(KXAN) — Aviation is possible through an understanding of physics and engineering, allowing humans to overcome gravity and cross long distances in a way that would have been just a dream for most of our history.

However, natural occurrences can impede aviation; one such occurrence is turbulence, an atmospheric phenomenon that induces chaotic changes to pitch, roll and yaw. Turbulence recently caused injuries to seven passengers on a Lufthansa flight from Texas to Germany, as well as a fatal injury to a passenger in a separate aircraft over New England only days later.

The Federal Aviation Administration recommends passengers remain seated during flights, with safety belts buckled, in order to protect against unexpected turbulence. According to the agency, turbulence caused at least 146 injuries between 2009-2021.

How does turbulence form?

According to the National Weather Service, there are four causes of turbulence: mechanical, thermal, pressure fronts and wind shear. The impact of turbulence is rated from light (slight strain against safety restraints) to extreme (may cause structural damage).

Mechanical turbulence is created by the friction between wind and obstacles on the ground. These obstacles (trees, mountains, buildings, etc.) impede the wind, creating eddies of fast-moving air downstream. Faster windspeed creates bigger eddies and more severe turbulence.

(Courtesy: NWS)

Thermal turbulence, caused by convection, happens below clouds and typically only impacts planes during takeoff and landing. Hot ground causes air to rise, cooling as it moves away from the heat, which leads to increased density and a resulting fall back down.

(Courtesy: NWS)

Frontal turbulence arises when low and high-pressure fronts collide — the warm air of a high-pressure front is lifted over the cold front. This collision can also form thunderstorms; if so, the turbulence will be more severe.

(Courtesy: NWS)

Wind shear, or the change in wind direction and/or speed, can also create turbulence. This can occur at much higher altitudes than the other three types, and can reach well above the flight ceiling of commercial aircraft.

That high-altitude wind shear occurs in “patches” around the jet streams, which are narrow bands of fast-moving air. This is called “clear air turbulence.”

Clear air turbulence forecast for March 3 at 41,000 feet. (Courtesy: NOAA)

Thunderstorms, according to the NWS, create strong vertical currents of air that can “displace an aircraft up or down vertically as much as 2000 to 6000 feet.” Usually, this kind of turbulence occurs at altitudes between 12,000 and 20,000 feet, and up to 20 miles away from the storm cell.

Turbulence, while still a major cause of injuries on flights, affects flight attendants at a much higher rate than passengers.

A 2021 NTSB report showed that 111 turbulence-related accidents occurred between 2009 and 2018 that resulted in at least one serious injury. But flight attendants — who are often up and moving — were most commonly hurt, accounting for 78.9% of those seriously injured.

Climate change is expected to make turbulence worse in the coming decades, experts say, though improvements in weather forecasting will help.

Sometimes, however, turbulence-causing weather phenomena is difficult to predict. In December, 11 passengers on a Hawaiian Airlines flight between Phoenix and Honolulu were seriously injured amid unusually severe turbulence, which the NTSB later determined to be caused by a cloud that “shot up” vertically toward the aircraft, leaving the crew with no time to deviate or warn passengers.

Another “turbulence event” in the skies over New England resulted in the death of a passenger on Friday, the NTSB said, though the nature of the passenger’s injury was not disclosed.

The Associated Press contributed to this report.