What Is Hail and Why Does It Form?
Hail is one of the most striking and destructive weather phenomena in South America, especially in regions like Mendoza (Argentina), southern Brazil, or the Bolivian highlands. But how does hail form? It's not simply frozen water falling from the sky: behind every hailstone lies a complex process within storm clouds, combining updrafts, supercooled water droplets, and sub-zero temperatures.
In this article, we explain step by step the journey a hailstone takes from being a tiny droplet to hitting the ground, and why some storms produce hail the size of a golf ball.
The Stage: Vertically Developed Clouds (Cumulonimbus)
Hail only forms in a specific type of cloud: the cumulonimbus, also known as a thunderstorm cloud. These clouds can reach heights of 15 to 20 kilometers, where temperatures drop below -40°C. Within them, two ingredients are key:
- Very strong updrafts: winds rising at speeds of 50 to 150 km/h, capable of holding ice particles aloft.
- Abundant moisture: large amounts of water vapor that condense rapidly as they rise.
These conditions are not exclusive to any region: they occur in severe storms worldwide, but in South America, the combination of the Andes mountain range and Atlantic humidity creates a perfect breeding ground for hail.
The Step-by-Step Process of Hail Formation
1. Condensation Nuclei and Supercooled Droplets
It all begins when water vapor condenses around microscopic particles (dust, pollen, salts), forming droplets. As these droplets rise to colder parts of the cloud, their temperature drops below 0°C, but they don't freeze immediately: they become supercooled water droplets, a liquid state at sub-zero temperatures.
2. The First Impact: The Hail Embryo Is Born
When a supercooled droplet collides with an ice nucleus (like a snow crystal or a frozen particle), it freezes instantly. This forms a small hail embryo, just a few millimeters in size. This embryo is the starting point for what will later become a hailstone.
3. Growth Through Collisions and Accretion
The embryo doesn't grow alone: it is carried by updrafts and collides with thousands of supercooled droplets. Each time a droplet hits it, it freezes onto its surface, adding a layer of ice. This process is called accretion. The stronger the updraft, the longer the stone stays in the cloud, and the more it grows.
4. The Up-and-Down Cycle: Layers of Translucent and Opaque Ice
Here lies the secret to hail's size and structure. Stones don't rise in a straight line: they are carried upward by currents, but when they become too heavy, they fall to lower parts of the cloud. If the current pushes them back up, they rise again. This cycle can repeat several times, and each time the stone gains a layer:
- Translucent ice: forms when the stone is in areas with abundant liquid water and freezing is slow, trapping air bubbles.
- Opaque ice: occurs when freezing is rapid, in colder and drier areas, creating a white, porous layer.
Cutting a hailstone in half reveals alternating rings, like those of a tree, telling the story of its journeys within the cloud.
5. Hail Falls When It's Too Heavy
Eventually, the stone reaches a size that the updraft can no longer support (generally over 2 cm in diameter). It then falls due to gravity. During its fall, it may partially melt if it passes through warm air layers, but if it's large enough, it reaches the ground as hail.
Why Are Some Hailstones Giant?
The world record for the largest hailstone fell in Vivian, South Dakota (USA), with a diameter of 20 cm. In South America, stones up to 10 cm have been recorded in Mendoza. For hail to reach extreme sizes, the following are needed:
- Updrafts exceeding 100 km/h, which hold the stone aloft for longer.
- High concentration of supercooled water in the cloud.
- A prolonged growth cycle, with multiple ascents and descents.
Hail in South America: Risk Zones and Season
In our region, the areas most prone to hail are:
- Mendoza and San Juan (Argentina): known as the 'hail corridor,' with frequent storms between October and March.
- Southern Brazil and Paraguay: where severe storms are common in spring and summer.
- Bolivian and Peruvian highlands: frequent hail during climatic transition periods.
Agriculture is one of the most affected sectors: a hailstorm can destroy vineyards, soybean crops, or fruit trees in minutes. That's why having early warnings and monitoring systems like those from Contingencias is vital to mitigate losses.
Practical Tips to Protect Yourself from Hail
Although we can't prevent hail from forming, we can reduce its impacts:
- At home: inspect roofs and gutters before storm season; use protections on skylights and vehicles.
- In the field: install hail nets on high-value crops and purchase agricultural insurance.
- When a warning is issued: seek shelter indoors, stay away from windows, and don't take cover under trees or metal structures.
- With the Contingencias app: enable notifications to receive hail alerts in advance and make informed decisions.
Conclusion: The Science Behind the Ice Falling from the Sky
Hail is a reminder of the atmosphere's power. Its formation involves complex physics, but understanding the process helps us respect its force and prepare better. The next time you see a storm cloud, remember that inside it, a journey of ice may be brewing that will end in your city or on your crops. Stay informed with tools like Contingencias and protect what you value most.