This article was written by Ph.D. Robert Baron, at the Aviation Consulting Group and first published here.

On any given day, at any given time, there is an aircraft somewhere in the world experiencing a turbulence event. Most of these events are relatively benign, while others cause significant injuries requiring flight diversions and emergency landings. The fact is, turbulence is an environmental phenomenon that’s here to stay. But what causes all the turbulence?

Turbulence is caused by disturbances in the air. A number of things can cause these disturbances, such as mountains (mountain wave turbulence), changes in wind speed or direction (windshear), rising air (convection), changes (bends) in the Jet stream, and vortices from other aircraft (wake turbulence). Temperature and pressure also play a role in the stability of the air.

Some of the worst turbulence I have personally experienced (and I am a very frequent flyer) occurred on a flight from Los Angeles to Fiji at the point where we crossed the equator. This area is known as the Inter Tropical Convergence Zone (ITCZ), and, depending on the time of the year, can be particularly violent. This occurs due to the meeting of the trade winds from the northern and southern hemispheres, which produces upward lifting and the potential for strong turbulence, as well as violent storms.

Since turbulence is a byproduct of atmospheric stability, it may come as no surprise that climate change may have an influence on the magnitude and/or frequency of turbulence events. In fact, this was illuminated recently in a peer-reviewed study by Dr. Paul Williams, a professor of Atmospheric Science at the University of Reading, UK. According to Dr. Williams, “Climate change is expected to strengthen the vertical wind shears at aircraft cruising altitudes within the atmospheric Jet streams… Climate modelling studies have indicated that the amount of moderate or-greater clear-air turbulence on transatlantic flight routes in winter will increase significantly in future as the climate changes…We find that the probability distributions for an ensemble of 21 clear-air turbulence diagnostics generally gain probability in their right-hand tails when the atmospheric carbon dioxide concentration is doubled. These results suggest that the prevalence of transatlantic wintertime clear-air turbulence will increase significantly in all aviation-relevant strength categories as the climate changes.” (the full report can be viewed here).

Turbulence comes in two forms; expected and unexpected. Expected turbulence gives the pilots a heads-up, or a warning, whereby they can take action, such as initiating a climb, descent, speed adjustment, or course deviation. On the other hand—unexpected turbulence—typically clear-air turbulence (turbulence that cannot be detected) is of the most concern. Clear-air turbulence is particularly troublesome in that there is little or no advance warning until it’s encountered, and it can cause significant aircraft upsets very quickly. Clear-air turbulence is typically (but not always) the cause of many of the in-flight upsets that result in passenger injuries. And the number of injuries is on the rise: According to the U.S. Federal Aviation Administration (FAA), there were 44 injuries in 2016, compared to 21 in 2015, a twofold increase. During the last 15 years, the lowest total was 12 in 2006, and the highest was 107 in 2009. During 2016, the U.S. National Transportation Safety Board (NTSB) investigated the following turbulence-related incidents:

• Dec. 13- A United Airlines flight from San Francisco to Denver encountered mountainwave turbulence at 20,000 feet. A flight attendant securing the galley was thrown to the ceiling and then struck her face on a counter that gashed her cheek and fractured a facial bone.

• Aug. 11- A JetBlue Airways flight from Boston to Sacramento encountered turbulence above South Dakota. Three flight attendants and 24 passengers suffered minor injuries, and the aircraft made an emergency landing in Rapid City.

• Aug. 3- An American Airlines flight descending into the Cayman Islands briefly encountered severe clear-air turbulence with no warning. The seatbelt sign had been on for 10 minutes, but five passengers and three crew members were injured. One passenger was taken to the hospital with a fractured vertebra.

• July 28- A JetBlue flight from JFK to Barbados encountered clear-air turbulence with no warning. A flight attendant had a pot of hot water scald her left shoulder and side with second-degree burns.

• July 26- A United Airlines flight encountered brief turbulence at about 16,500 feet while descending into Cleveland. A flight attendant fell in the galley and broke her left tibia.

• Jan. 15- A Southwest Airlines flight from Providence to Fort Lauderdale encountered brief turbulence over Titusville while beginning to descend. The seatbelt sign was illuminated, but a flight attendant securing the galley fell and broke her ankle.

Note: These were U.S. Airline incidents only. Globally, there were many more.

The U.S. National Aeronautics and Space Administration (NASA) also collects data on turbulence events through its voluntary and non-punitive Aviation Safety Reporting System (ASRS). This database provides verbatim, subjective, narratives submitted by cabin crews who have experienced turbulence events. Following are a few examples of those reports:

• “I was in my jumpseat with the seatbelt on but had to remove my shoulder harness and assume a brace position because the plane was being tossed so badly and the shoulder harness of my seatbelt was hurting me. I probably would have ended up with broken collar bones…”

• “…As I turned to hand him the cup of coffee, there was a jolt and the plane dropped. I hit the ceiling with my head, I landed by the center aft jumpseat. Flight Attendant X was partway in the center aft left jumpseat, while the passenger I was getting the coffee for landed next to me close to the center right jumpseat…The aircraft jolted again and dropped. I flew to the ceiling again, hitting it with my head…As I was falling to the floor, the plane dropped yet again. I hit the ceiling again, with my body this time, then landed on my posterior and left hand…The passenger that I had been getting the cup of coffee for was able to make it to a passenger seat near Flight Attendant Y. He noticed she was injured and helped apply pressure to her lacerated scalp…Flight Attendant Y came to the aft galley. She was covered in blood.”

• “…The moderate chop had started to get worse, and I was wondering if they would be able to finish the service…The two aisle Flight Attendants made it to the aft galley, when suddenly the plane dropped dramatically. The number 4 Flight Attendant seemed to have found a handhold near the galley, so she didn't go flying, but the number 2 Flight Attendant flew up high in the air, his body parallel to the aisle. There was a second, even more severe drop immediately afterward, and he flew up again, this time higher than the seatbacks, and came down very hard. He didn't move for a while, which scared me, but the turbulence was so bad, I was afraid to leave my jumpseat…This was the worst turbulence I have experienced in my 31-year career. Many people were screaming, a man got a small cut on his arm when the cart went airborne. The number 2 Flight Attendant became a helpless projectile in the cabin… Both of us have over 30 years flying experience, and are used to turbulence, but this event was so severe, our hands were shaking for about ten minutes afterwards.”

With the understanding that turbulence—particularly clear-air turbulence—will always be a significant threat to flight safety, the question is what can be done? For those whiteknuckled flyers, let me first assuage your fears that the aircraft itself can be torn apart by turbulence. It’s not going to happen. Aircraft are designed, tested, and certified to withstand even the most severe turbulence. On the other hand, the cabin occupants (i.e., cabin crew and passengers), well, that’s another story. Turbulence—especially unexpected turbulence—will continuously pose threats and risks to those in the cabin. Risk mitigation is the key.

We recently conducted a study with an international airline to look at threats encountered by cabin crews during the airline’s regularly scheduled flights. The results indicated that turbulence was the most significant threat in the cabin. Threats do not necessarily equate to events; rather, it means that without proper threat mitigation strategies, the turbulence has the potential to lead to consequential outcomes. Mitigation strategies may include something as simple as adhering to cabin crew instructions (such as remain seated with seatbelts fastened) during turbulence encounters. However, we found that many passengers blatantly disregarded cabin crew members instructions during turbulence.

In some cases, a lack of assertiveness by cabin crews may exacerbate passengers’ nonadherence to instructions. For instance, some cabin crews make the obligatory announcement to “remain seated with seatbelts fastened until the Captain has turned off the fasten seatbelt sign” but fail to strictly enforce the instructions. Interestingly, some airlines perform much better in this respect. It is notable that there is quite a bit of variation between, and within, airlines regarding enforcement of cabin safety instructions.

Passengers may not understand the effects of turbulence encounters. Anyone not buckled up, regardless of how much they weigh, will be subjected to gravitational forces that cannot be overcome. It is just the law of physics. This means that if a passenger is in the aisle, and the aircraft encounters significant negative g forces (rapid downward movement of the aircraft), that passenger, and anything else that’s not secured—such as backpacks, laptops, beverages, service carts, etc., will likely hit the ceiling of the aircraft. The same law of physics applies even when seated but not wearing a seatbelt (just being seated will not prevent you from hitting your head on the ceiling). This is why pilots and cabin crews always remind you to keep your seatbelt fastened any time you are seated.

So What’s The Bottom Line For Me, The Passenger?

The bottom line is that turbulence will always exist, and, as mentioned earlier, likely become more frequent and intense in the future. Some turbulence is expected, and the crew will provide forewarning. On the other hand, turbulence that is unexpected (i.e., clear-air turbulence) may happen suddenly and without warning. This can be the most dangerous type. Either way, here is what you need to do to stay safe:

1. Pay attention to cabin crew instructions. They know what they are doing. And they are there primarily for your safety.

2. During periods of bad turbulence, stow any personal items that may become projectiles (such as laptops). Expect that anything you are drinking will probably wind up on you and/or the person next to you. Finish your coffee or Bloody Mary and wait until smoother air for a refill.

3. Wear your seatbelt any time you are seated, even if it’s a smooth ride (things can get rough very quickly).

4. Wear your seatbelt any time you are seated

5. Wear your seatbelt any time you are seated

6. See items 3-5 above.

Any questions?

Dr. Robert Baron is the President and Chief Consultant of The Aviation Consulting Group. He performs extensive work in his core specializations of Human Factors (HF), Safety Management Systems (SMS), Crew Resource Management (CRM), and Line Operations Safety Audit (LOSA). He consults with, and provides training to, hundreds of aviation organizations on a worldwide basis.