Hail is precipitation composed of solid, irregular lumps of ice that fall from severe thunderstorms. Unlike sleet or freezing rain, hailstones form exclusively within the powerful updrafts of a thunderstorm cloud. The duration of any specific hailstorm is highly variable, depending on the characteristics of the parent storm system. While a hail-producing thunderstorm can last for hours, the actual time hail falls at any single location is typically much shorter. Understanding the factors that control this duration is important for assessing potential property damage.
The Average Duration of Hailfall
The experience of a hailstorm at ground level is often brief. For any single location, the time spent under falling hail usually ranges from five to fifteen minutes. This short period is often enough time for significant damage to occur, especially when large hailstones are involved.
Most individual thunderstorm cells that produce hail are short-lived, which limits the time a location spends under the storm’s hail core. Hail events lasting over thirty minutes are rare, generally associated with highly organized or slow-moving storm systems. The practical duration is the time from the first stone hitting the ground until the last, which is distinct from the total time the entire storm exists.
Storm Characteristics That Determine Length
The duration of hailfall is primarily dictated by the structure and movement of the parent thunderstorm. Storm type is a major factor. Ordinary single-cell thunderstorms produce the shortest hail events, typically lasting around thirty minutes, with hail falling in a quick burst as the storm passes over.
The most prolonged and severe hail events are linked to supercell thunderstorms. Supercells are characterized by a rotating updraft (mesocyclone) that allows the storm to sustain itself for many hours. This powerful upward current continuously feeds moisture, keeping the hail-producing mechanism active longer. This stability means the storm can produce a continuous stream of hail over a wider area, resulting in long tracks of damage called hail swaths.
The speed at which a storm travels is another determinant of hail duration. A fast-moving storm, traveling at 30 to 40 miles per hour, will drop hail over a specific point for only a few minutes before moving on. Conversely, a nearly stationary or very slow-moving storm can deposit hail over the same area for an extended period, sometimes approaching the half-hour mark. This prolonged exposure can lead to greater accumulation and more catastrophic damage.
Atmospheric conditions also sustain hail production by influencing the strength of the updraft. Strong wind shear (the change in wind speed or direction with height) helps separate the rising air (updraft) from the falling precipitation (downdraft). This separation is important because it prevents the rain and hail from falling back into and weakening the updraft prematurely. When the updraft is protected, it continues to lift water droplets high into the freezing layer, promoting hail growth and extending the duration of the event.
The Stages of a Hail-Producing Storm
All hail-producing thunderstorms follow a life cycle that sets the limits for the duration of hail production.
Developing Stage
This cycle begins as warm, moist air rises to form a towering cumulus cloud. During this initial phase, the cloud is dominated by strong updrafts, but ice particles are still growing and generally do not fall to the ground as hail.
Mature Stage
The storm transitions into the mature stage when precipitation, including hail, begins to fall from the cloud base. This stage is marked by the coexistence of strong updrafts and downdrafts (currents of air rushing downward with the precipitation). Hail growth is most vigorous during the mature stage because strong updrafts suspend large hailstones high in the cloud until they become too heavy to be supported. The length of this phase defines the maximum time a storm can produce hail.
Dissipation Stage
The final stage is dissipation, which begins as the downdraft completely overwhelms the updraft. The falling precipitation cuts off the supply of warm, moist air that fuels the storm’s growth. Once the updraft is eliminated, the hail-forming mechanism is shut down, and the storm rapidly weakens, leaving only light rain or dissipating entirely. The time from the beginning of the developing stage to the end of the dissipating stage for a single cell often lasts only about thirty minutes, which explains the brevity of many hailstorms.