Hail

• 1. Introduction

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  Hail killed more than 200 people in the Moradabad district of India on the 30th of April 1888. It is one of the highest hail-related death tolls on record, and it came from the sky in the form of balls and irregular lumps of ice. Each one is called a hailstone. This is a kind of solid atmospheric precipitation, born inside the towering clouds of thunderstorms. It is often confused with ice pellets, which Americans call sleet, but the two are not the same. Ice pellets generally fall in cold weather. Hail does something stranger: it grows during storms, layer by layer, while warm air rises hard beneath it. How does ice this large form when the surface is not even cold? Why does it cluster in certain corners of the planet and almost nowhere else? And what does it take to study something that falls from a cloud for only minutes at a time?

• 2. Onion Of Ice

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  A cross-section through a large hailstone shows an onion-like structure. Thick, translucent layers alternate with layers that are thin, white, and opaque, each at least 1 mm thick. Like other precipitation in cumulonimbus clouds, hail begins as water droplets. As those droplets rise and the temperature drops below freezing, they become supercooled water. They freeze on contact with condensation nuclei. The storm's updraft, with upwardly directed wind speeds as high as 110 mph, drives the forming hailstones up through the cloud. As a hailstone climbs, it passes through regions where humidity and supercooled water droplets vary. When it enters an area dense with water droplets, it captures them and gains a translucent layer. When it moves into a region of mostly water vapor, it picks up a layer of opaque white ice. The mode of growth itself shifts. In wet growth, the hailstone releases latent heat that keeps its outer layer liquid and sticky, so it can collide with smaller hailstones and clump into an irregular shape. In dry growth, that heat is not enough, and small air bubbles get trapped during rapid freezing, leaving the stone opaque. An older theory held that hailstones rode up and down repeatedly, refreezing on each ascent to build their layers. New research, drawing on theory and field study, has shown that a single trajectory through the storm is enough to explain the layered structure. The only real exception is a multicellular thunderstorm, where a stone may be ejected from a mother cell and captured by a more intense daughter cell.

• 3. What Makes A Hailstorm

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  Stronger updrafts make bigger hail. The more powerful the rising air in a storm, the longer it can hold a heavy stone aloft, which is why supercells produce far larger hailstones than weaker storms. A hailstone keeps rising until its mass can no longer be supported by the updraft, a process that may take at least 30 minutes in a storm whose top usually exceeds 10 km in height. Hail forms best where the freezing level sits below an altitude of 11000 ft. Dry air moving into a strong continental thunderstorm promotes evaporational cooling, which lowers the freezing level and gives hail a larger volume in which to grow. Growth becomes vanishingly small once air temperatures fall below -30 C, because supercooled water droplets become rare there. The largest hailstones tend to form some distance from the strongest updraft, where they have more time to grow. Within the cloud, hail is most likely at elevations above 20000 ft. Between 10000 ft and 20000 ft, 60% of hail remains inside the thunderstorm while 40% drifts into the clear air under the anvil. Below 10000 ft, it spreads evenly through and around the storm out to a distance of 2 nmi.

• 4. Where Hail Falls

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  Cheyenne, Wyoming is North America's most hail-prone city, averaging nine to ten hailstorms per season. It sits inside Hail Alley, the area where Colorado, Nebraska, and Wyoming meet, where hail falls between March and October during the afternoon and evening, mostly from May through September. Mountains explain a great deal of this geography. They force horizontal winds upward in a process called orographic lifting, which intensifies updrafts and makes hail more likely. Higher elevations also leave less time for stones to melt before reaching the ground. Mountainous northern India is one of the more common regions for large hail. The central region of Argentina, stretching from Mendoza eastward toward Cordoba, experiences some of the most frequent hailstorms in the world, with 10-30 storms per year on average. China, central Europe, and southern Australia all see significant hail, as do Croatia and Serbia in southeastern Europe. In Colombia, the high-elevation cities of Bogota and Medellin see frequent hailstorms. The high frequency across northern Argentina and its neighbors is tied to orographic forcing, moisture transport from the Amazon, and instability from temperature contrasts between the surface and the upper atmosphere. To the north of Hail Alley lies Alberta's Hailstorm Alley, just downwind of the Rocky Mountains.

• 5. Records And Hazards

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  The heaviest hailstone on record weighed 1.02 kg and fell in Gopalganj District, Bangladesh, on the 14th of April 1986. The largest diameter officially measured was 7.9 in, in Vivian, South Dakota, on the 23rd of July 2010, while the largest circumference, 18.74 in, was recorded in Aurora, Nebraska, on the 22nd of June 2003. For sheer frequency, nowhere rivals Kericho, Kenya, which averages hailstorms on 50 days a year and once reached a world record of 132 days of hail in a single year, helped by its elevation of 7,200 ft near the equator. These stones do real harm. In 2023, hailstorms cost the United States $46 billion in damage to cars, roofs, and crops, according to the Insurance Institute for Business & Home Safety. Wheat, corn, soybeans, and tobacco are the crops most sensitive to hail. Aircraft face serious danger too, since planes can be damaged within seconds once hailstones exceed 0.5 in in diameter. Massive stones have caused concussions and fatal head trauma to people caught outside. Some researchers have suggested a hailstorm killed several hundred nomads around the 9th century at Roopkund, Uttarakhand, India, though this is disputed. Deaths are rare in the modern era. In the United States, only three people are known to have been struck and killed by hail since modern records began.

• 6. Reading The Storm

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  Modern radar scans many angles around a site, and the reflectivity at each level is proportional to the precipitation rate there. Summing those values gives the Vertically Integrated Liquid, or VIL, which measures the liquid water in a cloud. VIL divided by the storm's vertical extent, known as VIL density, has a relationship with hail size, though it varies with conditions and is not highly accurate. Certain patterns in the data give the game away. The three-body scatter spike forms when radar energy strikes hail, deflects to the ground, bounces back to the hail, and only then returns to the radar. That detour adds time, so the echo appears farther from the radar than the hail truly is, drawing a cone of weaker reflectivities. More recently, the polarization properties of radar returns have been used to tell hail from heavy rain, and differential reflectivity combined with horizontal reflectivity has produced several hail classification algorithms. Visible satellite imagery is beginning to detect hail, but false alarm rates remain high. On the ground, size is best measured with a ruler. In its absence, people compare stones to coins, though comparisons to hen's eggs, peas, and marbles are imprecise. Terminal velocity rises with size: a 1 cm stone falls at about 9 m/s, while an 8 cm stone reaches 48 m/s. Because hailstones are not perfect spheres, their drag coefficient, and therefore their speed, is hard to calculate.

• 7. Deep Accumulations

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  On the 5th of June 2015, hail up to four feet deep fell on a single city block in Denver, Colorado. The stones, described as between the size of bumble bees and ping pong balls, came with rain and high winds and fell only in that one spot, leaving the surrounding area untouched. It lasted one and a half hours, between 10:00 pm and 11:30 pm. Tractors brought in to clear it filled more than 30 dump truck loads. A National Weather Service meteorologist in Boulder called it a very interesting phenomenon, saying, "We saw the storm stall. It produced copious amounts of hail in one small area. It's a meteorological thing." Such depth is rare because it requires a storm to become nearly stationary while producing hail, something that tends to happen in mountainous areas. On the 29th of July 2010, a foot of hail accumulated in Boulder County, Colorado. Narrow zones where hail piles up are called hail streaks or hail swaths, detectable by satellite after a storm passes. Depths of up to 18 in have been reported, and accumulating hail behaves much like snow, restricting transport and infrastructure over a smaller area. It can also block drains and cause flooding, turning into a snow-like slush carried downhill in floodwater. One stubborn obstacle to research is that hail depth, unlike diameter, is not commonly reported, leaving forecasters short of data to verify their methods.

• 8. Fighting The Sky

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  During the Middle Ages, people across Europe rang church bells and fired cannons to ward off hail and protect their crops. Updated versions of that idea survive today as modern hail cannons. After World War II, cloud seeding was tried to remove the hail threat, especially across the Soviet Union, where deploying silver iodide into clouds with rockets and artillery shells was claimed to cut crop damage by 70-98%. Those results were never reproduced in randomized trials run in the West. Between 1965 and 2005, hail suppression programs were undertaken by 15 countries. The science itself is still moving forward. Beginning in the spring of 2025, scientists in the United States launched a field study called the In-situ Collaborative Experiment for Collection of Hail In the Plains, or ICECHIP. Working across two regions known as hail alleys, the Great Plains and the Front Range of the Rocky Mountains in Colorado and Wyoming, it is the world's largest field campaign ever devoted to hail. It brings together 100 scientists from four countries and 11 states, all chasing the questions about hail formation that the layered onion of ice has not yet given up.