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.
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.