During a thunderstorm, strong updrafts of warm air and downdrafts of cold air begin to circulate in storm cell clouds. The updrafts carry water droplets up above freezing altitude, where they form droplets of ice. Then, downdrafts carry the frozen droplets into warmer air, causing it to partially thaw. There, it runs into more water before being picked up again by another updraft, refreezing and growing larger when it passes through cold air again.
If this cycle repeats enough times, the frozen droplet gets bigger and bigger until it’s finally heavy enough to drop to the ground as hail. The stronger a storm cell is, the longer it can recycle these ice droplets—producing larger and larger hailstones, sometimes getting as large as golf balls or grapefruits.