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A hailstorm is an unusual weather phenomenon in which balls of ice, called hail, fall from the sky. The ice balls are nothing more than solid precipitation that will form under certain conditions.

Fast Facts about Hail

  • Hail is formed at high altitudes within massive clouds when supercooled water droplets adhere to each other and form layers of ice.
  • The average velocity of a falling hailstone is approximately 106 miles per hour (mph).

What causes a hailstorm to occur?

Several conditions are required in the atmosphere in order for hailstorms to occur. Highly developed Cumulonimbus clouds need to be present. These are the massive anvil or mushroom shaped clouds that are seen during thunderstorms which can reach heights up to 65,000 feet. There must also be strong currents of air ascending through these clouds. These currents are commonly known as updrafts. The updrafts contain ice particles, as a large number of water droplets become solid ice at the low temperatures found at high altitudes within the massive clouds. The last remaining condition is that the clouds will need to contain high concentrations of supercooled liquid water.

How does hail form?

A hailstone begins as a water droplet that is swept up by an updraft inside of a thundercloud. Inside the cloud, there are a large number of other supercooled water droplets already present. These supercooled particles will adhere to the water droplet's surface, forming layers of ice around it. As the water droplet reaches higher elevations within the cloud it comes into contact with more and more supercooled particles. This is because it is at the highest parts of the cloud, where the temperature is too low (at least 32 degrees Fahrenheit) for water molecules to remain in either a liquid or gaseous state. The hail embryo will grow larger and larger as it reaches higher altitudes in the updraft.

The hailstone will reach a size and weight where gravity will begin to act on it and pull it down. However, this is not necessarily the end of its formation, as it could be pulled into another strong updraft and remain in the upper part of the cloud. A stone the size of a golf ball would need an updraft flowing at 60 miles per hour (mph) to keep it elevated in the cloud. The size the hailstone reaches depends on the amount of time it spends surrounded by supercooled water droplets, but eventually gravity causes the stone to fall to the Earth.

During this process hailstones can become considerably large. In 2010, the largest hailstone found in the United States (Vivian, South Dakota) was 8 inches in diameter, 18.5 inches in circumference, and weighed approximately two pounds. As gravity takes over, they will fall to Earth at approximately 106 miles per hour. The exact velocity each stone falls at will vary depending on several conditions, such as weight, air friction and collisions with other suspended objects.

Hail in Film and Culture

  • In the opening scenes of "The Day after Tomorrow", a storm dropping massive hailstones causes immense amounts of damage in New York City.