What is the name of the largest hailstone recorded in the United States by diameter?

The largest hailstone recorded in the United States by diameter is the Vivian hailstone, which measured 20.3 centimetres across in South Dakota on July 23, 2010.

The stone fell near Vivian during a severe thunderstorm. It had a circumference of 47.6 centimetres and weighed 0.879 kilograms when officially measured after recovery. Its size made it one of the most notable hailstones ever documented in North America.

Hail grows inside powerful thunderstorm updrafts. Supercooled water droplets freeze onto an ice embryo, and repeated lifting through regions of liquid water can build layered ice. A stronger updraft can keep a growing stone suspended longer, although the final size also depends on moisture and storm structure.

The Vivian stone is often called the largest U.S. hailstone by diameter. Records based on diameter, circumference, mass, and location can differ, so the measurement category matters.