Elevated humidity. The air entering the crankcase is picked up between the throttle body and the air filter. Not suggesting liquid water is getting past the air filter but ... the elevated humidity of the air injested while driving in rain does also enter the crankcase. Add to this the cooling effect of cold water washing across the outter surfaces of the oil pan and you know have a more favorable condition for condensation to form on the inside surfaces of the oil pan that are above the oil level. More so is the case when driving in snow. Water condensation is a major component of soft sludge. Ideally, engine oil temperature should be maintained close to the boiling temperature of water. With lots of cold water, air, or snow contacting the oil pan, this is much less likely to happen.