Why do some thunderstorms produce hail, but others do not?

Dear Tom,
Why do some thunderstorms produce hail, but others do not? What factors determine the size of hailstones?

Roy Lane,
Boerne, Texas

Dear Roy,
Hail is likely to occur in most thunderstorms, but it never reaches the ground. Hail requires intense upward air motions within thunderstorms and consequently occurs most often in severe thunderstorms, especially those in the continental interiors of the mid-latitudes. Hailstones will keep growing until their mass can no longer be supported by the updrafts of the thunderstorm, at which time they begin to fall. The plains of Colorado, Wyoming and western Kansas and Nebraska experience the greatest number of hailstorms in the United States. Cheyenne, Wyoming, with an average of nine days of hail per year, is the country’s “hail capital.”