Hellman wrote:
I fly a motor glider out of the San Francisco Bay Area and frequently
head over the Sierras to the good soaring. My question has to do with
lift on the Sierras themselves.

I've occasionally been able to soar on them, but most of the time find
them marginal at best. Clouds which would indicate great lift on the
mountain ranges to the east (which are more barren and therefore
better thermal generators) often have little or no lift under them.
There are exceptions and I've had some fantastic times on the Sierras
themselvess, but I'm talking about most of the time. Which leads to
three questions:

Perhaps what you are seeing is airmass convergence effects.

I can't speak about the Sierras, but your description matches clouds we
often see in our area. The clouds are clearly formed by, or at least
organized by, convergence zones. Some zones are lines, so the clouds
form a street; some zones are irregular areas and just a clump of clouds
form. Early in the day, the soaring is weak under these "convergence
clouds"; later in the day, the soaring is stronger. The original pattern
of lines and clumps may change or disappear completely as the thermals
strengthen and overwhelm the effect of the valley wind convergences.

--
Eric Greenwell - Washington State, USA
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