The reactions inside aircraft vary from occupants feeling slight strain against their seat belts and unsecured items being slightly displaced, through to occupants being forced violently against seat-belts, and unsecured items being being tossed about. It is usually felt at its mildest in the flight deck and is generally more severe in the aft section. It can occur when no clouds are visible. Aircraft radars can't detect it. It is common at high altitudes, where cruising airline suddenly enter turbulent areas.
Turbulence is the leading cause of in-flight injuries.
There are countless reports of occupants who were seriously injured while moving about the passenger cabin when clear air turbulence is encountered. The causes Thermals - Heat from the sun makes warm air masses rise and cold ones sink. Mountains - Air passes over mountains and causes turbulence as it flows above the air on the other side. In-flight turbulence is the leading cause of injuries to passengers and crew. Occupants injured during turbulence are usually not wearing seatbelts, ignoring recommendations to keep seatbelts fastened even when the signs are not illuminated.