Hail is frozen precipitation, while a tornado is a violently rotating column of air extending from a thunderstorm to the ground. There is a frequent connection between the two, but hail itself is not a guarantee that a tornado is occurring or will form. Hail is a product of a powerful thunderstorm, and these storms often possess the ingredients necessary to spawn a tornado. Therefore, falling large hail serves as a warning that conditions are favorable for severe weather.
Hail Indicates Storm Severity
The presence of large hailstones (one inch in diameter or greater) is a reliable indicator of a highly organized and severe thunderstorm. The size of the hail correlates directly with the vertical wind speed inside the storm’s updraft. It takes an extremely strong upward current of air to hold a hailstone aloft long enough for it to grow before its weight overcomes the wind and it falls.
A thunderstorm capable of producing large hail has intense energy, which means it can also generate severe straight-line winds and strong rotation. The National Weather Service issues a “Severe Thunderstorm Warning” when hail of one inch or greater is expected, or wind gusts exceed 58 miles per hour. While this warning includes the potential for localized tornadoes, most storms that drop hail do not actually produce a tornado.
How Supercell Structure Produces Both Hail and Tornadoes
Both large hail and most intense tornadoes are byproducts of a specific and powerful type of thunderstorm known as a supercell. The defining characteristic of a supercell is a deep, persistent, and rotating updraft, called a mesocyclone. This rotation is introduced by wind shear—a change in wind speed and direction with height—which tilts horizontal rolling air into a vertical column.
The mesocyclone is the engine that supports both hail growth and tornado formation. The intense vertical winds within this rotating updraft keep hailstones suspended high above the freezing level, allowing them to cycle and grow layers of ice. Simultaneously, the persistent rotation lower in the storm creates the environment where a tornado can descend. The hail and the tornado are not directly dependent, but they are both manifestations of the supercell’s exceptional power.
Definitive Indicators of a Tornado
Since hail is an unreliable sign of a tornado, people should look for other, more direct indicators to confirm an imminent threat. The most definitive visual sign is a rotating, funnel-shaped cloud extending downward from the base of the storm. A wall cloud, which is a localized lowering of the cloud base underneath the main updraft, is also a strong visual indicator that a tornado may be developing.
If the funnel cloud reaches the ground, the base often becomes obscured by a swirling “debris cloud” of dirt and objects being lifted by the intense winds. Auditory signs include a continuous, loud roar, often described as similar to a freight train or jet engine. These cues, combined with a rapidly darkening or sometimes greenish sky, indicate that a tornado is either occurring or imminent, and immediate shelter should be taken.