There is also an illusion caused by rain on the canopy that makes the
glider appear higher than it is - causing the pilot to flare higher
than normal. When you have rain on the windscreen, you should gently
fly it on; it is a surprise when you find that the height you expect
to touch the ground is not correct. Also, because you think you're
higher, you tend to fly a lower approach, so add a few feet for
obstacle clearance in rain.

There will also be a tendancy for the inside of the canopy to mist up
more, so open the vents.