Just like hiking, sooner or later you're going to get wet, either from internal sweat or external leakage. Don't fight it. Resistance is futile. You can be wet and still warm. Just keep moving.

Exceptions: Very short rides and very cold temps. Of course if it's very cold it won't be raining, it will be snowing and that's another animal.
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If you only travel on sunny days you will never reach your destination.*

* May not apply at certain latitudes in Canada and elsewhere.