On Serebii's Swift Swim page, it says "When rainy, The Pokémon’s Speed doubles. However, Speed will not double on the turn weather becomes Heavy Rain." So, what's the difference between Rain and Heavy Rain?
>Normal rain: Rain falls. Found in all places where rain is found on the field.
Heavy rain: Rains falls heavily, at a lower angle than in the other. Found occasionally on Route 212.