A girl younger than me sent me a text message saying; It's raining heavily and horribly. How should I reply to her?

See if you are interested in her. She was just trying to protect. If you don't want to have an affair with her, don't say "Don't be afraid, I'm here". You can just say "Yes, it's summer, it often thunders and rains, so it's safer to take an umbrella when you go out". Just don't be ambiguous and don't give her some concern that will make her think.