clear-air turbulence

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Atmospheric turbulence that occurs under tranquil and cloudless conditions and subjects aircraft to strong updrafts and downdrafts.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun The erratic movement of air masses in the absence of any visual cues, especially such movement that causes unexpected turbulence to an aircraft.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun strong turbulence in an otherwise cloudless region that subjects aircraft to violent updrafts or downdrafts