How did Jack and his followers respond to the rain? What was Ralph's concern? What more did this reveal about the difference between Jack and Ralph?

It was not so much the rain but rather a storm with thunder and lightning. Jack knew the boys would be scared so he orders them to dance and chant. Jack also wanted the boys to forget about home and the old life. This works as a diversion for the boys, They forget their fears in a communal frenzy. Ralph knows what Jack is doing. Ralph would rather have proper shelters and "civilized" reaction to situations.