Sumatra's mountain range works like a stamping press: when easterly winds hit the slopes, they leave a chain of spinning imprints in the air behind the island. These vortices, like blanks on a conveyor belt, detach from the land and drift out over the ocean. Over ten years, from 2008 to 2017, 1141 such vortices were born off the coasts of Sumatra, the Malay Peninsula, and Java.

Previously, scientists had only short-term observations, and it was unclear whether vortices were always this frequent. Now, a decade of statistics has confirmed that about 70% of vortices are born during easterly winds. And near Sumatra's southern tip, a particular wind can quadruple their number. Why the effect is so strong there remains a mystery, but solving it could improve storm forecasting.