Hailstorms are most common in the warm spring and summer months. When thunderstorms and tornadoes increase, hailstorms usually follow close behind.
Cumulonimbus clouds — those tall, threatening clouds often called "thunderclouds" — are where hailstones begin to form. The temperatures high up in these clouds can be quite cold, causing moisture in the air to change into ice crystals. These ice crystals — sometimes called "soft hail" or "snow pellets" — are called "graupel" by meteorologists.
As graupel forms, the ice crystals become heavier and begin to fall downward. When weather conditions are right for hail, very strong winds — called "updrafts" — will push the graupel back up into the upper parts of the thunderclouds.
As the graupel ascends back into the upper parts of the thunderclouds, it accumulates more moisture and gets bigger. This process is called "accretion." As layers of ice are added, the graupel turns into hailstones that get bigger and again begin to fall toward the Earth.
When the hailstones grow big enough to overcome the power of the updrafts, they will fall to Earth as hail. How big hailstones get depends largely on how strong the winds are.
Updrafts of 20 miles per hour usually result in pea-sized hail (up to 0.5 inches in diameter), while updrafts of 40 miles per hour can make hailstones the size of quarters (up to 0.75 inches in diameter).
Very large hailstones require incredibly strong updrafts. Hail the size of a golf ball (up to 1.75 inches in diameter) needs updrafts of about 55 miles per hour. Softball-sized hail (4 or more inches in diameter) only forms when updrafts exceed 100 miles per hour.
The largest hailstone ever recorded in the United States fell in South Dakota on July 23, 2010. It weighed 1.93 pounds and measured 8 inches in diameter and 18.62 inches in circumference! Can you imagine the damage that hailstone could have caused?
Hailstorms usually don't last long — only about 5 to 10 minutes — but they can cause a lot of damage in that time. In addition to damage caused to automobiles, airplanes, skylights, and roofs, hail also regularly destroys farmers' crops.
Hailstorms are most common in the Great Plains states, especially northern Colorado and southern Wyoming. If a hailstorm breaks out where you live, get inside as quickly as possible and stay away from windows.