Hail is a form of solid precipitation that falls as lumps of ice, created by strong updrafts within severe thunderstorms. The National Weather Service (NWS) officially uses one inch as the minimum threshold for hail to be classified as “severe.” Knowing this size is important, as it signals a danger to both people and property, prompting the issuance of Severe Thunderstorm Warnings.
Visualizing the 1-Inch Standard
A one-inch hailstone is most commonly compared to the diameter of a U.S. quarter coin. Although a quarter measures slightly less than a full inch, it provides the public with a universally recognizable object for quick visual reference. This size represents a significant change in potential impact, as smaller hail, such as dime or nickel size, typically causes minimal damage.
Hailstones are formed by layers of ice and liquid water freezing onto a nucleus, often resulting in a layered internal structure. The appearance of a one-inch hailstone can vary, sometimes forming a smooth, spherical shape, or more often taking on an irregular, lumpy form. Its texture may be opaque or have a milky white appearance due to tiny air bubbles trapped within the ice layers.
Reporting and Measuring Hail Size
Accurately determining the diameter of hail is crucial for official reporting and insurance claims. The most precise method involves using a standard ruler or measuring tape to find the largest dimension of the stone. This physical measurement is significantly more reliable than a visual estimate.
Trained weather spotters and meteorologists utilize specialized tools to document the size and concentration of hailstones. When reporting to the NWS, the protocol is to record the diameter of the single largest stone found, even if most of the hail was smaller. Reporting the size in inches or using the quarter comparison helps ensure consistency and accuracy.
The public uses the quarter analogy for immediate estimation to determine if the event qualifies as severe. For official verification of a Severe Thunderstorm Warning, the report must confirm hail one inch or greater. This systematic documentation provides meteorologists with valuable data for storm analysis and future warning improvements.
Damage Potential of 1-Inch Hail
The designation of one-inch hail as “severe” is based on its capability to cause measurable damage to property. Hailstones of this size fall with enough force to create small dents in the sheet metal of vehicles. The impact force is sufficient to bruise the underlayment beneath asphalt shingles, leading to the dislodging of protective granules.
Older or weaker asphalt shingles are particularly susceptible to this damage, which can shorten the lifespan of the roof and potentially lead to leaks. One-inch hail can also damage other exterior elements, such as denting gutters and downspouts or causing minor damage to vinyl siding. For vegetation, this size hail can shred leaves and break small, tender tree branches.
One-inch hail poses a safety risk to any person or pet caught outside during the storm. While typically not life-threatening, the size and speed of the ice can cause painful bruising or minor injury upon impact. This potential for harm to both life and property is why the one-inch diameter serves as the official metric for a severe weather threat.