Hail
A hailstone measures at least 5 millimeters in diameter to be officially classified as such. This threshold distinguishes it from ice pellets, often called sleet in American English, which are smaller and translucent. The Meteorological Service of Canada issues severe thunderstorm warnings when hail reaches this size or larger. In the United States, the National Weather Service adopted a 1-inch diameter threshold for severe hail effective January 2010. Before that date, the threshold was set at 3/4 inch. These definitions vary by country based on local sensitivity to damage. Grape-growing regions may consider smaller stones damaging compared to urban areas. A cross-section through a large hailstone reveals an onion-like structure with alternating layers of transparent and opaque ice. Transparent layers form during wet growth where latent heat keeps the exterior liquid. Opaque white layers appear during dry growth when air bubbles become trapped during rapid freezing.
Hail forms within strong updrafts inside cumulonimbus clouds that reach heights greater than 10 kilometers. These upwardly directed wind speeds can exceed 100 kilometers per hour. As water droplets rise above the freezing level, they become supercooled and freeze upon contact with condensation nuclei. The storm's updraft carries forming hailstones upward into areas with varying humidity and supercooled water concentrations. When encountering high water droplet concentration, the stone captures them to acquire a translucent layer. Moving into vapor-rich zones results in an opaque white ice layer. This process continues until the stone's mass overcomes the updraft force. The entire cycle may take at least 30 minutes depending on updraft strength. Once weight exceeds lift, the stone falls toward Earth while potentially continuing to grow through collision with other stones.
Hail occurs most frequently within continental interiors at mid-latitudes rather than tropical regions despite higher thunderstorm frequency there. Mountain ranges intensify updrafts through orographic lifting, making hail more common along elevations like northern India. One of the highest recorded death tolls occurred in 1888 across mountainous northern India. In North America, the region where Colorado, Nebraska, and Wyoming meet is known as Hail Alley. Cheyenne, Wyoming averages nine to ten hailstorms per season. South America experiences frequent storms particularly in central Argentina extending from Mendoza to Córdoba. This area sees 10 to 30 storms annually. Southern Chile receives persistent hail from mid-April through October. Colombia's cities Bogotá and Medellín see frequent events due to their elevation. Some Mediterranean countries register maximum hail frequency during the Fall season.
Weather radar serves as a primary tool for detecting hail-producing thunderstorms but requires atmospheric condition knowledge for accuracy. Modern radar scans multiple angles around a site to measure reflectivity values proportional to precipitation rates. Summing these reflectivities creates Vertically Integrated Liquid data indicating cloud water content. VIL divided by vertical storm extent yields VIL density which correlates with hail size though not always accurately. The three body scatter spike pattern appears when radar energy hits hail, deflects to ground, then returns to the radar creating weaker echoes behind the core. Polarization properties differentiate between hail and heavy rain using differential reflectivity combined with horizontal reflectivity. Visible satellite imagery shows promise despite high false alarm rates. Beginning Spring 2025 scientists launched ICECHIP, the world's largest field campaign studying hail processes. This project involves 100 scientists from four countries across two hail alley regions: the Great Plains and Colorado Wyoming Front Range.
Hail caused $46 billion in damage to cars, roofs, and crops in the United States alone during 2023 according to the Insurance Institute for Business & Home Safety. Aircraft suffer serious damage within seconds when struck by stones exceeding 2 centimeters diameter. Automobiles face severe denting and shattered windshields unless parked under cover. Wheat, corn, soybeans, and tobacco represent the most sensitive crops to hail damage. Massive stones have caused concussions or fatal head trauma to unprotected individuals. Historical records show a severe hailstorm killed more than 200 people in Moradabad district India on the 30th of April 1888. Modern fatalities remain rare; only three deaths occurred in the US since record keeping began despite billions in annual damage. Some researchers suggest a storm caused hundreds of deaths among nomads around Roopkund Uttarakhand India during the 9th century though this remains disputed.
Narrow zones where hail accumulates on ground are known as hail streaks detectable by satellite after storms pass. Depths up to one foot have been reported covering landscapes resembling snow accumulation. On the 5th of June 2015, four feet of hail fell on one city block in Denver Colorado between 10:00 pm and 11:30 pm. Tractors filled more than 30 dump truck loads clearing that single area. The largest recorded hailstone measured 7 inches in diameter with 48 centimeters circumference falling in Vivian South Dakota the 23rd of July 2010. Another record shows 6.1 inch diameter from Aurora Nebraska the 22nd of June 2003. Kericho Kenya holds the greatest average precipitation record experiencing hailstorms 50 days annually. This location reached 132 days of hail in one year due to its equatorial position and elevation. Megacryometeors remain unclassified as official hail records since they lack thunderstorm association.
During the Middle Ages European communities rang church bells and fired cannons attempting to prevent crop damage from hail. Modern versions exist today but effectiveness remains debated. Cloud seeding programs deployed silver iodide via rockets and artillery shells across the Soviet Union claiming 70 to 98 percent reduction in crop damage after World War II. These results failed replication in randomized trials conducted by Western nations. Hail suppression programs have been undertaken by fifteen countries between 1965 and 2005. Research continues into predicting accumulation events through synoptic weather patterns radar data and lightning characteristics. A joint project between University of Colorado and National Weather Service aims to build public databases for hail depth verification. Unlike diameter measurements which are common, depth data remains scarce hindering operational method development.
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Common questions
What is the minimum diameter for a hailstone to be officially classified as hail?
A hailstone measures at least 5 millimeters in diameter to be officially classified as such. This threshold distinguishes it from ice pellets, often called sleet in American English, which are smaller and translucent.
When did the National Weather Service change its severe hail threshold to 1 inch?
The United States National Weather Service adopted a 1-inch diameter threshold for severe hail effective January 2010. Before that date, the threshold was set at 3/4 inch.
Where does hail occur most frequently within continental interiors at mid-latitudes?
Hail occurs most frequently within continental interiors at mid-latitudes rather than tropical regions despite higher thunderstorm frequency there. Mountain ranges intensify updrafts through orographic lifting, making hail more common along elevations like northern India.
How much damage did hail cause to cars roofs and crops in the United States during 2023?
Hail caused $46 billion in damage to cars roofs and crops in the United States alone during 2023 according to the Insurance Institute for Business & Home Safety. Aircraft suffer serious damage within seconds when struck by stones exceeding 2 centimeters diameter.
What is the largest recorded hailstone diameter and where did it fall on the 23rd of July 2010?
The largest recorded hailstone measured 7 inches in diameter with 48 centimeters circumference falling in Vivian South Dakota the 23rd of July 2010. Another record shows 6.1 inch diameter from Aurora Nebraska the 22nd of June 2003.