Generally, the bigger the aircraft the greater the turbulence in its wake. This wake can continue for some distance behind the aircraft, and while you may feel nothing in the aircraft in which you are travelling, passengers in another aircraft passing through the same altitude may fly through your wake and feel some turbulence. Pilots and air traffic controllers are aware of wake turbulence and that it can lead to accidents. For example, a small aircraft that is following too closely behind a big jet on a final landing approach may fly into the jet's wake and encounter vortexes so strong that it loses control. The worst conditions for wake turbulence are stable air and light winds.
Strong winds help to disperse the wake.
Clear air turbulence occurs in clear air above the friction layer, at around 600 metres, where the effects of surface friction are noticeably reduced. It is invisible and cannot be detected by conventional aircraft weather radar. Jet streams and CAT can be forecast and appear on the meteorological forms given to aircrews at pre-flight briefing. Other meteorological conditions also help cause bumpy rides. Turbulence occurs in the friction layer and is caused by thermals and surface winds blowing around obstacles such as buildings and hills.