Effect of rain on gliders in flight

Rain on the wings will almost certainly increase the stall speed - by
how much depends on the particular airfoil. As a rule of thumb, older
wood & fabric or metal gliders suffer only a little degradation, early
glass (like my Open Cirrus) suffers quite a lot, newer glass suffers
something in between.

This means that if you are landing with wet wings you need a higher
approach speed to retain the same safety margin over the stall. Your
instructor will advise you (but maybe later in your training - now
you've got too much to think about just following the tow plane).

*Never* winch launch with wet wings, as this could lead to a stall and
spin off the wire. I wouldn't aerotow with anything more than mildly
damp wings either.