My bike is XTC820 (Giant). Why does the brake produce a lot of friction before it rains? Usually not, but it will rain.

When a disc brake bicycle rides in rainy days, rain and moisture will cause uneven friction between discs, so it will make a rubbing sound. It doesn't matter. Although the braking effect of disc brakes is not as good as that of dry weather, it is more reliable to use disc brakes in rainy days.

Disc brake bicycles should be cleaned after riding in rainy days to avoid unnecessary wear and tear caused by rust and mud sticking to the disc or disc.