Of all the precipitation types a pilot may encounter, hail stands apart as a uniquely violent product of the atmosphere. Unlike rain or snow, hail is born inside the most intense thunderstorms nature produces, carried aloft and grown by powerful updrafts before being hurled earthward. Understanding exactly how hailstones form, what atmospheric conditions are required, and why their presence signals a severe thunderstorm is essential knowledge for every pilot — from the private certificate candidate to the seasoned airline captain. The FAA Aviation Weather Handbook (FAA-H-8083-28B), Chapter 14, devotes specific attention to hail as a precipitation type and singles it out as one of the greatest thunderstorm hazards to aircraft, competing directly with turbulence for that grim distinction.
This article traces hail from its microscopic beginnings as a supercooled water droplet to a potentially softball-sized ice projectile, explains the thunderstorm characteristics that favor its formation, and lays out the operational rules every aviator must know to avoid a potentially catastrophic encounter.
How Precipitation Forms: The Foundation
Before examining hail specifically, it helps to understand what all precipitation requires. According to FAA-H-8083-28B, three ingredients must be present: water vapor, sufficient lift to condense that vapor into cloud droplets, and a growth process that allows those tiny droplets to become large and heavy enough to fall. A key benchmark from the handbook is that significant precipitation generally requires clouds at least 4,000 feet thick. The heavier the precipitation, the deeper the cloud is likely to be — and hail-producing thunderstorms are among the deepest cloud systems on Earth, routinely reaching 40,000 feet or higher.
Cloud droplets are extraordinarily small. An average droplet falling from a cloud base near 4,000 feet would take roughly 48 hours to reach the ground and would evaporate long before completing that journey. Two processes allow droplets to grow large enough to survive the fall: the collision-coalescence (warm rain) process, associated with warm clouds and high freezing levels common in tropical and maritime air masses, and the ice crystal process, dominant in mid- and high-latitude clouds where both supercooled liquid water and ice crystals coexist. Hail formation is an extreme extension of the ice crystal process, turbocharged by the violent dynamics of a severe thunderstorm.
How Hailstones Form
Hail is defined by FAA-H-8083-28B as precipitation in the form of balls or other irregular lumps of ice produced by thunderstorms. The process begins above the freezing level, where supercooled liquid water droplets — water that remains liquid even though it is colder than 0 °C — exist in abundance. When a droplet finally freezes, it acts as a nucleus. Additional supercooled droplets collide with it, latch on, and freeze in turn, causing the hailstone to grow layer by layer, much like an onion. Cut a large hailstone in half and you will often see concentric rings of clear and opaque ice, each ring representing a cycle of growth under different temperature and moisture conditions.
The key to growing large hailstones is the thunderstorm's updraft. In a severe thunderstorm, updrafts are extremely strong, capable of suspending hailstones aloft far longer than in an ordinary storm. This powerful column of rising air suspends hailstones aloft, giving them more time to accumulate additional layers of ice. The handbook identifies four thunderstorm characteristics that are especially favorable to hail formation:
- Strong updrafts — keep the hailstone suspended and recirculating through regions of supercooled water.
- Large Supercooled Liquid Water Content (SLWC) — provides the raw material for each new ice layer.
- Large cloud-drop sizes — larger individual droplets contribute more mass with each collision.
- Great vertical height — a taller storm provides a longer updraft column and more time for growth.
Eventually, the hailstone grows too massive for even the strongest updraft to support, and it falls. If temperatures between the cloud base and the ground are above 0 °C, the outer layers begin to melt. Small hailstones may melt entirely into raindrops before reaching the surface; larger stones survive the descent and strike the ground intact.
Why Hail Signals Severe Thunderstorms
The connection between hail and storm severity is not coincidental — it is structural. Ordinary air-mass thunderstorms produce modest updrafts and limited cloud depth. They rarely generate hail, and when they do, the stones are small and melt before reaching the surface. Large hail, particularly hailstones 0.75 inches in diameter or larger, is one of the official NWS criteria for a severe thunderstorm precisely because producing stones of that size requires updraft velocities and storm depths that are extraordinary by any measure.
The FAA handbook notes that hailstones can range from pea-sized (0.25 inches) to larger than a softball (4.5 inches). As a real-world reference point outside the handbook, the National Weather Service's record U.S. hailstone, collected at Vivian, South Dakota on July 23, 2010, measured 8 inches in diameter, 18.62 inches in circumference, and weighed 1.93 pounds. While such extremes are rare, even modest hailstones — a marble or golf ball in size — can inflict serious structural damage on an aircraft in seconds.
The Aviation Hazard: What Hail Does to Aircraft
The handbook states bluntly that hail competes with turbulence as the greatest thunderstorm hazard to aircraft. Hailstones 0.75 inches in diameter and larger can cause significant damage and make an aircraft difficult to control. The specific threats include:
- Windshield damage or penetration, which can incapacitate the crew.
- Deformation of leading edges on wings, horizontal stabilizers, and propeller blades, degrading aerodynamic performance.
- Engine ingestion, which can cause compressor stalls, surges, or complete engine failure on turbine-powered aircraft.
- Loss of pitot-static accuracy if pitot tubes are damaged or blocked.
- Structural damage to radomes, antennas, and airframe skin.
A critical operational point: rain at the surface does not mean the absence of hail aloft. Because smaller hailstones melt as they descend through above-freezing air, the surface observation may show only rain while large hail exists at altitude. Pilots who use surface weather reports as their sole indicator of hail risk are making a dangerous assumption.
Hail Beyond the Storm: Swaths and Clear-Air Encounters
One of the most counterintuitive and dangerous aspects of hail is that it can fall in clear air several miles from the visible thunderstorm. The storm's anvil — the characteristic flat, spreading top of a mature cumulonimbus — acts as a launch ramp for hailstones ejected by the updraft. Pilots flying what appears to be a safe distance from a storm, particularly beneath the anvil, can suddenly be struck by hail without warning. The FAA handbook specifically warns pilots to anticipate possible hail with any thunderstorm, especially beneath the anvil of a large cumulonimbus.
Hail also falls in organized patterns called hail swaths — corridors of hail damage on the surface that can range from a few acres to large areas covering many square miles. Accumulations can be deep enough to require snowplows for removal, and hail drifts have been reported. This underscores the enormous quantity of ice a single severe thunderstorm can produce.
Geographic and Altitude Patterns
In the United States, hail is most common across the Great Plains region east of the Rocky Mountains — the area sometimes called Hail Alley — where dry air from the Rockies interacts with moist Gulf air and strong wind shear to produce supercell thunderstorms. Globally, hail is most frequent in the interior of continents within the mid-latitudes and at higher elevations within the tropics. Hail is also more common aloft than at the surface for the same reason that rain may replace it below the freezing level: smaller stones melt on the way down.
Key Numbers and Rules
- Significant precipitation requires clouds at least 4,000 feet thick.
- Hailstone diameter ranges from 0.25 inches (pea) to over 4.5 inches (softball).
- Hailstones 0.75 inches or larger can cause significant aircraft damage and control difficulty.
- Hail can be encountered in clear air several miles from a thunderstorm, especially under the anvil.
- Hail swaths can cover large areas on the surface, sometimes requiring snowplows for removal.
- Rain at the surface does not rule out hail aloft — smaller stones melt before reaching the ground.
- As a real-world reference (not from the FAA handbook), the record U.S. hailstone (Vivian, SD, July 23, 2010) was 8 inches in diameter and weighed 1.93 pounds.