The insurance industry is grappling with a new kind of weather risk that’s increasingly driving its biggest loss category.

While no single weather event caused more than $10 billion in losses for insurers last year, there were 37 thunderstorms that each cost at least $1 billion, according to a report by Aon Plc. That’s more than ever before and way above the average of 14 such storms in a single year, the insurance broker said.