When you are sleeping next to somebody special, and it starts to rain, or is already raining, you are given a free pass to kiss them passionately and relentlessly, even if they are asleep. When they awake, they are not allowed to stop you as long as it's raining, or at least until they respond in kind. The intention is to take advantage of the romantic setting before it is gone, even if only one of you is awake to notice it in the first place. Rain Kisses work best in a region that rains infrequently.
Every time it rains, Scott wishes Ann were in his arms, so he could wake her up with a passionate attack of rain kisses. She claims she would enjoy it, but has not had the chance prove it yet.