Associated with jet streams is a phenomenon known as clear-air turbulence ( CAT ) , caused by vertical and horizontal wind shear caused by jet streams . The CAT is strongest on the cold air side of the jet , next to and just under the axis of the jet . Clear-air turbulence can cause aircraft to plunge and so present a passenger safety hazard that has caused fatal accidents , such as the death of one passenger on United Airlines Flight 826
United Airlines Flight 826
United Airlines Flight 826
Jet stream