There are different causes of turbulence

There are a few bumps and drops.  The seat belt light comes on.  The voice from the cockpit announces that there is a little turbulence ahead and it is time to sit down and buckle in.  Those who fly often are very familiar with the process.

To understand turbulence it might be easier  to think of the sky as a large ocean of air.  There are waves of air moving in different directions, colliding, and shifting. When there are clouds one can see the movement.  When there are no clouds the movement seems to come from nowhere and is very mysterious.

There are several natural causes of turbulence.

* Wind shears
Wind shears are created at the boundaries where the winds are moving in different directions and at different speeds.  This kind of turbulence is most likely to be encountered by large air-crafts  that are traveling at high altitudes. It is normally associated with weather around a jet stream.

Wind shears are key components of  the formation of hail and tornadoes.

There are three basic types of wind shears.  Direction wind shears, speed wind shears and the combination of both speed and directional wind shears.

* Convective currents
These current are a result of the sun heating the ground.  This causes the air to rise.  As it cools it forms clouds.  What we see are white fluffy clouds.  In all actuality the clouds are stirring and churning inside  with the active movement to air.  As the plane passes through the clouds it rides the air waves. It is much like a boat slamming against large waves

In the evening, when the sun sets, this type of turbulence tends to settle down some.

* Obstructions to wind flow
These deal with the normal flow of air.  For example, when flying over a mountain range there are currents and eddies created by the mountains and the air responding to them. As the air passes over the tops of the mountains and dips back into the valleys it creates turbulence.

Airplanes are equipped with special monitors and meters to help detect and avoid turbulence whenever possible. There are times when it comes as a complete surprise.

Pilots identify turbulence in different degrees.
* Light turbulence – slight or erratic changes in altitude that do not last long
* Light chop – rapid and somewhat rhythmic bumpiness without changes in altitude
* Moderate turbulence – Greater intensity than light turbulence.  The aircraft remains in control, with variations in altitude and speed control.
* Moderate chop – more intense version of the light chop
* Severe turbulence – large and abrupt changes in altitude and speed.  Sometimes the aircraft is out of control.

* Extreme turbulence – aircraft is violently toss and is impossible to control

With all this being said, very few airplanes have serious damage due to turbulence. It is important to note that the most dangerous part of most airplane trips is the drive to the airport.