Hail forms inside tall thunderstorm clouds called cumulonimbus clouds, where air rises quickly and temperatures drop below freezing. These storms can lift tiny water drops high into the cloud before they have time to fall as rain. When the drops freeze and collect more water, they can grow into solid balls of ice called hailstones.
Understanding hail helps us predict storm hazards that can damage crops, cars, roofs, and aircraft.
Inside the storm, strong updrafts carry small ice particles above the freezing line, where supercooled water droplets freeze onto them. Each trip through wetter and colder parts of the cloud can add a new layer of ice, like rings in a tree trunk. A hailstone falls when gravity becomes stronger than the upward force of the storm's updraft.
The largest hailstones can reach softball size, but even small hail can be dangerous in severe thunderstorms.
Understanding How Hail Forms Inside Thunderstorms
Cloud water can remain liquid even when its temperature is below zero degrees Celsius. This happens because very pure, tiny droplets need a surface on which to begin freezing. An ice crystal, dust grain, or growing hail particle can provide that surface.
When supercooled droplets hit the particle, they freeze onto it. If freezing happens almost at once, tiny air bubbles get trapped in the ice. This makes a cloudy white layer.
If a large amount of liquid water spreads over the surface before it freezes, the layer can become clearer and smoother. The two kinds of ice record different conditions inside the storm.
A hailstone does not follow one simple path from the cloud top to the ground. Air inside a severe storm can move in several directions. A rising current may carry the stone upward, while a falling current may pull it lower.
Near the lower part of the cloud, the hailstone can pass through regions with many liquid droplets. Higher up, it enters colder regions where those droplets freeze. This repeated travel acts like a conveyor system.
Stronger rising air can keep a larger particle in the cloud for longer, giving it more chances to collect ice. Storms with weak rising air usually produce smaller hail or only heavy rain.
The layers inside a hailstone are useful evidence, but they are not a simple clock that counts one trip through the storm at a time. One layer can form during a change in liquid water supply, air temperature, or freezing speed. A cut hailstone may show alternating clear and white bands.
Clear bands often formed where liquid water was plentiful. White bands often formed where droplets froze quickly and trapped air. Scientists can study these bands to infer the changing environment around the stone.
The shape matters too. Some hailstones are round, while others have lumps because ice collected unevenly on different sides.
Hail is different from sleet and freezing rain. Hail grows within a thunderstorm, often during warm seasons when the ground air can be quite warm. Sleet usually begins as snow, melts in a warmer layer, then refreezes into small pellets before reaching the ground.
Freezing rain stays liquid until it hits a cold surface. Weather radar helps meteorologists locate intense storm cores that may contain hail. Very strong radar echoes can signal many ice particles, though radar cannot measure every hailstone perfectly.
During a hail warning, people should move indoors, avoid windows, and protect vehicles when possible. Students should pay attention to the link between air motion, temperature, collisions, and gravity, since hail forms only when all of these processes work together.
Key Facts
- Hail forms in cumulonimbus thunderstorms with strong updrafts and cold upper regions.
- The freezing line is the height in the atmosphere where temperature drops to 0°C or 32°F.
- Supercooled water droplets are liquid drops below 0°C that freeze when they hit ice.
- A hailstone grows when ice collects in layers as it moves up and down inside a storm.
- Hail falls when weight becomes greater than the lift from the updraft.
- Weight can be calculated with W = mg, where W is weight, m is mass, and g is gravitational acceleration.
Vocabulary
- Cumulonimbus cloud
- A tall thunderstorm cloud that can produce heavy rain, lightning, strong winds, and hail.
- Updraft
- A rising current of air inside a storm that can lift water drops and ice particles upward.
- Freezing line
- The altitude in the atmosphere where the temperature changes from above freezing to below freezing.
- Supercooled water
- Liquid water that remains unfrozen even though its temperature is below 0°C.
- Hailstone
- A ball or lump of ice that forms in a thunderstorm and falls to the ground.
Common Mistakes to Avoid
- Thinking hail is just frozen rain is wrong because hail grows in layers inside thunderstorm updrafts, while freezing rain forms when raindrops freeze near the ground or on surfaces.
- Assuming hail only forms in winter is wrong because the ground can be warm while the upper part of a thunderstorm is far below freezing.
- Ignoring the role of updrafts is wrong because strong rising air is needed to keep hailstones inside the cloud long enough to grow.
- Thinking every thunderstorm makes large hail is wrong because large hail requires especially strong updrafts, abundant supercooled water, and enough time for repeated ice layering.
Practice Questions
- 1 A hailstone has a mass of 0.020 kg. Using g = 9.8 m/s², calculate its weight with W = mg.
- 2 The temperature at the ground is 24°C and drops by 6°C per kilometer of height. At what height will the air reach 0°C?
- 3 Explain why a stronger updraft can produce larger hailstones than a weaker updraft.