Why Now Daily.

Published

Hail Size and Damage: How Storm Reports Classify It

Storm reports classify hail by the largest stone's diameter, while actual damage also depends on fall speed, horizontal wind, stone shape and density, exposure duration, impact angle and the age, material and condition of what is struck.

Timeline

  1. During the storm: Stay inside, away from windows and skylights; do not go out to collect or measure hail.
  2. After it is safe: Measure the largest representative stone across its widest dimension and photograph it with a ruler.
  3. Report context: Give time, location, largest diameter, duration, wind-driven direction and observed damage to the local weather service.

Hail forms in thunderstorms when strong updrafts carry ice embryos through regions containing supercooled liquid water. Droplets freeze onto the growing stones until they become too heavy for the updraft or are expelled from it. Strong, persistent updrafts can support larger hail, especially in supercells. Melting below the freezing level may reduce size before impact, so the stones reaching one neighborhood can differ from those detected higher in the storm. [1][2][3]

The National Weather Service reports hail by diameter, preferably measured with a ruler across the stone's widest dimension after it is safe to go outside. Common comparisons provide quick estimates: a quarter is about one inch, a golf ball about 1.75 inches and a baseball about 2.75 inches. Reports should give the largest observed stone and any damage. 'Marble size' is discouraged because marbles vary too much. [2][4][5]

In the United States, a thunderstorm meets the severe criterion when it produces hail at least one inch in diameter, damaging wind gusts of at least 58 mph or a tornado. That threshold supports warning operations; it is not a promise that smaller hail causes no damage or that every one-inch event produces the same loss. Tender crops, skylights, vehicles, aircraft and different roofing materials have different vulnerabilities. [5][6]

Impact energy generally increases as stones become larger and faster, but diameter alone is incomplete. Shape, density, melting and local airflow affect fall speed. Strong horizontal winds can drive hail at an angle, striking siding and windows and increasing relative impact speed on exposed surfaces. NOAA's hail guidance notes that wind-driven hail can damage walls, break glass and cause severe injury, even when a vertical fall-speed estimate would suggest less exposure. [1][6][7]

Roof performance depends on material, installation, slope, age, prior weathering and impact angle as well as hail size. A NOAA-reviewed assessment notes that aging and weathering of asphalt shingles can lower the size threshold at which damage occurs. Conversely, an impact-resistant product may reduce damage without making a roof invulnerable. A storm report can identify observed effects, but a photograph of hail size alone cannot diagnose hidden roof damage. [7][8]

Safety comes before measurement. During a severe thunderstorm, stay in a sturdy building and away from windows, skylights and exterior doors. Do not run outside while hail is falling to protect a vehicle or retrieve stones. When the storm and lightning threat have passed, photograph representative stones beside a ruler, note whether they melted before measurement, and record duration, wind direction, accumulation and specific damage without climbing onto a wet roof. [4][6][9]

A useful public report includes exact location, time, measured largest diameter and a plain description of damage, submitted through the method requested by the local National Weather Service office. Separate hail effects from straight-line wind when possible, because both can occur together. For property decisions, document conditions from the ground and use a qualified professional; storm-warning categories and object comparisons are communication tools, not engineering assessments or insurance determinations. [4][5][6]

Sources

  1. NOAA National Severe Storms Laboratory — Hail Basics
  2. NOAA National Severe Storms Laboratory — Hail Forecasting
  3. National Weather Service — Thunderstorm Life Cycle
  4. National Weather Service — Weather Spotter Field Guide: What to Report
  5. National Weather Service — Hail Diameter Size and Wind Descriptors
  6. National Weather Service — Severe Thunderstorm Safety
  7. NOAA National Severe Storms Laboratory — Understanding Hail in the Earth System
  8. National Weather Service Tampa Bay — Hail Hazard Impacts
  9. National Weather Service — Weather Spotter Field Guide: Safety First

Related stories