The question of whether large hail signals an approaching tornado is a common public concern regarding severe weather. While large hailstones indicate an extremely powerful thunderstorm, their presence does not guarantee that a tornado is occurring or will form. Both phenomena originate from the same type of violent storm structure, making large hail an urgent warning sign that conditions are favorable for a tornado.
Defining Large Hail and Tornado Indicators
Meteorologists define severe hail as any hailstone with a diameter of 1 inch or greater, roughly the size of a quarter. Hail this size indicates a powerful updraft within the storm cloud. Updrafts are columns of rapidly rising air that suspend water droplets high into the atmosphere, allowing them to freeze and grow into hailstones.
The larger the hailstone, the faster the air must rush upward to keep it aloft, sometimes reaching speeds over 100 miles per hour. This strong vertical movement is required for tornado formation, as it provides the energy to organize rotation. When large hail falls, it signals that the parent thunderstorm is intense enough to potentially produce a tornado. Weather services issue a severe thunderstorm warning for large hail, which requires immediate caution.
The Supercell Structure: The Common Link
The common origin for both large hail and the most destructive tornadoes is a supercell thunderstorm. A supercell is characterized by a persistent, rotating updraft, called a mesocyclone. This rotation is introduced by wind shear, where wind speed and direction change significantly at different altitudes, creating a horizontal spin that the storm’s updraft tilts vertically.
This rotating column of air makes the supercell long-lived and powerful, sustaining both severe hailstones and tornadoes. The powerful updraft continuously cycles hailstones through the storm’s cold upper regions, allowing them to grow large before their weight overcomes the upward force. This rotational energy also sets the stage for a tornado to descend to the ground. Nearly all of the strongest tornadoes develop within a supercell.
Why Hail Does Not Always Mean a Tornado
Despite the shared environment, most supercell thunderstorms that produce large hail do not produce a tornado. The distinction between a severe hailstorm and a tornadic storm lies in the specific atmospheric conditions present in the lowest 500 meters. Although the storm may have a strong rotating updraft aloft, a tornado requires the rotation to tighten and extend all the way to the ground.
This process, known as tornadogenesis, depends on the balance of low-level wind shear and the air flowing around the storm’s base. If the storm’s rear flank downdraft (RFD)—a surge of rain-cooled air—undercuts the updraft, it disrupts the flow of warm, moist air needed to sustain ground-level rotation. This cold air chokes the storm’s inflow, preventing tornado formation, even while large hail falls. Only about 25 to 30 percent of supercells produce a tornado.
Actionable Safety Measures
Since large hail confirms the presence of a powerful, potentially tornadic storm structure, immediate action is necessary. If you observe large hail, seek shelter in a sturdy building and move to an interior room away from windows and glass doors. Closing blinds and curtains offers protection from potential broken glass if strong winds or hail break a window.
If driving and unable to reach a secure building, pull over safely and cover your head and neck with your arms or a blanket. Your vehicle offers reasonable protection, but the windshield is typically the strongest glass, so angle the car to minimize impact on side windows. Monitor local weather alerts, listening for a specific Tornado Warning, which means a tornado has been sighted or indicated by radar.