On a warm summer afternoon, why would cumulus clouds and thunderstorms form in one part of a town and not in another?
Usually such clouds are not in towns. They are in the upper atmosphere. Please read:
http://en.wikipedia.org/wiki/Cumulus_cloud
THEN TRY FERRETING HERE:
http://www.google.com/search?hl=en&lr=&as_qdr=all&q=allintitle%3A++%22cumulus+cloud+formation%22+