Bill Daniels wrote:

You don't look down to see height changes, you look out at an angle. You're
not looking for changes in the size of objects, you look for changes in
angles. It's just like we judge height on final approach to landing. I can
judge the strength of thermals visually up to 1000 meters or so and I bet
the birds can do a lot better.

Bill Daniels

Well, the changes in (apparent) size of objets is nothing else than a change
in an angle. I agree that looking for such an angle just below the glider is
not what will maximize the change for a given height change. If your method
is by watching the change in the angle of the directions of some fixed ground
feature and the horizon, it can easily be shown that the maximum rate of change
is obtained by looking at some feature at 45 degrees below the horizon. In this
case, the change rate, in radians per climbing meter, is 1/(2*height), at 1000
meters the rate of change is of 1.7 minute per meter, in order to see a 1
degree change when climbing at 2 m/s, you have to wait 35 seconds.
Difficult but workable. I should try it in my next flight, although I think I
will not be able to perceive changes below 10 degrees, when looking at 45 degrees.
Looking toward a more horizontal direction should provide better senitivity, as
the fixed feature and the horizon are together in the visual field, but the
rate of change of the angle is much lower. When looking in the same direction
as on final approach, i.e. the direction of the 1/10 slope, the rate of change
is 1/(10*height) radians per climbing meter, 5 times lower than at 45 degrees,
you have to wait nearly 3 minutes climbing at 2 m/s to see a 1 degree change
at 1000 m.