Why do tornadoes not touch down that often in big cities?
Tornadoes require three factors to grow: vertical movement, which generally comes from thunderstorms; a significant variation in both wind speed and direction within the thunderstorm or air mass; and plenty of space for the rotation to develop. That need for wide open spaces is why you hear about tornadoes so much in the plain states and flat areas of the Midwest and West. Tornadoes are unusual in cities because they lack the open space to develop. Still, it’s not entirely impossible. On May 12, 1997, a tornado struck very near downtown Miami, Florida, and lasted about 15 minutes. Some people were injured and it caused $525,000 in damages. That tornado developed over open water, which provided the space for it to grow, and then moved onshore.