Hail Facts

Have you ever observed a thunderstorm that included hail? Maybe you've even been caught outside in a storm with hail. Aside from its basic appearance, did you ever take the time to consider anything else about hail? Here are a few facts about this memorable weather phenomenon.

Appearance

Hail can range from the size of a pea to the size of a grapefruit or larger. The largest hailstone recorded had a diameter of 8.7 inches and a circumference of 18.75 inches. The heaviest hailstone recorded weighed 1.65 pounds. Typically a hailstone has a few concentric layers. They can either be milky or clear. The milky color comes from tiny bubbles that are trapped in the hail.

When the atmospheric temperature is low and the supercooled cloud droplets are small, the droplets will freeze as soon as they hit the hailstone. When the atmosphere temperature is higher but still below zero degrees Celsius and the average drop size is larger, the cloud has a higher liquid water content and a clear layer will form. The rate that drops freeze is dependent on their size. When drops freeze at a slow rate, the trapped air will escape. On some occasions, hailstones can shed water above the freezing level. This occurs because the intrinsic heat that is released when the water is frozen produces a hailstone that is warmer than its surrounding environment.

When a hailstone is lifted into the air multiple times by the updrafts of a storm, it will have multiple layers. However, this is not always an essential component for the creation of larger hailstones.

Fall Speed

Hail's terminal velocity (m/s) is expressed as aDb. D stands for the diameter of the hailstone (mm). The letters a and b are factual constants. The constant b has a range of 0.5 to 1. For example, a hailstone that is 10 mm in diameter falls at approximately 9 m/s.

Occurrence

The occurrence for the most frequent hail is in Kericho, Kenya, which once had 132 days with hail in a single year. However, Kericho hardly ever has occurrence of large hail. Large hail measuring 2 cm or more in diameter occurs most frequently in the central Great Plains area of the United States. This is a result of the severe thunderstorms, particularly the supercells, that occur in this area on a regular basis. The area receives 7 to 9 days with hail annually.

The factors for regular hail occurrence include a significant uplift of easterly flow over a terrain, small distance between the ground and the freezing level and convection that is driven by thermal components. All of these factors will decrease the possibility of the hail melting.

Hail Damage

Typically hail damage occurs when there are intense, sporadic storms. During the past 60 years, there has been a consistent upward trend for hail damage in the United States. Throughout this time period, there have been some fluctuations, with peaks occurring in both the mid 1970s and mid 1990s.

Each year, around 24 people suffer from hail injuries in the United States. It is very rare that hail ever causes a fatality. Typically hail damages agricultural crops, not people. The damage that it does to these crops can cost hundreds of millions of dollars every year.

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