Six people were treated for severe injuries after the plane made an emergency landing in Bangkok, Thailand, CBS News previously reported. 

The incident has drawn attention to the dangers turbulence can pose. One type of turbulence, known as clear-air turbulence, can be especially difficult for pilots.