... It's raining.

Our parade was being seriously rained on...

There are several scientific explanations for this calorie rule. First off, rain generally implies cold weather, or at least less-than-ideal-going-outside weather. Facing the elements requires an immense output of energy and extreme patience, not only to shield oneself from the rain, but also to endure the cold and unpleasant outside environment. One burns so many calories during such strenuous times that it's highly necessary to over-consume in order to make up for this deficit.

Furthermore, it's scientifically proven that rain and overcast weather conditions can lead to seasonal depression. So "when the weather outside is frightful," who cares about delightful fires? Eat, drink, and be merry! It's good for your health!