I've noticed that there seems to be a time in the afternoon when the
lift gets "soft" and previously reliable clouds become much less
reliable. Then, after an hour or so, the lift seems to pick up again,
not usually as strong but good enough to get home.

My totally uneducated guess is that at some point, the reserve of lift
that is breaking loose and forming thermals gets exhausted and isn't
replenished fast enough, so it takes a while to build up again and
start making reliable thermals.

This is really noticable where I often fly in Illinois, where you have
to slow down and be real careful not to get too low mid afternoon,
even though the day still looks great with cu's all over the place -
around 2 to 3 in the afternoon it gets real soft and you have to slow
down and stay high and wait for the lift to cycle back on. Then you
can stay up till the sun goes down!

Kirk
66