Remembering the last time I was on a plane I've thought of a question about an encounter we had with turbulence...
We're flying almost at cruise altitude, I'm guessing we're at 25,000 to 30,000ft with low level cloud(overcast) at around 2,000 to 5,000ft.
Looking out the window I spy below us an area of bumpy cloud(an area where the cloud rises up in lumps!) in the otherwise flat cloud and correctly guess thatmeans we're going to encounter turbulence as we pass over it.
Which as it happened turned out to be quite bumpy, but nothing serious.
My question is what meteorological effect can cause such a relativly small area of low level cloud to be "bumpy" and then have enough force to force an effect 20,000ft above it?
Would the bumpiness be caused by something under the cloud? And why would it be so localised? As the rest of the flight, before and after was completely smoooth.
I'm presuming it was a thermal of some kind but, again, why so localised?