Southwestern China’s busiest airport was mopping up and struggling to resume operations on Sunday after the biggest hailstorm in a decade grounded flights, damaged aircraft and stranded passengers.

Hailstones, some as big as fists, battered Kunming Changshui International Airport in Kunming, Yunnan province, when the storm struck early on Saturday.

The storm lashed runways, terminal roofs and some of the more than 110 aircraft at the facility on Saturday morning, with water seen seeping down...