Hail forms inside strong thunderstorms when water droplets are lifted high into cold regions of the atmosphere and freeze into ice. It matters because hail can damage crops, cars, roofs, aircraft, and power lines in only a few minutes. Understanding hail helps meteorologists warn communities about severe storms.
It also shows how temperature, moisture, and rising air work together in the atmosphere.
The main engine of hail formation is a powerful updraft inside a cumulonimbus cloud. Small ice particles grow as they collide with supercooled water droplets, which are liquid even though their temperature is below 0 degrees Celsius. If the updraft is strong enough, the growing hailstone is carried upward many times, adding layers of ice before it finally falls.
Large hail usually means the storm has intense rising air and a deep freezing layer.
Understanding What Causes Hail
A hailstone needs a starting point called a hail embryo. This may be a tiny ice crystal, a frozen raindrop, or a small pellet of soft ice. Once an embryo enters a region filled with supercooled droplets, those droplets strike it and freeze.
The details of that freezing affect the hailstone's texture. When droplets freeze very quickly, air becomes trapped in the ice. This makes a cloudy, white layer.
When water spreads over the surface before freezing, it makes clearer, denser ice. Cutting open a large hailstone can reveal alternating clear and cloudy rings, much like growth rings in a tree.
The storm does not lift air at the same speed everywhere. Some parts of the updraft rise rapidly, while nearby areas may have weaker rising air or falling air. A developing hailstone can move through these regions several times.
It may be carried upward, drift outward, fall slightly, then enter another rising current. Each trip exposes it to different amounts of liquid water and different temperatures.
This is why hailstones are often uneven or lumpy rather than perfectly round. Collisions between hailstones can break them apart, while collisions with droplets usually help them grow.
Temperature changes across the cloud control whether the outer surface stays dry or becomes coated with liquid water. In a very cold region, droplets tend to freeze on contact. This is called dry growth and it creates lighter, more opaque ice.
In a somewhat warmer region below freezing, many droplets arrive so quickly that freezing cannot happen at once. A thin liquid coating forms first, then freezes later. This wet growth can make hail grow faster and form hard, clear layers.
The cloud must contain enough liquid water for this process. A powerful updraft without much moisture will not produce large hail efficiently.
Hail can begin melting before it reaches the ground. A thick layer of warm air below the cloud may shrink a hailstone or melt it completely into rain. This explains why a storm can produce hail in one town but only heavy rain a few kilometres away.
Weather radar helps forecasters identify parts of storms where hail is likely. Large ice particles often give a strong radar signal, though radar cannot measure every stone at the ground.
When studying hail, pay attention to vertical motion, temperature with height, and available liquid water. These three factors work together to determine the size, structure, and survival of the falling ice.
Key Facts
- Hail forms in cumulonimbus clouds with strong updrafts and cold upper regions.
- Water freezes at 0 degrees Celsius, but supercooled droplets can remain liquid below 0 degrees Celsius until they hit ice.
- A hailstone grows by accretion, which means supercooled droplets freeze onto its surface.
- Stronger updrafts can support larger hailstones before gravity pulls them down.
- Weight force on a hailstone is Fg = mg, where m is mass and g is about 9.8 m/s^2.
- A hailstone falls when its weight becomes greater than the upward forces from the storm updraft.
Vocabulary
- Hail
- Hail is precipitation made of balls or lumps of ice that form inside strong thunderstorms.
- Cumulonimbus cloud
- A cumulonimbus cloud is a tall thunderstorm cloud that can produce heavy rain, lightning, strong winds, and hail.
- Updraft
- An updraft is a rising current of air inside a storm that can lift water droplets and ice particles upward.
- Supercooled water
- Supercooled water is liquid water that remains unfrozen even though its temperature is below 0 degrees Celsius.
- Accretion
- Accretion is the growth of a hailstone as liquid droplets freeze onto its surface.
Common Mistakes to Avoid
- Thinking hail is just frozen raindrops from the ground, which is wrong because hail grows high inside thunderstorm clouds through repeated freezing and lifting.
- Assuming hail only forms in winter, which is wrong because hail often forms in spring and summer thunderstorms when cloud tops are very cold.
- Believing bigger hail means colder air at the ground, which is wrong because hail size depends more on updraft strength and storm structure than surface temperature alone.
- Ignoring supercooled water, which is wrong because hail grows mainly when below-freezing liquid droplets strike ice and freeze on contact.
Practice Questions
- 1 A hailstone has a mass of 0.020 kg. Calculate its weight using Fg = mg with g = 9.8 m/s^2.
- 2 The freezing level in a storm is 3.0 km above the ground, and a hail embryo is lifted to 9.0 km. How far above the freezing level is it lifted?
- 3 Explain why a thunderstorm with a stronger updraft can produce larger hail than a storm with a weak updraft.