Air turbulence causes injuries to an average of 58 passengers per year, according to the Federal Aviation Administration. The rough ride occurs when airplanes fly into turbulent air steams. Sometimes, pilots can avoid turbulence by calculating the point at which the air streams hit another aircraft. Often, it is unavoidable.
Turbulence feels like driving on a bumpy road when you are flying. However, its cause lies within the natural airflow of the troposphere. Air currents move as the air on Earth is cooled and heated. This is known as convective heating. The mixing of warm and cool air forms fluffy white clouds, which form turbulent barriers for planes. The turbulence caused strictly by clouds is generally limited to lower altitudes.
Storms pose a threat to airplanes for a variety of reasons, such as hail and lightning. Though storms tend to linger below the 20,000 feet, the turbulence that comes with storms can often rise up as high as 50,000 feet. These violent up and downdrafts can force a plane off its course.
Flights just east of the Rocky Mountains often experience turbulent winds known as mountain waves. These are formed when perpendicular easterly winds blow against the mountainside. This sends the drafts upward, where they collide with flight paths. However, pilots are occasionally able to spot the presence of such drafts in the formation of lenticular clouds, which are formed when the winds blow upwards at existing clouds to reshape them.
This is also known as "clear air turbulence" and is most common during the first flights of the day. With no indication of turbulence, planes are often shaken without warning. Clear air turbulence is invisible and is caused by the collision of hot and cold jet streams in the troposphere.