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What Is Hail?
Hail is a large frozen raindrop produced by intense thunderstorms where snow and rain coexist. As the snowflakes fall, liquid water freezes onto them forming ice pellets that will continue to grow as more and more droplets accumulate. Upon reaching the bottom of the cloud, some of the ice pellets are carried by the updraft back up to the top of the storm. As the ice pellets once again fall through the cloud, another layer of ice is added and the hail stone grows even larger.
Typically the stronger the updraft, the more times a hail stone repeats this cycle and consequently, the larger it grows. Once the hail stone becomes too heavy to be supported by the updraft, it falls out of the cloud toward the surface. The hail stone reaches the ground as ice since it is not in the warm air below the thunderstorm long enough to melt before reaching the ground.
Where Does Hail Occur?
Hailstorms are most frequent in the southern and central plains states, where warm moist air off of the Gulf of Mexico and cold dry air from Canada collide, thereby spawning violent thunderstorms. This region, known as hail alley, lies predominantly within the states of Texas, Oklahoma, Colorado, Kansas, Nebraska and Wyoming. While their domain of greatest frequency is in the plains states, hailstorms have been observed just about everywhere thunderstorms occur.
The Dangers of Hail
Hail might seem like a minor concern, but it can shatter windows, leave pockmarks in siding and destroy or damage roof coverings. In an average year, hail causes more than $1.6 billion worth of damage to residential roofs in the United States, making it year in and year out, one of the most costly natural disasters. The costliest hailstorm in the United States took place in Denver in July 1990 with damage of $625 million.
The Power of Hail
The combination of gravity and a downward wind known as a downburst (a common occurrence during severe thunderstorms) can propel a hailstone at speeds upwards of 90 mph. At such excessive speeds, large hailstones have been known to penetrate straight through roof coverings and the deck to which they are attached. Although the majority of hailstorms are not quite so severe, even moderate hailstorms can damage buildings, automobiles, crops and other personal property.
The Largest Hailstone
How large can hail get? Fortunately, most hail is small -- usually less than two inches in diameter. The largest hailstone ever recorded fell in Coffeyville, Kansas on September 3, 1970. It measured about 17.5 inches in circumference (over 5.6 inches in diameter) and weighed more than 26 ounces (almost 2 pounds)!