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.
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.