The US may not have a monopoly on tornadoes, but it certainly dominates the market, with three-quarters of the world’s twisters occurring in a region known as “tornado alley” across eight states of the central US. While some religious thinkers may see them as the judgment of the almighty, meteorologists are using computer models to find out how terrain affects tornado formation.
Research from Purdue University in Indiana, published in the Proceedings of the National Academy of Sciences, compares North America with South America, which has a similar number of severe thunderstorms but far fewer tornadoes. The researchers used computer models to alter the texture of terrain, changing plains to forests or flattening mountain ranges. These changes have a significant effect on wind shear, the difference in wind speed with altitude. Like smoke rings, tornadoes need a particular combination of factors to form.
“The wind shear in the kilometre of air above the ground is a critical ingredient for tornadoes,” says researcher Dan Chavas.
The researchers found that without the Andes, South America might have had its own tornado alley. The surface roughness of the mountains prevents the wind speed patterns that typically cause tornadoes. This is why only its neighbour to the north has the dubious blessing of a region generating a steady stream of twisters.
