Summer thunderstorms become much more fierce when they collide with a
city than they would otherwise be in the open countryside, according
to research led by Princeton engineers.

http://theanalystmagazine.com/pr/n223.html

Alexandros A. Ntelekos and James A. Smith of Princeton University's
School of Engineering and Applied Science based their conclusion on
computer models and detailed observations of an extreme thunderstorm
that hit Baltimore in July of 2004.