What are some natural events mentioned in Robert Bartlett's book "The Natural and the Supernatural in the Middle Ages"?
It's so easy to find info on Google!
http://www.google.com/search?q=natural+events+in+middle+ages&rlz=1C5CHFA_enUS705US707&oq=natural+events+in+middle+ages&aqs=chrome..69i57j0.6717j0j7&sourceid=chrome&ie=UTF-8
Choose any of these that are referred to in that book. I haven't read it, so make sure you already have.