How do you explain why the A/S increases on thermal entry?

Brian:

I'm not comfortable with that explanation. The glider goes "up" only
in relation to the earth, not in relation to the airmass. In fact, the
glider is steady in the airmass -- descending at 2 kts (more or less)
-- and we only seek out the rising airmass because it puts us higher
relative to the earth. Your explanation makes it sound as if the
glider is staying steady relative to the earth while the airmass
accelerates from 2 to 6 kts. (If this is so, it is only very
momentary.)

I think Terry and Burt have the explanation that is both
aerodynamically accurate and something a student can grasp relatively
quickly and easily.

Thanks to all. Fred