How hail is made: Instability is key

People in parts of the Inland Northwest were surprised by large hailstones on Thursday afternoon. But hail actually isn’t uncommon during summer thunderstorms, forecasters said.

The conditions that cause hail are most common between April and September.

“In the spring months, we get cold weather systems that create instability and cause hail,” said Laurie Nisbet of the National Weather Service in Spokane. “And in the summer, we get cold conditions following hot weather, which also causes hail.”

Water droplets in the bottom of a cumulonimbus cloud are carried into the cloud’s top layer, where they are frozen in the high altitude. The frozen droplets then fall back to the bottom of the cloud, where more water droplets stick on. The hailstone is then carried back up to the below-freezing temperatures, causing it to grow.

This cycle continues until the air inside of the cloud can no longer support its weight. The hailstones are supported by drafts of air caused by instability inside of the cloud.

“Instability makes it so that even after air is pushed up, it will keep going as opposed to falling back down,” Nisbet said. “If conditions are stable, pushed air goes up at first, but then it comes down.”

Many factors can cause the unstable conditions that lead to hail, though.

“Daytime heating, forcing air particles up mountains with wind, cold fronts,” Nisbet said. “All these things create instability.”