The idea that hail is a guaranteed warning sign for an approaching tornado is a common misconception. Hail does not always mean a tornado is coming, though the two phenomena frequently occur within the same powerful storm systems. Hail indicates a thunderstorm has reached a severe level, resulting from vertical motion and temperature. A tornado, however, requires rotation. Understanding the separate physical mechanisms that create hail and tornadoes clarifies why one can exist without the other.
Hail Does Not Guarantee a Tornado
While hail confirms a storm is intense, it is not a direct predictor of a tornado’s formation. Hail is a common feature of nearly all strong thunderstorms, but only a small percentage of these storms ever produce a tornado. The vast majority of hailstorms pass without rotation reaching the ground. The belief linking the two exists because both require a highly energetic and organized thunderstorm. However, they rely on distinct atmospheric dynamics. The presence of hail merely signals the storm possesses the extreme vertical lift needed to suspend ice particles, which is a necessary, but insufficient, condition for a tornado.
The Physics of Hail Formation
Hail formation is driven by strong vertical air currents, known as updrafts, within a thunderstorm. Water droplets are carried upward by these powerful updrafts into the extremely cold regions of the cloud, well above the freezing level. The droplets freeze and become ice embryos. These initial ice particles then grow by colliding with and collecting supercooled water droplets. The stronger the updraft, the longer the hailstone can be suspended in the cloud, allowing it to collect more ice and grow larger.
When the weight of the hailstone finally overcomes the force of the updraft, it falls to the Earth. A relatively low height of the atmospheric freezing level also favors hail formation. A lower freezing level gives the hailstones a greater vertical distance to grow before they are too heavy for the updraft to support. The intensity of the updraft, which can sometimes reach speeds exceeding 100 miles per hour, is the primary control on the ultimate size of the hailstone.
The Mechanics of Tornado Development
Tornado development depends on a specific sequence of rotational mechanics, not just vertical lift. The process begins with wind shear, which is a significant change in wind speed or direction with increasing height. This wind shear creates a horizontal, spinning tube of air in the lower atmosphere. As a strong thunderstorm develops, its powerful updraft encounters this horizontally rotating air. The updraft lifts and tilts the spinning tube from a horizontal to a vertical orientation.
This process creates a column of rotating air within the storm, known as a mesocyclone, which is the signature of a supercell thunderstorm. The mesocyclone represents the deep, persistent rotation necessary for a tornado. The actual tornado forms when this vertical rotation tightens and stretches closer to the ground. This process dramatically increases the wind speed due to the conservation of angular momentum. While the mesocyclone is a strong indicator, only about 20% of supercells that contain one will produce a tornado.
When Hail and Tornadoes Coincide
Hail and tornadoes are often observed together because both are products of the supercell, the most powerful and long-lived type of thunderstorm. A supercell is defined by its persistent, rotating updraft, or mesocyclone, which provides the unique environment for both phenomena. The strong vertical lift of the updraft supports the growth of very large hail, while the rotation is the precursor to a tornado.
Large hail, particularly stones larger than one inch in diameter, signals that the supercell’s updraft is robust and well-organized. This organized structure, fueled by strong wind shear, has the potential to spin up a tornado. However, the hail often falls to the northeast of the main updraft, while the tornado forms near the center of rotation. The presence of hail should be viewed as confirmation that a severe, potentially tornadic, storm is overhead. It is a warning to seek shelter because the storm possesses the intense structure and energy capable of producing violent weather.