KEEP YOURSELF DRY
It’s much harder to regulate your temperature when you’re losing heat to evaporation. Whether it’s because your clothes are sweaty or through rain, excessive moisture can render even the best warm layers useless.
The most breathable waterproofing available is the
humble umbrella. In the right conditions, you just can’t beat it, and it means your upper layers don’t have to be so closely sealed against the elements. (They aren’t so good at dealing with wind though.)
If a brolly is impractical, think layering: warm layer underneath, breathable layer in the middle, waterproof membrane at the top. Then you’ll manage the moisture as effectively as possible.