Storm clouds contain violent updrafts and downdrafts that carry air and moisture up and down inside the cloud.
Creating A Storm
Even on a hot summer day on the ground, the top of a storm cloud may be forty thousand feet high, in air well below freezing. As the warm updraft in the cloud carries moisture to the top of the cloud, the moisture freezes around dust or other particles in the air.
As the ice particles get bigger or get caught in a downdraft, they drop through the cloud, picking up more moisture as they go.
From Ice To Rain
Most rain drops begin as ice particles that fall from clouds and either melt or evaporate before they reach the ground. But in the tall, violent storm clouds of summer, falling ice particles often get caught in updrafts before they leave the cloud and are carried back up again.
When an ice particle returns to the freezing temperatures above, the moisture it picked up on the way down turns into a layer of ice and the small ice particle becomes a hailstone.
Layers Of Ice
Each time the hailstone falls through the cloud and is carried back to the top, it accumulates another layer of ice. If you cut open a large hailstone, you can see the layers inside. How big a hailstone gets depends on the strength of the updraft.
The largest hailstone ever recorded was over five inches in diameter, weighed over one and a half pounds, and fell on Coffeyville, Kansas in September of 1970.
It's ironic that the largest form of frozen precipitation falls during the warmest part of the year, but that's when we get the biggest storm clouds with the strongest updrafts.