Turbulence occurs when a mass of air moving at a specific speed meets another air mass moving at a different speed. It can be created by several different conditions, including thunderstorms, warm or cold fronts, mountain waves, jet streams and atmospheric pressures.
The most common and potentially the most dangerous type of turbulence is referred to as "Clear-air turbulence" or "CAT." Unlike other forms of turbulence, it occurs in calm, blue skies. Since it cannot be foreseen, there is no warning. It is typically mildly felt in the flight deck and more severely in the aft section. It also cannot be detected using aircraft radars and is common at high altitudes.
There are varying intensities of turbulence. Light turbulence leads to slight changes in altitude. Light chop causes rapid, rhythmic bumpiness with slight changes in altitude. Moderate turbulence is similar to light turbulence but the changes in altitude are more intense. Moderate chop causes rapid jolts and bumps with unnoticeable changes in altitude. Severe turbulence leads to large changes in altitude that can cause the aircraft to temporarily lose control. During extreme turbulence the airplane is tossed about and is difficult to control.
Generally, turbulence is harmless and pilots have been trained to deal with it. However, during severe turbulence, life-threatening injuries may occur.