According to Harrison, it’s still a pretty big mystery why some storms stay local and are fairly transient, and why others go global and persist for weeks. But so far, “we know that dust storm season happens at the same time every year—early spring to summer—and the season creates these strong temperature gradients every year.” But it’s still unclear why some years experience regional storms, and some experience global storms. Generally, storms start in the northern hemisphere and are funneled south through topographic basins to the equator. If they cross the equator and make it to southern hemisphere, toward the end of the polar cap, they have a good chance of blowing up into something much more massive. “That’s what we saw happen this year,” she says.