Hail

The Underestimated Peril

Hail is one of the most underappreciated perils in catastrophe modelling — underappreciated in the sense that its aggregate annual insured losses rival those of many far more prominent perils. In the United States alone, hail causes insured losses averaging USD 10–15 billion per year, making it a larger contributor to annual average loss than earthquake for most U.S. property insurers. Yet hail receives far less attention in industry discussions than hurricanes or earthquakes, partly because individual hail events rarely produce the dramatic single-event losses associated with those perils, and partly because the science of hail modelling has matured more recently.

This relative obscurity makes hail knowledge particularly valuable for a cat modelling professional. Portfolios with significant exposure to the U.S. Great Plains, the European hail belt (Germany, Austria, Switzerland, northern Italy), and Australia's east coast carry material hail risk that must be properly quantified and managed.

The Physics of Hail Formation

Hail forms within severe convective storms — thunderstorms with strong vertical updrafts. Understanding hail formation requires understanding the atmospheric conditions that produce these storms.

Convective Available Potential Energy (CAPE)

Convective Available Potential Energy (CAPE) is the single most important atmospheric parameter for severe storm and hail development. It measures the buoyancy energy available to a parcel of air rising through the atmosphere — the greater the CAPE, the more vigorously air will rise if lifting begins. High CAPE environments produce strong updrafts capable of supporting large hailstones aloft. CAPE values above 2,000 joules per kilogram are associated with severe convective weather; values above 4,000 J/kg indicate conditions capable of producing very large hail.

CAPE develops when the lower atmosphere is warm and moist (high dewpoint temperatures) while the upper atmosphere is cold. This combination creates an unstable vertical temperature profile — warm air near the surface wants to rise because it is buoyant relative to the cold air above. The U.S. Great Plains is the global epicentre of high-CAPE environments because warm, moist air from the Gulf of Mexico regularly flows northward beneath cold, dry air descending from the Rocky Mountains, creating perfect hail-producing conditions from April through September.

The Hail Growth Process

Within a severe thunderstorm, hail forms through the following process:

1. Ice nucleus formation: Small ice particles form in the upper reaches of the storm cloud, typically at altitudes where temperatures are well below freezing.
2. Updraft suspension: Strong updrafts carry these ice particles upward and keep them suspended within the cloud. The strength of the updraft determines how large a hailstone can grow — only updrafts exceeding the terminal velocity of the hailstone can keep it aloft.
3. Supercooled water accretion: As the ice particle travels through regions of the cloud containing supercooled water droplets (liquid water below 0°C), these droplets freeze onto its surface, adding layers of ice. The stone grows outward layer by layer — like an onion — and cross-sections of large hailstones reveal these growth rings.
4. Recirculation: In the most intense storms — particularly supercells — hailstones can be recirculated multiple times through the updraft, growing with each pass. This recirculation mechanism is responsible for the very largest hailstones: the U.S. record hailstone, found in Vivian, South Dakota in 2010, measured 20cm (8 inches) in diameter and weighed nearly 1kg.
5. Fallout: Eventually the hailstone grows too heavy for the updraft to support, or is carried to the edge of the updraft, and falls to the ground. The size at which it reaches the surface depends on the updraft strength, the distance fallen, and the air temperature through which it falls (smaller stones may melt partially before reaching the ground).

Supercell Thunderstorms

The vast majority of large, damaging hail is produced by supercell thunderstorms — a distinct, rotating storm type characterised by a persistent, deep rotating updraft called a mesocyclone. Supercells are also responsible for the majority of significant tornadoes, and a supercell capable of producing large hail often poses a concurrent tornado threat. The rotation in a supercell organises the updraft and allows it to persist for hours, whereas ordinary thunderstorms have updrafts that last only 20–30 minutes before being disrupted by their own downdrafts.

Supercell formation requires not just high CAPE but also significant wind shear — a change in wind speed and direction with height. Wind shear provides the rotation that organises the updraft into a mesocyclone. The combination of high CAPE and high wind shear — both common across the U.S. Great Plains — explains why that region produces the world's most frequent and intense severe convective storms.

Hail Intensity Measures

Unlike earthquake (measured in ground acceleration) or hurricane (measured in wind speed), hail intensity is measured primarily by hailstone diameter — the maximum size of hailstones reaching the ground. The relationship between hailstone size and damage is non-linear and depends on the vulnerability of the surface being struck:

• Less than 2cm (pea-size): Generally causes minimal or no damage to well-maintained roofs
• 2–3cm (marble to quarter-size): Can dent soft metals (gutters, aluminium cladding), damage older or degraded roofing
• 3–5cm (golf ball-size): Causes significant damage to most roofing types, dents vehicles, breaks skylights
• 5–7cm (tennis ball-size): Causes severe damage to most roofing and cladding materials; structural damage to older buildings
• Greater than 7cm (baseball-size and above): Causes catastrophic damage to essentially all exposed surfaces; life-threatening to unprotected individuals

A secondary intensity parameter is hail kinetic energy — a function of both hailstone size and fall velocity. At the same diameter, a dense stone falling at high velocity causes more damage than a porous stone falling slowly. Some cat models incorporate kinetic energy alongside diameter as a dual intensity measure.

The Hail Hazard Footprint

A hail event produces a swath — an elongated strip of land along which hail falls, aligned with the storm's direction of travel. Swaths can extend from a few kilometres to several hundred kilometres in length, but are typically narrow (5–40km wide), reflecting the concentrated nature of the convective system producing the hail. This narrow swath geometry has important implications for modelling and for exposure management:

• A single hailstorm may cause severe losses across one neighbourhood while leaving adjacent areas untouched
• Multiple swaths from a single convective outbreak can affect a large region non-uniformly — some areas hit by multiple swaths, others unaffected
• The spatial variability of hail damage within a swath can be high — localised zones of very large hail (hail cores) may be surrounded by areas of smaller hail

Hail Detection and Observation Data

Quantifying hail intensity across a swath is challenging because hail observation networks are sparse — reports come from weather stations, trained storm spotters, and increasingly from crowdsourcing and smartphone apps. The primary tool for estimating hail size and areal extent is weather radar. Dual-polarisation radar — which transmits pulses in both horizontal and vertical planes — can distinguish hail from rain and estimate hailstone size from the differential reflectivity signal. The MESH (Maximum Estimated Size of Hail) algorithm converts radar reflectivity into an estimated maximum hailstone size, producing gridded hail footprints that are the primary input to many hail cat models.

Hail Vulnerability — How Buildings Are Damaged

Hail damage is primarily cosmetic and envelope-related — it affects the exterior surfaces of buildings rather than their structural systems. This distinguishes hail fundamentally from earthquake (primarily structural) and hurricane wind (envelope and structural). The key targets of hail damage are:

Roofing

Roofing is the primary driver of hail losses. The response of a roof to hail impact depends on the roofing material, its age and condition, and the size and density of hailstones:

• Asphalt shingles: The dominant residential roofing material in North America. Hail impact dislodges the mineral granule surface, exposing the underlying asphalt to UV degradation and accelerating weathering. Functional damage (actual loss of waterproofing capacity) occurs at larger hail sizes; cosmetic damage (loss of granules without functional impairment) occurs at smaller sizes. Distinguishing functional from cosmetic damage is a major insurance claims challenge.
• Metal roofing: Dents easily but retains waterproofing function unless severely deformed. Claims are primarily cosmetic — dents are visible but the roof still functions. Insurance coverage of cosmetic metal roof damage is a contested area.
• Tile roofing (clay, concrete, slate): Can crack or shatter under large hail, particularly when aged and brittle. Once cracked, tiles allow water intrusion and must be replaced.
• Built-up and single-ply membrane roofing (flat commercial roofs): Hail can puncture membranes, particularly older or degraded ones, allowing water infiltration into commercial buildings.

Vehicles

Motor vehicles are highly vulnerable to hail damage — body panels dent readily even from relatively small hailstones (2–3cm), and windshields can crack or shatter. Vehicle losses often represent 30–50% of total insured hail losses in major events. The concentration of vehicles in outdoor car parks, dealerships, and residential driveways creates significant exposure accumulation that is often poorly captured in property cat models (which focus on buildings rather than vehicles).

Solar Panels

The rapid growth of rooftop and utility-scale solar installations has created a new and growing source of hail exposure. Solar panels are designed to withstand moderate hail but can crack under large or very dense hail. The May 2023 hailstorm in Texas caused significant damage to utility-scale solar farms, highlighting an emerging exposure that was not considered in original cat model designs and is still being incorporated into updated models.

Skylights, Glazing, and HVAC

Skylights, glazed facades, and HVAC equipment on commercial rooftops are all vulnerable to hail impact. Broken skylights allow immediate water intrusion causing damage disproportionate to the cost of the skylight itself. HVAC units — particularly their aluminium fins — are readily damaged by hail, causing both direct repair costs and business interruption from loss of cooling.

The Hail Modelling Framework

Hail cat models follow the same general framework as other peril models but with several distinctive features:

Stochastic Event Generation

Hail events are generated stochastically from historical radar and observational data. The key challenge is that the historical hail record — even with modern radar networks — extends only a few decades in most regions. For the U.S., the NOAA Storm Data database provides storm reports back to the 1950s, but early records are heavily biased toward populated areas. This limits the ability to estimate long return period losses with confidence, particularly in regions with sparse historical observations.

Modern hail models use reanalysis datasets — historical recreations of atmospheric conditions using numerical weather models — to supplement the observational record. By simulating the atmospheric conditions that produce hail (CAPE, wind shear, moisture) at every point on a grid back to the 1970s or 1980s, modellers can identify hail-conducive conditions even where no observations exist.

Spatial Correlation and Multi-Event Years

Unlike hurricanes (where a single large event dominates annual losses) or earthquakes (where a single event can be catastrophic), hail losses accumulate across many events in a single year. A portfolio with exposure across the U.S. Midwest might experience 20–30 hail events per year, each causing relatively modest losses, that together constitute a significant aggregate annual loss. This means the Aggregate Exceedance Probability (AEP) curve is often more relevant than the OEP curve for hail — it is the aggregate of many events that creates the financial risk, not a single catastrophic occurrence.

Secondary Perils and Convective Outbreaks

Hail rarely occurs in isolation. Severe convective systems — particularly supercell outbreaks — produce hail, tornadoes, damaging straight-line winds, and heavy rainfall simultaneously across the same region. The April 2011 Super Outbreak produced 758 tornadoes across 21 states over three days, accompanied by widespread hail and wind damage. Cat models for severe convective storm (SCS) risk increasingly treat these perils as a combined hazard rather than modelling each independently — because the underlying atmospheric drivers are common and the losses are correlated.

Geographic Distribution of Hail Risk

Hail risk is not globally uniform — it is concentrated in regions where the atmospheric ingredients for severe convective storms regularly converge:

• United States Great Plains and Midwest: The highest frequency of large hail globally. The intersection of Gulf moisture, Rocky Mountain cold air, and strong wind shear creates ideal conditions from April–September. The corridor from Texas to Nebraska accounts for the majority of U.S. hail losses.
• European hail belt: Germany, Austria, Switzerland, northern Italy, and parts of France experience significant hail during summer months. The combination of Alpine orography (mountains triggering convection), Mediterranean moisture, and cold upper-level troughs creates conditions for severe hail. Germany alone experiences average annual hail insured losses of EUR 1–2 billion.
• Australia: The Sydney and Melbourne regions are periodically affected by severe hailstorms. The April 1999 Sydney hailstorm caused AUD 1.7 billion in insured losses — the costliest natural disaster in Australian insurance history at the time — demonstrating that hail can produce catastrophic single-event losses even outside North America.
• China, India, and South Asia: Significant hail occurs but insurance penetration is low, making the protection gap very large. As insurance markets develop in these regions, hail exposure will become an increasingly material modelling challenge.

Climate Change and Hail

The relationship between climate change and hail frequency and severity is more complex and uncertain than for flood or hurricane. Several competing mechanisms are at work:

• Higher CAPE: A warmer, moister lower atmosphere increases CAPE, providing more energy for severe convection and potentially larger hailstones
• Higher freezing levels: As the atmosphere warms, the altitude at which temperatures drop below freezing rises. Hailstones have a longer distance to fall through warm air before reaching the ground, giving more time to melt — potentially reducing surface hail size even if larger stones form higher in the atmosphere
• Changed wind shear: Climate models project reduced upper-level wind shear in some regions as the pole-to-equator temperature gradient weakens — which could suppress supercell formation even if CAPE increases

The net effect remains genuinely uncertain. Current scientific consensus suggests that while overall hail frequency may decrease slightly in some regions, the frequency of very large hail may increase — a pattern of fewer but more intense events consistent with broader trends in extreme weather.