Tuesday, March 2, 2010
Why was my flight so bumpy? Part II
Well, as long as I am terrorizing all of you with tales of flight turbulence, I might as well be complete!
In my last blog I talked about the number-one source of turbulence in the skies, shear-induced turbulence. When such turbulence occurs outside of clouds it is often called clear air turbulence or CAT.
But there is another source of aircraft bumps that is often felt around here...mountain wave turbulence. Ever notice that when flying across the Rockies that sometimes the plane is rocking over and immediately to the east of the mountain crest? Or have you experienced lots of bumps flying into Denver? Mountain Wave Turbulence could be the cause!
As shown in the figure, when air moves over mountains, waves can form. Quite simply, when air is pushed up it often oscillates up and down. These oscillations can spread downwind or can propagate vertically. As these waves move up into less dense air then can amplify causing increased shear (and turbulence) or they can break (producing strong turbulence). One sign of such mountain waves are lenticular clouds (see picture at the top). Mountain wave turbulence can sometimes become severe, but generally it is light to moderate in intensity. But every once in a while you hear about jets crossing the Rockies that are jolted by the strong stuff, with people and belongings being thrown to the ceiling.
Interestingly, the Cascades generally don't produce strong mountain wave turbulence because they are too narrow, but there are exceptions to the rule. During strong easterly flow and Enumclaw-style windstorms (see my book for details), moderate to severe turbulence can occur over and to the immediate west of the Cascades.
Another important source of turbulence...but one not as frequent here in the Northwest... is convection....big cumulus and thunderstorms. With strong up and downward motion and large wind shears, such convection can produce moderate to severe turbulence. But pilots know this and generally keep a safe distance, something made easier by weather radar on most big planes.
Remember the NW Weather Workshop!!
Posted by Cliff Mass at 7:32 PM