Clear-Air Turbulence is the name given for a form of turbulence that often takes pilots by surprise; literally “out of the blue,” as they say.
As the name suggests, this form of turbulence takes place in what visually appears to be calm and friendly skies; its difficulty in advance detection, both visually and by technological means, makes it scarier and more dangerous.
Though it is often caused by two bodies of air moving at different speeds and colliding, a recent study1 found that increased levels of CO2, due primarily from climate change, will likely increase the occurrences of clear-air turbulence by double by 2050.
Source 1: Warming Planet Means Bumpier Flights, Scientific American, 8 April 2013. Retrieved 29 February 2016.