well, With regard to grammar, the right answer to “what” question should be a noun, but it is not always so. For many, there’s a more or less unwritten code here, I mean some words or idioms were established by long usage. Take”what’s the weather like today?”, usually we say “it’s clear/windy/rainy etc” instead of “wind, rain”. In this case, you can’t say the answer is wrong though. No doubt, you can definitely say ”there’s heavy rain”. What I wanna say here is that we ought to focus on both grammar and usage for sure, but I prefer to respect and learn more usage based on reasonable grammar. :)