Eight people are dead after tornadoes swirled through Oklahoma yesterday. The twisters touched down in at least three cities: Lone Grove, Edmond and nearby Oklahoma City. Fourteen people were seriously injured in Lone Grove, the worst-hit city, where all the fatalities occurred.

Tornadoes are common in Oklahoma, but more so in the spring, National Weather Service meteorologist Rick Smith told the Associated Press. Here's why, according to a ScientificAmerican.com Ask the Experts column from 2005 on why Kansas, Texas, and Oklahoma get more twisters: Tornadoes form when warm, moist air near the ground collides with dry air higher up. When the winds over the central plains bring these two temperature and moisture patterns together, tornadoes can occur.

The Gulf of Mexico to the south and the Rockies to the west prove a violent combination when the Gulf’s moist, warm winds collide with the mountains’ cool, dry ones. In fact, the plains are best place in the world for tornadoes to form, because of the expanse and height of the Rockies.

Although a 1999 tornado killed 36 people when it touched down in Oklahoma City, twisters are less common in densely populated areas than in wide-open spaces, we reported last year. Why? Cities are small, and tornadoes thrive in open country. City tornadoes aren’t unheard of, though: tornadoes injured 200 people in the Washington, D.C. suburbs last year, and a twister tore through a section of Brooklyn in 2007.

1999 Oklahoma tornado/OAR/ERL/National Severe Storms Laboratory via Wikimedia Commons