What Is Hail? Definition, Formation, and Facts

What Is Hail Formation
Hail is a type of solid precipitation that falls during thunderstorms.

Hail is a type of solid precipitation associated with thunderstorms that consists of irregular lumps or rounded balls of ice. This article explores the definition of hail, its formation process, the factors influencing its development, its varying sizes, and how it differs from other forms of precipitation like graupel, sleet, and snow.

What Is Hail?

Hail is a type of solid precipitation that forms during thunderstorms. It consists of balls or irregular lumps of ice, known as hailstones. Unlike snow and sleet, hail is most common in the warmer months when strong thunderstorms are more frequent.

How Does Hail Form?

The formation of hail is a complex process involving several steps, primarily occurring within cumulonimbus clouds during thunderstorms. Here’s a step-by-step breakdown:

Hail Cross-Section Rings
The cross-section of a hailstone reveals concentric rings. (ERZ, CC 3.0 Unported)
  1. Updrafts: It begins with strong updrafts – upward moving air currents – that carry raindrops into the colder regions of the cloud, where temperatures are below freezing.
  2. Freezing: Once the raindrops reach these cold areas, they freeze into tiny ice particles.
  3. Accumulation of Layers: These ice particles then undergo a series of ascents and descents within the cloud, carried by the storm’s updrafts and gravity. As they move, they collide with supercooled water droplets – liquid water below freezing temperature. This causes a process called accretion, where the supercooled droplets freeze onto the ice particle, making it grow in size. Dry growth also occurs, where the outer surface freezes yet accumulates more mass. Wet layers are translucent, while dry layers are opaque and contain trapped air bubbles.
  4. Formation of Hailstones: This process repeats, with the hailstone growing larger with each cycle until it becomes too heavy for the updrafts to support. At this point, it falls to the ground as hail.

Factors Required for Hail Formation

The key factors for hail formation include:

  • Sufficient Cloud Height: Higher clouds allow more time for hailstones to grow before falling. So, cumulonimbus clouds are the type that produce hail.
  • Strong Updrafts: Powerful updrafts lift the hailstones high into the cloud for further growth.
  • Abundance of Supercooled Water: Clouds containing a lot of moisture provide the material for the hailstones to grow.

While lightning does not cause hail storms, cloud-to-ground lightning is common when they occur.

Relationship Between Hail and Tornadoes

Hail and tornadoes are both weather phenomena commonly associated with severe thunderstorms, particularly those involving supercells, which are highly organized, rotating thunderstorms. While they are distinct events, they often occur in the same storm.

Hail Green Clouds
These green clouds produced pea-sized hail in Michigan (NOAA).
  1. Formation Conditions: Both hail and tornadoes typically form in environments where there is strong instability in the atmosphere, a significant difference in wind speed and direction with altitude (wind shear), and ample moisture. These conditions favor the development of powerful thunderstorms capable of producing both hail and tornadoes. A green sky is indicative of conditions ripe for hail or tornado formation.
  2. Indicator of Storm Intensity: The presence of large hail in a storm indicates the potential for tornado formation. This is because the same strong updrafts needed to support large hailstones are also a key component in the development of tornadoes.
  3. Spatial and Temporal Relationship: In a typical supercell thunderstorm, hail often falls near the storm’s updraft region, while tornadoes usually form in the downdraft area, near the rear of the storm. However, tornadoes can occur before, after, or at the same time as hail, depending on the storm’s dynamics.
  4. Predictability Challenges: Despite the correlation, not all storms that produce hail will produce tornadoes, and vice versa.

Areas Prone to Hail

Regions where these conditions frequently converge are more prone to hail. Here’s an overview:

  • North America: The Great Plains in the United States, known as “Hail Alley,” experiences the most hailstorms, particularly in Colorado, Nebraska, and Wyoming. However, much of central Canada and the United States encounters hail.
  • Central and Southern Europe: Countries like Italy, Germany, and France frequently experience hailstorms, particularly in the spring and summer months. The combination of warm ground temperatures and cold upper air currents in these regions creates ideal conditions for hail formation.
  • South and Southeast Asia: Parts of India, Bangladesh, and Nepal are known for significant hail events, often associated with pre-monsoon and monsoon thunderstorms. The Himalayan region, in particular, sees frequent hailstorms due to its high altitude and the presence of moisture-laden air currents.
  • East Africa: Regions around Kenya, Ethiopia, and Uganda experience hail, mainly due to the convergence of warm, moist air from the Indian Ocean and cooler air from higher elevations. This leads to strong updrafts necessary for hail formation. Kericho, Kenya experiences an average of 50 days of hail per year and holds the record for the most days of hail in a single year (132 days).
  • South America: Argentina and Brazil experience significant hail. The Pampas region in Argentina, in particular, is akin to the Great Plains in the USA and sees some of the most intense hailstorms in the world.
  • Australia: Parts of eastern Australia, including Queensland and New South Wales, frequently encounter hail, especially during the spring and summer months. The region’s climate, characterized by hot, humid days and cold fronts, fosters the development of severe thunderstorms that can produce hail.

Speed of Hail

Hailstones fall at speeds varying from 44 to 72 mph (70 to 116 km/h). Factors affecting the fall speed include:

  • Size and Weight of the Hailstone: Larger, heavier hailstones fall faster.
  • Shape and Density: Irregular shapes slow down the fall.
  • Wind and Updrafts: Air currents influence the trajectory and speed.

Size of Hail

Hailstones range from pea-sized (1/4 inch or 6 mm) to as large as grapefruits (4.5 inches or 114 mm). For perspective, common comparisons include:

  • Pea-sized (1/4 inch)
  • Marble-sized (3/4 inch)
  • Golf ball-sized (1.75 inches)
  • Tennis ball-sized (2.5 inches)
  • Grapefruit-sized (4.5 inches)

Difference Between Hail, Graupel, Sleet, and Snow

While hail, graupel, sleet, and snow are all forms of frozen water, there are difference between them:

  • Hail: Hail forms in thunderstorms with repeated cycles of ascending, freezing, and accreting. Hailstones are larger than graupel, sleet, or snowflakes.
  • Graupel: Also known as soft hail, graupel forms when supercooled water droplets coat a snowflake, creating a soft, spongy ball. Graupel is smaller than hail and forms in weaker updrafts.
  • Sleet: Sleet or ice pellets form when raindrops freeze as they fall through a cold layer of air near the ground. Sleet is smaller and more translucent than hail.
  • Snow: Snow forms when water vapor in a cloud freezes directly into ice crystals, bypassing the liquid phase. Snowflakes have intricate, symmetrical patterns and are much softer and less dense than hail.

Of course, the other significant difference is that graupel, sleet, and snow are generally cold weather events. Thunderstorms that produce hail do sometimes occur in winter, but mainly they are warm weather phenomena.

Hail Suppression

Some countries experiment with hail suppression methods, such as cloud seeding. This involves releasing substances like silver iodide into clouds to prevent hailstones from growing large. In the middle ages, efforts involved ringing church bells and firing cannons. The thinking was that the vibrations would prevent large hail formation. However, thunder produces greater sonic waves and does not dispel hail.

Interesting Hail Facts

  1. Record-Setting Hailstone: The largest hailstone in terms of diameter ever recorded fell in Vivian, South Dakota, USA, on July 23, 2010. It measured 8 inches in diameter and weighed nearly 2 pounds.
  2. Hail in the Tropics: While hail mostly occurs in temperate regions, it falls in tropical areas as well. Usually, it happens in higher altitudes where the atmosphere is cooler.
  3. Hail in the Sahara Desert: While rare, hail occurs even in desert regions, including the Sahara Desert.
  4. Hail Swaths: Sometimes hail falls in paths known as “swaths,” which can be more than 10 miles long and 5 miles wide.
  5. Annual Hail “Seasons”: In certain parts of the world, there are specific “hail seasons.” For example, in North America, hail is most common between May and September, whereas in India, hail is more frequent during the spring months.
  6. Economic Impact: Hail causes billions of dollars in damage each year, affecting crops, vehicles, and buildings. In agricultural regions, severe hailstorms destroy entire harvests.
  7. Ancient Hailstorms: Historical accounts of hail date back thousands of years. One early record was for Roopkund, India, in 9th century AD, where large hailstones reportedly killed hundreds of people.
  8. Variation in Hailstone Shape: Hailstones are not perfectly round. Many have spikes or irregular shapes due to the way they collide and freeze with other hailstones or water droplets in the cloud.
  9. Layers in Hailstones: Cutting open a hailstone reveals layers of ice, much like the rings of a tree. These layers tell a story about how the hailstone formed and traveled within the cloud.
  10. Varied Colors: While most hailstones are white or translucent, some have layers of different colors, indicating the number of times they’ve ascended and descended in the cloud.
  11. Hailstones and Air Pockets: Some hailstones have small air pockets or inclusions, where air got trapped during the dry formation process.
  12. Rapid Formation: Hailstones can form quickly, sometimes in just 5 to 10 minutes.
  13. Hail in Mythology and Culture: Hail features into mythology and folklore, often as a form of divine retribution or a sign from the gods. For example, some Native American tribes believed that hailstones were the frozen tears of the gods.
  14. Wildlife and Hail: Hail significantly impacts wildlife. Birds, for example, are injured or killed by large hailstones. Hail devastates habitats and food sources.


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