The phrase “it’s raining cats and dogs” is what we call an idiom. An idiom is a phrase that cannot be taken seriously because it is meant to add emphasis to the literal meaning of what is trying to be said. This idiom means that it is raining really hard outside. This cannot be taken seriously because it is not physically possible or humane for somebody to literally make cats and dogs fall out of the sky like rain. Because this visually and symbolically hints at the fact it is raining hard, this phrase is not meant for an implication to be emitted that if one was to look outside, cats and dogs would be falling at a constant rate from the sky. Therefore, this idiom is only a symbol that it is raining hard outside.