How Does Hail Form? — In Plain Words

General information only.

The short answer

Hail is a form of precipitation consisting of solid ice that forms inside thunderstorm updrafts. That is how NOAA's National Severe Storms Laboratory (NSSL) defines it on its Hail Basics page.

NOAA's JetStream page says the same thing in its own words: hail is precipitation that is formed when updrafts in thunderstorms carry raindrops upward into extremely cold areas of the atmosphere.

An updraft is the rising air inside a thunderstorm. Hail is made up in that rising air, not at the ground.

How a hailstone forms and grows

Hailstones are formed when raindrops are carried upward by thunderstorm updrafts into extremely cold areas of the atmosphere and freeze.

Hailstones then grow by colliding with liquid water drops that freeze onto the hailstone's surface.

The JetStream page words the growing step this way: hailstones grow by collision with supercooled water drops. JetStream explains that supercooled drops are liquid drops surrounded by air that is below freezing, which is a common occurrence in thunderstorms.

Cloudy ice and clear ice

If the water freezes instantaneously when colliding with the hailstone, cloudy ice will form, as air bubbles will be trapped in the newly formed ice.

However, if the water freezes slowly, the air bubbles can escape and the new ice will be clear.

The JetStream page gives these two kinds of growth names. In wet growth, the hailstone is in a region where the air temperature is just below freezing, not super cold; the water does not immediately freeze, liquid water spreads across the tumbling hailstone and slowly freezes, air bubbles can escape, and the result is a layer of clear ice. In dry growth, the air temperature is well below freezing, the water droplet immediately freezes as it collides, the air bubbles are frozen in place, and the result is cloudy ice.

These two kinds of growth are what produce the layered look of hail, as JetStream states it. The next section gives NSSL's fuller explanation of those layers — and an important correction.

The big correction: where the layers really come from

NSSL states this correction directly, and this page prints it in NSSL's framing: Hailstones can have layers of clear and cloudy ice if the hailstone encounters different temperature and liquid water content conditions in the thunderstorm. The conditions experienced by the hailstone can change as it passes horizontally across or near an updraft. The layers, however, do not occur simply due to the hailstone going through up and down cycles inside a thunderstorm.

In plain words: the layers record the different temperature and liquid-water conditions the stone passed through in different parts of the storm. They are not simply a count of trips up and down through a freezing level.

NSSL adds why the simple up-and-down picture does not fit: the winds inside a thunderstorm aren't simply up and down. Horizontal winds exist, from either a rotating updraft, like in supercell thunderstorms, or from the surrounding environment's horizontal winds.

NSSL also corrects a second part of the popular picture: hailstones do not grow from being lofted to the top of the thunderstorm. At very high altitudes, the air is cold enough (below −40 °F) that all liquid water will have frozen into ice, and hailstones need liquid water to grow to an appreciable size.

When hail falls

The hail falls when the thunderstorm's updraft can no longer support the weight of the hailstone, which can occur if the stone becomes large enough or the updraft weakens.

NSSL also states it this way: hail falls when it becomes heavy enough to overcome the strength of the thunderstorm updraft and is pulled toward the earth by gravity.

Smaller hailstones can be blown away from the updraft by horizontal winds, so larger hail typically falls closer to the updraft than smaller hail.

If the winds near the surface are strong enough, hail can fall at an angle or even nearly sideways.

How fast hail falls

NSSL says this is a very complicated answer. The fall speed of hail primarily depends on the size of the hailstone, the friction between the hailstone and surrounding air, the local wind conditions (both horizontal and vertical), and the degree of melting of the hailstone.

NSSL gives these expected fall speeds:

• For small hailstones (under 1 inch in diameter), the expected fall speed is between 9 and 25 mph.
• For hailstones that one would typically see in a severe thunderstorm (1 inch to 1.75 inch in diameter), the expected fall speed is between 25 and 40 mph.
• In the strongest supercells that produce some of the largest hail one might expect to see (2 inches to 4 inches in diameter), the expected fall speed is between 44 and 72 mph.
• However, it is possible for very large hailstones (diameters exceeding 4 inches) to fall at over 100 mph.

NSSL is careful about how sure these numbers are, and this page keeps its wording: there is much uncertainty in these estimates due to variability in the hailstone's shape, degree of melting, fall orientation, and the environmental conditions.

NSSL also notes that early research assumed hailstones fell like solid ice spheres and showed very high fall speeds, even for very small hailstones, while recent research outside of NSSL using 3-D printed casts of real hailstones suspended in a vertical wind tunnel has repeatedly shown that natural hailstones fall more slowly than solid ice spheres.

Where hail happens most: hail alley

Although Florida has the most thunderstorms, NSSL says Nebraska, Colorado, and Wyoming usually have the most hailstorms. The area where these three states meet — “hail alley” — averages seven to nine hail days per year.

The JetStream page names a different third state. JetStream says: while Florida has the most thunderstorms, New Mexico, Colorado, and Wyoming usually have the most hail storms. JetStream adds that the freezing level in the Florida thunderstorms is so high, the hail often melts before reaching the ground.

This page prints both wordings, each attributed to its own page, and does not merge them into one list.

NSSL names other parts of the world that have damaging hailstorms: China, Russia, India and northern Italy.

Hail swaths

When viewed from the air, it is evident that hail falls in paths known as hail swaths. These occur as storms move while the hail is falling out. They can range in size from a few acres to an area 10 miles wide and 100 miles long.

Some storms, instead of producing large hail, produce copious amounts of small hail. Storms like these have produced hail drifts that, when captured in clogged drainage channels, formed piles of hail several feet deep.

As a fact stated by NSSL: hail that completely covers roadways is especially hazardous because, if deep enough, a vehicle's tires may not touch the roadway at all, with the vehicle instead driving on the hail, which acts exactly like an icy roadway in the winter.

What hail can do — stated as facts

Both NOAA pages state this fact: hail can damage aircraft, homes and cars, and can be deadly to livestock and people.

The JetStream page gives one dated example: one of the people killed during the March 28, 2000, tornado in Fort Worth was killed when struck by grapefruit-size hail.

The largest U.S. hailstone

The largest hailstone recovered in the United States fell in Vivian, South Dakota, on June 23, 2010, with a diameter of 8 inches and a circumference of 18.62 inches. It weighed 1 lb 15 oz.

Hail sizes compared to everyday objects

Hail size is often estimated by comparing it to a known object. NSSL lists these comparisons:

NSSL also states that most hailstorms are made up of a mix of different sizes.

Where the two NOAA pages differ

The two source pages do not word everything the same way. This page prints each wording with its page named, instead of blending them:

• Hail alley states: NSSL names Nebraska, Colorado, and Wyoming. JetStream names New Mexico, Colorado, and Wyoming. Both lists are printed in the hail alley section above.
• The hailstone's path: The JetStream page describes a path in which the updraft catches the hailstone and moves it back into the cloud, where it once again collides with water and grows, and says this process may be repeated several times. The NSSL page expressly corrects the layer explanation: the layers do not occur simply due to the hailstone going through up and down cycles inside a thunderstorm. NSSL's correction is the one printed in full, in the correction section above.
• Two object sizes: In NSSL's estimating list, softball = 4 inches and grapefruit = 4 1/2 inches. In JetStream's updraft table, grapefruit = 4 inches and softball = 4 1/2 inches. The size table on this page follows NSSL's estimating list, as printed above.

A different topic

Sleet, graupel, and snow are different kinds of frozen precipitation — a different topic, not covered on this page.

General information only.