Hello how may I help you? I'm trying to find out a weeather related question, I guess it would be weather related? Yes, I can help you with that. What was the biggest cloud ever? As far as widest vs heaviest? I guess widest. Just because a cloud is wider doesn't mean it holds more water. Maybe you want the heaviest instead? I wasn't really thinking of the amount of water in it, I was thinking how far did it go on? It's hard to say where one ends and another begains. How about the biggest storm vs tallest? I don't understand biggest storm vs. tallest? It's not much different. Four miles across is the answer your likely looking for.