I'm running a reasonably fast laptop: P4 2.53g, Geforce 4 graphics. I'm patched up to the gills and have done the graphics tweaks. Terrain is about 3, effects 2, clouds 1, aircraft highest at 4. I put scenery at 1 because I hate all those little autogen boxes that are supposed to be houses, and I'm in a high-flying Bomber, anyway.
On a normal campaign mission I'm getting no lower than about 26 fps.
The other night, I moved the aircraft visuals up to 5, and did a normal mission which kept me around a comfortable 21 fps.
The next mission, it was raining, and after a reasonable take-off and warp, I came out at 4 fps, which increased to 12 and back to 4 in waves. Fortunately, I got a good bomb drop, and warped home, but I had to give up on landing, because the fps wouldn't hold at 12 long enough to get lined up before the 4 fps kicked in again.

Is this normal with bad weather?
And is bad weather an "effect" - meaning I should lower the effects graphic down to 1?

Now that tip is an absolute gem. Thank you very much.

The rain is very nice eye-candy, but my frame rate is more important.