Hailstorms occur anywhere thunderstorms do, but in the United States, they occur with the greatest frequency in the southern and central-plains states. "Hail alley," an area lying predominantly within the states of Texas, Oklahoma, Colorado, Kansas, Nebraska and Wyoming, is marked by the collision of warm moist air off the Gulf of Mexico and cold dry air from Canada.
Hail is most frequent in Nebraska, Wyoming and Colorado; however, severe hail, larger than three-fourths of an inch, is most frequent in Oklahoma.
Locales within hail alley average seven to nine hail days per year. The high hail rates of this area are explained by the low-lying freezing levels, the area of the atmosphere at 32 degrees Fahrenheit or less. Freezing levels in the high plains are much closer to the ground than they are at sea level, where hail has plenty of time to melt before reaching the ground.
Other parts of the world that have damaging hailstorms include Russia, western China and northern India, as well as the Alpine regions of northern Italy, the Andes mountains of South America, the mountainous regions of East Africa and New South Wales in Australia. Hail falls in paths known as hail swaths, which range in size from a few acres to 10 miles wide and 100 miles long.