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

It's useful to look at extremes here.

If a plane was going straight down (extreme case) and flew into a 10
knot thermal, it's speed would increase by 10 knots instantaneously.

If a plane was going straight and level, and flew into a thermal, it's
speed would increase by zero knots. no increase.

For any rate of descent, the plane's speed would increase upon entry
into a thermal by some value between 0 and 10 knots, varying based on
it's rate of descent/angle when it penetrates the thermal.

nafod40 wrote:
It's useful to look at extremes here.

If a plane was going straight down (extreme case) and flew into a 10
knot thermal, it's speed would increase by 10 knots instantaneously.

If a plane was going straight and level, and flew into a thermal, it's
speed would increase by zero knots. no increase.

For any rate of descent, the plane's speed would increase upon entry
into a thermal by some value between 0 and 10 knots, varying based on
it's rate of descent/angle when it penetrates the thermal.

As I understand what you're saying, the portion of AS increase results
from the increase in relative wind due to the component of the thermal
in line with the direction of flight, since the glider is descending at
an angle. I don't buy it, and here's why (It's been a long time since I
did trig but here goes.):
38:1 glider has a glide slope of about 1.5 degrees in still air flies
into a 10 kt thermal with an IAS of 50 kts.
The component of the thermals upward velocity in the direction of flight
is sin1.5x10kts=0.26 kts or 50.26 kts IAS.
I see a lot bigger jump than this (like my ASI would show a quarter knot
dif!).
OK, you math profs can tear me to shreds now.

Shawn