What is in-flight turbulence, and when does it become dangerous for passengers and crews?

abcnews.go.com:

The death of a British man and injuries impacting dozens of other people aboard a Singapore Airlines flight have highlighted the potential dangers of flying through extreme turbulence

NEW YORK — The death of a British man and injuries impacting dozens of other people aboard a Singapore Airlines flight that hit severe turbulence Tuesday highlighted the potential dangers of flying through unstable air.

The exact cause of the 73-year-old man’s death is under investigation. Authorities said he may have suffered a heart attack, though that hasn’t been confirmed. Based on witness accounts, the number of injuries and the airliner's sharp descent, experts point to the significant safety hazards that in-flight turbulence poses to airline passengers and crews.

While turbulence-related fatalities are quite rare, injuries have piled up over the years. Some meteorologists and aviation analysts note that reports of turbulence encounters also have been increasing and point to the potential impacts that climate change may have on flying conditions.

Most incidents of planes hitting bumpy air are minor, however, and airlines have made steady improvements to reduce accident rates from turbulence over time. Experts advise air travelers to stay vigilant, stressing the importance of wearing a seat belt whenever possible as a first line of protection.

Turbulence is essentially unstable air that moves in a non-predictable fashion. Most people associate it with heavy storms. But the most dangerous type is clear-air turbulence, which often occurs with no visible warning in the sky ahead.

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