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Topics : Storms

Hail

The pelting chunks of ice known as hail take shape several miles above ground inside strong thunderstorms. Here, the updrafts can reach speeds of more than 110 miles per hour (176 kilometers per hour).

Even though temperatures at this height are below freezing, most of the cloud's moisture is supercooled. This means it remains liquid because it lacks a surface on which to freeze. As soon as a supercooled droplet collides with potential nuclei—ice crystals, frozen raindrops, dust, or salt—it freezes quickly.

When many supercooled droplets are present, they coat the hailstone and then freeze into a clear layer of ice. If there are fewer supercooled droplets, they freeze on the growing mass individually, trapping small air bubbles in a patchwork that makes this part of the hailstone opaque. These differing rates of accumulation and freezing lead to the onion-like layers often visible in cross sections of large hailstones.

Smaller hail tends to fall in large batches near the heart of a storm. Larger stones, which are fewer in number, typically get centrifuged toward the edge of a storm. The biggest hailstones are aggregates of smaller stones, which gives them a spiky, irregular shape.


A thunderstorm that pummeled Aurora, Nebraska, on June 22, 2003, left several grapefruit-sized hailstones like this one in its wake, including a new record holder.

NCAR scientist Nancy Knight, who has studied hail for over 30 years, has analyzed the hailstones vying for a place in U.S. record books. For many years, the largest known U.S. hailstone was one captured in Coffeyville, Kansas, on September 3, 1970. A new size champion, fell in Aurora, Nebraska, on June 23, 2003. It has a diameter of 7 inches (17.8 centimeters) and a circumference of 18.75 inches (47.63 cm). However, the Coffeyville stone remains the heaviest U.S. hailstone in the official record, at 1.65 pounds (0.75 kilograms).

Threats to life and property

Only a few people have been killed by hail in recent decades in North America, but a number are injured each year. Many more have died in China and other countries in areas where shelter from intense hailstorms is unavailable or inadequate.

Hail's biggest property threat is to automobiles and crops. A single round of large hail can ruin thousands of cars. Windblown hail can slice corn and other plants to ribbons. Small hailstones are actually the most dangerous type for some crops, because they tend to fall more densely and the wind blows them across plants more readily.

Hail swaths can affect areas up to 10 miles (16 km) wide and 100 miles (160 km) long. Many hailstorms are quite localized, affecting a much smaller area.

Prevention and detection

In medieval times, Europeans tried to ward off hail by ringing church bells and firing cannons. After World War II, scientists across the world began to experiment with cloud seeding to combat hail.

Today, some farmers and state or local governments arrange for aircraft to deposit water-attracting particles into storms. The idea is to form more droplets and disperse the moisture that would otherwise congeal into hailstones. Despite decades of research at NCAR and elsewhere, the jury is still out on whether this technique works. A 2003 report from the National Academy of Sciences summarizes the evidence for and against weather modification.

Some types of weather radar can detect hail as it falls. NCAR has built a dual-polarization radar called S-Pol that sends out two kinds of signals, one aligned horizontally and the other vertically. This allows the radar to distinguish the flattened, hamburger-bun shape of large raindrops from the more spherical signal produce by a tumbling hailstone.

Based on research at NCAR and elsewhere, the National Weather Service plans to add dual polarization to its network of Doppler radars over the next few years.

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