Mike Ash writes:

The difference is that a glider's energy source is so UNreliable that no
sane pilot would ever count on it being there, and the glide performance
is necessarily so large, thus a safe landing spot is always kept within
range.

Is it reasonable to think of altitude as the glider equivalent of fuel, or is
that too much of a simplification to be useful? In the sense of a resource
that must be carefully managed, I mean. In a powered aircraft, you can "buy"
more altitude in exchange for fuel, if you run low, but in a glider, you have
only what nature has chosen to provide, although I suppose you can search for
naturally occurring "wellsprings" of altitude from which you can draw to
extend your flight.