Summer skies can unleash surprising fury. While we often associate summer storms with heavy rain and dramatic lightning, sometimes they deliver something harder: ice. Hailstones, ranging from pea-sized pellets to grapefruit-sized monsters, seem out of place during the warmest months. How does frozen precipitation form and fall when the ground temperature is high? The answer lies high above us, within the turbulent heart of powerful thunderstorms.
The Thunderstorm Engine: Setting the Stage
Hail doesn’t just happen; it requires a specific type of storm – typically a cumulonimbus cloud, the towering giants often called thunderheads. These clouds are immense vertical structures, sometimes reaching altitudes of over 50,000 feet, well into the freezing levels of the atmosphere. The key ingredient for hail formation within these clouds is a powerful updraft. This is a strong current of warm, moist air rising rapidly from the ground up into the cloud.
Think of the updraft as the storm’s engine and elevator. It carries water vapor high into the atmosphere where temperatures are well below freezing (0°C or 32°F). Alongside the updraft, thunderstorms also feature downdrafts – currents of cool air and precipitation falling towards the ground. The intense circulation pattern created by these opposing air currents is the volatile environment where hail is born and nurtured.
The Birthplace: Supercooled Water and Ice Nuclei
High up in the storm cloud, even where temperatures are far below freezing, water doesn’t always instantly turn to solid ice. Liquid water existing below the freezing point is called supercooled water. It’s in a delicate, unstable state, ready to freeze upon contact with something solid.
This ‘something solid’ comes in the form of tiny particles called ice nuclei or freezing nuclei. These can be specks of dust, pollen, salt, or even bacteria swept up into the cloud by the updraft. When a supercooled water droplet collides with an ice nucleus, it freezes almost instantly, forming a tiny ice crystal or a frozen droplet. This minuscule piece of ice is the embryo of a hailstone.
Growing Up Fast: The Role of Accretion and Updrafts
Once an ice embryo forms, the powerful updraft plays a critical role in its growth. Instead of falling out of the cloud, the light embryo is suspended or even lifted higher by the rising air current. As it tumbles and travels within the cloud, it encounters vast numbers of supercooled water droplets.
This is where the process of accretion occurs. The supercooled droplets freeze on contact with the ice embryo, adding successive layers of ice, much like rolling a small snowball down a hill makes it bigger. The stronger the updraft, the longer it can suspend the growing hailstone and the higher it can lift it.
The journey isn’t always straight up. Hailstones often get caught in cycles, rising in the updraft, collecting supercooled water, then perhaps falling slightly or being tossed into areas with different droplet concentrations or temperatures before being caught by the updraft again. Each trip through a moisture-rich area adds another layer.
Understanding Hailstone Layers
If you were ever brave enough (and it landed safely) to cut open a large hailstone, you might notice it has layers, somewhat like an onion. These layers can tell a story about the hailstone’s turbulent journey within the storm cloud. The layers alternate between clear or milky/opaque ice:
- Clear Ice: This tends to form in the lower, warmer (though still below freezing) parts of the hail-growth region where there’s a high concentration of supercooled water. Droplets hit the hailstone and spread out, freezing relatively slowly, trapping fewer air bubbles and resulting in clear ice.
- Opaque or Milky Ice: This forms in the higher, colder parts of the cloud where supercooled water content might be lower. Droplets freeze almost instantly upon impact, trapping tiny air bubbles which scatter light and give the ice a cloudy or white appearance.
The number and thickness of these layers give scientists clues about the hailstone’s path and the conditions it encountered within the storm.
Important Safety Information: Hail, especially large hail, falls at significant speeds and poses a real danger. It can shatter windows, severely damage vehicles and buildings, and cause serious injury to people and animals caught unprotected. During a hailstorm, immediately seek shelter inside a sturdy building, staying away from windows and skylights. Do not take shelter under trees, as lightning is often a concurrent threat.
The Inevitable Fall: When Gravity Wins
A hailstone continues its cycle of rising and falling, growing larger with each pass through supercooled water regions, as long as the updraft is strong enough to support its increasing weight. Eventually, one of two things happens:
- The hailstone grows so large and heavy that the updraft can no longer hold it aloft.
- The hailstone gets tossed out of the main updraft area, perhaps into a weaker updraft region or into a downdraft.
Once gravity overcomes the updraft’s lift, the hailstone begins its descent, falling through the cloud and eventually reaching the ground. The speed at which it falls depends on its size, shape, and factors like air resistance and any melting that occurs as it passes through warmer air layers closer to the surface. Larger hailstones obviously fall much faster and hit with greater force.
Why Size Matters: Factors Influencing Hailstone Dimensions
Not all thunderstorms produce hail, and those that do don’t always produce large hail. Several factors must align perfectly for giant hail to form:
- Strong Updrafts: This is the most critical factor. Stronger updrafts can suspend heavier stones for longer periods, allowing them more time to grow and lifting them higher into colder regions rich in supercooled water. Updrafts in severe storms can exceed 100 mph.
- High Supercooled Water Content: More available “building material” means faster growth.
- Sufficient Residence Time: The hailstone needs to spend enough time in the growth region (supported by the updraft) to accumulate significant size.
- Storm Structure: Certain storm structures, like supercells, are particularly efficient hail producers due to their organized and persistent powerful updrafts.
The absence or weakness of any of these factors typically results in smaller hail or no hail at all.
Where and When Does Hail Happen Most?
While hail can technically occur anywhere a strong thunderstorm forms, certain regions are more prone to it due to favorable atmospheric conditions. In the United States, the central plains (often called “Hail Alley”), encompassing parts of Texas, Oklahoma, Kansas, Nebraska, Colorado, and Wyoming, see frequent hail events, particularly during the spring and summer months.
Globally, hail is common in mid-latitude continental interiors where intense surface heating can lead to strong atmospheric instability and powerful storm development. The connection to summer is clear: warmer surface temperatures fuel the powerful updrafts needed to loft particles high into the freezing levels and keep them suspended long enough to grow into sizable hailstones.
So, the next time you hear the unexpected clatter of ice against your window on a warm day, remember the incredible journey those hailstones took. Born from tiny nuclei high in a freezing cloud, nurtured by powerful updrafts and supercooled water, they grew layer by layer in a turbulent dance within the storm before finally succumbing to gravity. Hail is a dramatic reminder of the immense power and complex processes occurring miles above us, turning summer warmth into falling ice.
