I ride in everything, though there are icy conditions where that may not be the smartest thing to do. Unless you have health issues, 100+ heat won't hurt you -- just drink plenty of water. Rain definitely won't hurt you however heavy -- it's not like taking a shower in the morning is traumatizing.
Real lightning would get me to reconsider, but that's very rare where I live. I've never been in hail on a bike as large as Harhir's -- that would get me off. Once it's even decent marble sized, that stuff can hurt.