ASK BRANDON: Why Are Some Hailstones Spiky?

The Science Behind How Hailstones Are Formed


Facebook Photo: Kristen Dunn – Watertown, SD – July 11, 2017

Facebook Comment: Kristen Dunn: “I’m intrigued by the shape of the top one. Isn’t hail normally round?


Great question! In short, the answer is no. But in order to get to why, we need to break down the science of how hail is formed!

Hail can come in all shapes and sizes. The size of the hail is dependent on the strength of the storm: The stronger the storm, the larger the potential hail is. In a thunderstorm, while there are several moving parts, there are two main components: an updraft, and a downdraft. Just as it sounds, in an updraft air is moving upwards while in a downdraft air moves downwards. The stronger the storm is, the stronger the updraft is pulling air from the surface all the way into the upper atmosphere.

As the updraft becomes stronger, the thunderstorm grows taller, taking the storm higher into the atmosphere. The higher we get in the atmosphere, the colder the air gets. Inside clouds there are supercooled water droplets, which are sucked upward in the updraft to reach altitudes where temperatures are well below freezing. As the air cools, some of these water droplets freeze while others will remain in a liquid state, even though the temperatures are below freezing. After they freeze, they collide with other supercooled water droplets and sometimes other ice crystals, allowing them to grow in size.  At some point, the ice crystals and water droplets will move into the downdraft falling towards the surface.

Many of these will fall to the ground, likely melting into raindrops by the time they reach the surface. However, the others will be sucked back up into the updraft where they will go through the cycle again, for another “ride” similar to ferris wheel! As they make another trip, they will collide with more supercooled water droplets, other ice crystals, or even dust and will freeze on contact to create even larger hailstones. This cycle can be repeated several times, creating larger and larger hail. When the hail finally becomes too heavy for the updraft to support, gravity does its job and they come crashing down to the ground.

Hail is considered “severe” when it reaches one inch in diameter. That is roughly the size of a quarter. In order for one inch hail to form, the updraft speed in a thunderstorm must be around 35 to 40 mph. For golf ball sized hail (1.75”) the updraft needs to be 50+ mph and for baseball sized hail (2.75 inches), the updraft must be at least 100 mph!

As mentioned above there are a couple of ways to create hail but it breaks down into two types: riming hail & aggregation hail. Riming hail is the kind that is formed when the supercooled water droplets freeze on contact with the hailstone. This is similar to if you were to roll a snowball. The longer you roll it, the larger it gets. This is the hail that looks like a perfect sphere, similar to the second hailstone from the left in the picture below. Aggregation hail is the kind that is formed when the hailstones collide with other hailstones as it goes through the updraft. This would be as if you took several snowballs and pressed them to a much larger snowball. This creates that gnarly, spiky look. An example of this would be the first and third hailstones in the picture below.

Photo Credit: Megan Simon – Watertown, SD – July 11, 2017

DID YOU KNOW? The largest hailstone ever recorded fell right here in South Dakota? Seven years ago, on July 23 of 2010 the hailstone (pictured below) fell in Vivian, South Dakota as a very strong storm pushed through the area. The hail weighed in at nearly TWO POUNDS and measured a diameter of eight inches, which is roughly the size of a VOLLEYBALL! Initial estimates indicate that the updraft strength on that storm likely ranged between 160 to 180 mph to create that hailstone. Incredible!


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Brandon Spinner
Chief Meteorologist
Twitter: @wxSpinner89
Facebook: Meteorologist Brandon Spinner

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