Turbulence is a routine experience for airline travellers, whereas violent turbulence is a rare and potentially dangerous occurence characterized by frequent violent changes in the aircraft's speed and altitude. (Ted S. Warren/Associated Press)

Turbulence is a common occurrence during air travel. While it can be frightening or nauseating for some travellers, it's usually as routine as the in-flight movie.

"Violent" or "extreme" turbulence, however, is another matter completely.

What is violent turbulence?

Violent turbulence is characterized by frequent violent changes in the aircraft's speed and altitude, causing the feeling that the plane is being "thrown around." It is likely that the pilot will lose control of the aircraft during this kind of turbulence, and damage to the plane is possible.

Normal turbulence, by contrast, is characterized by small, temporary changes in speed and altitude. It often creates a sensation somewhat like driving along a bumpy road, and it's so common that more experienced flyers sometimes claim to not even notice that it's happening.

What causes it?

All forms of turbulence can be caused by a number of factors, including heat, jet streams and other objects near the plane. Violent turbulence is often attributed to a combination of these elements that make the air even more unstable.

Flying over mountain ranges can also cause extreme turbulence. As air passes over the mountains, it can take on a wave-like motion. Rapid changes in the wind disrupts the flow of the jet stream, which is the flow of air currents roughly 11 kilometres above the Earth.

A severe thunderstorm can also cause violent turbulence for many of the same reasons, and pilots tend to avoid flying in thunderstorms for that very reason.

High altitudes can cause "clear air turbulence," which is harder to predict. The higher the altitude, the less likely pilots will be able to gauge whether turbulence is likely based on cloud patterns. It is also much more difficult to detect by radar.

Without sufficient warning, aircraft can enter turbulence very suddenly. Clear air turbulence also feels more severe to passengers than it does to the crew on the flight deck, which can make for a stressful flight.

How common is it?

Violent turbulence is not particularly common at all, and experienced pilots know how to avoid it. Air traffic control bodies are constantly monitoring weather reports and considering which of the factors that can cause extreme turbulence are likely to interfere with a flight. Aircraft are usually routed around masses of highly unstable air.

What are the guidelines for pilots when it comes to flying near thunderstorms?

Transport Canada advises pilots never to think of a thunderstorm as "light." It advises pilots should avoid thunderstorms.

The agency notes that whenever possible, pilots should:

• Not land or take off when a thunderstorm is approaching. The sudden wind shift of the gust front or low-level turbulence could result in loss of control.
• Not attempt to fly under a thunderstorm. Turbulence under the storm could be disastrous.
• Not fly into a cloud mass containing embedded thunderstorms without airborne radar.

The agency further advises pilots who have entered thunderstorms to focus on their instruments instead of looking outside because intense lightning can cause temporary blindness. Transport Canada also says pilots should not try to turn back once they've entered a storm and to let the aircraft ride the turbulence rather than trying to maintain a rigid altitude.

Pilots entering turbulence should slow to turbulence penetration speed — the speed at which the plane can best handle rocky conditions and avoid incurring serious damage to the aircraft.

What if I experience violent turbulence on a flight?

No matter what degree of turbulence occurs, it is important to keep your seatbelt on. The Canadian Transportation Agency website says that standard safety briefings are expected on aircrafts after the flight crew has notified passengers of potential turbulence.

The CTA also recommends that passengers keep their seatbelts fastened throughout the entire flight so as to minimize potential injuries from unexpected and potentially severe turbulence.

And remember: airplanes are built to withstand the forces of nature — even extreme turbulence.