Large hail is common across the United States and occurs most frequently in the Spring and Summer months. When warm, dry air moves from the Rocky Mountains over hot, moist air originating from the Gulf of Mexico, explosive thunderstorm development often occurs across the central and eastern US. As water vapor rises and condenses into towering thunderstorm clouds, the air cools and can condense into small water droplets. If the rising thunderstorm updraft grows taller still, some of that water vapor will become ice.
As the ice falls with gravity, it can get covered with the small water droplets. As the ice, called hail, is pulled by gravity and plummets to Earth, it may encounter additional strong updraft winds. These winds moving upward if strong enough, can cause the hail stone to shoot back up into the cold, frozen part of the thunderstorm. Any liquid water that clings to the stone now also becomes frozen, causing the stone to grow. This process can repeat many times over causing hail stones to grow from smaller than a pea to larger than softballs. Eventually, the stone is too heavy to be lifted by the updraft wind and is tossed down. Perhaps onto unfortunate people below.
In the United States, at least 3 deaths have been attributed directly to falling hail with the most recent being an unfortunate boater on Lake Worth, Texas in 2000. The largest stone known to have fallen in the United States was in Vivian, South Dakota in 2010. It weighed almost 2 pounds and measured 8 inches in diameter, almost as big as a regulation volleyball. But deadly hail isn’t isolated to the United States. On April 14th, 1986 the largest documented hail stones in the world fell in the Gopalganj district of Bangladesh in an area known as the Deccan Plateau. 96 people were killed in the storm, many of those believed to have been directly due to hail. The largest stone was believed to have weight at least 2.25 pounds (around 1 kg).

So just how big can hail get?

It’s not unreasonable to suggest that we most likely did not find the largest hail stone that has fallen. There’s a lot of ground and only so many folks looking! And it’s even more unlikely that we found what we did before any melting (or sublimating) could occur to shrink the stone. So, it’s entirely possible that perhaps a stone as large as 10 or 12 inches in diameter could have fallen in the United States or elsewhere and its size perhaps reaching 2.5 or 3 pounds (1.2-1.4 kg). While we have not seen such a stone to date, its not unreasonable to suggest such a thing likely existed.
So how much force would that be? First, we need to find out how fast the stone will be travelling. Taking into account the mass of the proposed stone (3 pounds), assuming it is spherical (It won’t be, but we can’t predict the shape!), and assuming common values for air density near the ground (sea level), we can predict a falling stone of these dimensions would reach its terminal, or maximum falling, velocity to be around 86 miles per hour (138 kph). Horizontal winds in thunderstorms could blow the stone much faster (indeed, a stone that becomes caught in a tornadic vortex could move many times faster!) but this is a good estimate. Assuming the object impacted is deformed by a half inch, (eg a hail stone hitting a car dents it 0.5”) the peak impact force is around 204 Newtons. A common AR-15 bullet, the 55 grain .223 round, has a muzzle velocity of around 3239 feet/s.  Assuming similar circumstances, the peak impact force for the bullet is 175 Newtons. It would be misguided to suggest the hail stone is more dangerous than the bullet. Indeed, the much smaller cross section of the bullet means the force is much more concentrated. But it certainly sheds light on just how dangerous a very large hail stone can be, certainly to deadly result.

For Wxchasing,
Meteorologist Logan Poole