How Does Hail Form?
QUICK ANSWER
Hail forms when ice particles in a thunderstorm collide with supercooled water droplets that freeze on contact. As the particle is carried through the cloud by updrafts, it accumulates layers of ice. Each pass through the storm adds to the hailstone. When too heavy for the updraft, it falls as hail.
Hailstones are nature's ice onions, growing in layered rings as they're carried through thunderstorm clouds. The formation mechanism involves remarkable physics: supercooled water droplets that freeze instantly on contact with ice, multiple trips through the cloud collecting more layers each time, and the eventual fall when the hailstone outgrows the updrafts. Understanding hail formation explains why hailstones have distinctive internal structure when cut open.
How do hailstones grow?
Hailstones grow through a process called accretion. A small ice particle (called a hail embryo) is carried into a region of the cloud containing supercooled water droplets. When a supercooled droplet hits the ice, it freezes on contact, adding mass to the growing hailstone. The hailstone continues collecting droplets as long as it stays in the supercooled water region. Updrafts can lift the hailstone higher into the cloud, then it falls back down, sometimes cycling up and down many times. Each pass adds more ice, growing the hailstone progressively larger.
Why do hailstones have layers?
Hailstones have visible layers because they form in different conditions during their growth. In some parts of the cloud, droplets freeze instantly into opaque white ice that traps air bubbles. In other parts, droplets coat the hailstone in a thin layer of liquid that freezes more slowly into clearer ice without bubbles. As the hailstone cycles between different cloud regions, alternating layers of clear and opaque ice accumulate. Cut a large hailstone open and you can count the layers, similar to tree rings, sometimes revealing the storm's structure during the stone's formation.
How big can hailstones get?
The largest recorded hailstone fell in Vivian, South Dakota on July 23, 2010, measuring 8 inches in diameter and weighing 1.94 pounds. That hailstone broke previous records for both diameter and weight. Other very large hailstones include a 7.0-inch stone from Aurora, Nebraska in 2003 and several recorded over 4 inches. Hail of 1 inch or larger is considered severe by the National Weather Service and triggers warnings. Hailstones over 3 inches require updrafts of 80+ mph; 4-inch stones require 100+ mph. The physical limits depend on what updraft speeds storms can produce.
What determines hail formation success?
Several factors determine whether hail formation produces small or large stones. The strength of updrafts determines how long hailstones can grow before falling. The amount of supercooled water available determines growth rate. The temperature range within the cloud affects what type of ice layers form. The time a hailstone spends in growth regions affects total accumulation. The storm's geometry (how tilted updrafts and downdrafts are) affects whether hailstones can stay aloft long. All these factors must align for very large hail. Most storms producing severe hail are supercells with all conditions favorable.
How do hailstones grow?
Hailstones grow through a process called accretion. A small ice particle (called a hail embryo) is carried into a region of the cloud containing supercooled water droplets. When a supercooled droplet hits the ice, it freezes on contact, adding mass to the growing hailstone. The hailstone continues collecting droplets as long as it stays in the supercooled water region. Updrafts can lift the hailstone higher into the cloud, then it falls back down, sometimes cycling up and down many times. Each pass adds more ice, growing the hailstone progressively larger.
Why do hailstones have layers?
Hailstones have visible layers because they form in different conditions during their growth. In some parts of the cloud, droplets freeze instantly into opaque white ice that traps air bubbles. In other parts, droplets coat the hailstone in a thin layer of liquid that freezes more slowly into clearer ice without bubbles. As the hailstone cycles between different cloud regions, alternating layers of clear and opaque ice accumulate. Cut a large hailstone open and you can count the layers, similar to tree rings, sometimes revealing the storm's structure during the stone's formation.
How big can hailstones get?
The largest recorded hailstone fell in Vivian, South Dakota on July 23, 2010, measuring 8 inches in diameter and weighing 1.94 pounds. That hailstone broke previous records for both diameter and weight. Other very large hailstones include a 7.0-inch stone from Aurora, Nebraska in 2003 and several recorded over 4 inches. Hail of 1 inch or larger is considered severe by the National Weather Service and triggers warnings. Hailstones over 3 inches require updrafts of 80+ mph; 4-inch stones require 100+ mph. The physical limits depend on what updraft speeds storms can produce.
What determines hail formation success?
Several factors determine whether hail formation produces small or large stones. The strength of updrafts determines how long hailstones can grow before falling. The amount of supercooled water available determines growth rate. The temperature range within the cloud affects what type of ice layers form. The time a hailstone spends in growth regions affects total accumulation. The storm's geometry (how tilted updrafts and downdrafts are) affects whether hailstones can stay aloft long. All these factors must align for very large hail. Most storms producing severe hail are supercells with all conditions favorable.
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