How Does Hail Form?

How does hail form? | RainViewer Blog

On a hot sunny day, we can suddenly witness such an unusual weather phenomenon as hail. If it catches you by surprise right on the street, then better quickly find shelter. Large hailstones are extremely dangerous and can cause bruises or even serious injuries. This icy precipitation is also a threat to agriculture because it can destroy crops. Fortunately, hail storms occur quite rarely. And yet, how does hail form? What are the conditions for its formation? We will answer these questions in this article.

What Is Hail and What Does It Consist Of?

Researchers agree that hail forms as a result of heavy rainfall and thunderstorms, typically with dark gray clouds. You can easily recognize such clouds by their unusual shape.

Hail clouds over mandan Image source: SkySpyPhotos

A special variant of thunderstorms often produces particularly large hail - the supercell thunderstorms. This is a thunderstorm with a rotating and exceptionally strong updraft that persists for a long time. The updraft area is also called “mesocyclone” and has a diameter of about a mile. Supercells produce not only hail but also tornadoes.

When viewed from the air, you can see hail falling in trajectories known as hail swaths. They appear as the storm moves through the hail fall. Sizes of hail swaths can vary from several acres to an area 10 miles wide and 100 miles long.

Hail swath on the ground near Airdrie, Alberta Canada. Image source: Weatherlogics

Hail size typically ranges from 0.03 to 0.7 inches in diameter. However, sometimes scientists recorded abnormally large hailstones weighing several pounds. Here are some hail-related records:

  • In 1939, the inhabitants of India witnessed hailstones that weighed over 6 pounds.
  • In 1970, the citizens of Coffeyville, Kansas saw a huge hailstone with the diameter of 17.5 inches and the weight of 1.3 pounds. It was the world’s heaviest hailstone.
  • In 1986, pieces of ice weighing more than 2 pounds fell on the inhabitants of Bangladesh from the clouds.
  • In 2010, the US state of South Dakota experienced the biggest hailstorm on record. The largest hailstone was about 8 inches in diameter.

The largest recorded hailstone in the United States Image source: Wikipedia

According to the National Weather Service, large hail should be a minimum of one inch in diameter. Hailstones this size or bigger can incur severe damage to property and crops.

Here are more interesting facts about hail:

  • During rain showers with hail, the sky may turn green. Of the entire spectrum of sunlight, hailstones most often reflect a green shade.
  • It is hard to define the speed hail falls from clouds. The National Severe Storms Laboratory estimates that a baseball-sized hailstone (2 ¾ inches) can gain a speed of about 100 miles per hour.
  • Hail typically falls in 5-10 minutes because of its quick movement and short bursts of energy. On rare occasions, it can last between 30 minutes and 1 hour.
  • Bacteria, insects, and even small animals and fish can sometimes hide inside hailstones.

Found South Florida Frog hailstone" Image source: Imgur

Hail Forms and Shapes

External factors such as temperature, liquid water content, and wind speed impact the structure, shape, and color of hailstones. The most common shape is a cone. Hailstones have layered structures of alternating cloudy ice and clear ice. This is explained by the changing conditions that surround the hailstone when it passes horizontally through or near the updraft. Additional factors are winds inside a thunderstorm cloud, which do not have only up and down cycles. Horizontal winds arise either from the rotating upward flow, as in supercell thunderstorms, or from horizontal ambient winds.

Large hailstone with concentric rings Image source: ERZ, CC BY-SA 3.0, via Wikimedia Commons

Sometimes you can often observe the shape of a ball when there are layers of solid ice around the snow core in the center of the hailstone. It is also possible to see air bubbles of cloudy ice inside clear ice. It happens when water instantly freezes after the collision with a hailstone.

More rarely, you can witness the shapes of plates, crystals, ellipses, or parallelepipeds. Sometimes this unusual weather phenomenon surprises with its outlines, resembling a flower or a star. Such a variety of forms and shapes occurs due to the wind blowing on the hailstone that has appeared in the air. Additionally, when the coating of hailstones does not fully freeze, they can stick to other hailstones as a result of wet growth, causing extraordinary shapes.

Star-shaped hailstones near Corning, California Image source: The Weather Channel

How Does Hail Work

Let’s take a closer look at the hail formation process. A hailstone grows when several conditions are met:

  • the presence of an embryo (an ice crystal less than 0.3 inches in size),
  • a thunderstorm cloud,
  • the air temperature above +68°F,
  • a large zone of supercooled water drops,
  • powerful updrafts of air.

Сaught by the updraft, an ice crystal turns into hail as it freezes in the upper atmosphere. It may remain in the cloud for up to 10-15 minutes, gradually growing in size. Hail growth can be either “dry” or “wet”. It depends on the number of supercooled drops in the cloud, the size of the hail, the temperature of the hail surface, and the surrounding air.

Dry growth occurs when there are few water droplets around and the surface of the hail is very cold as a result. Water vapor from the surrounding air sublimates on it, forming a cloudy ice layer. At this stage, the hail grows rather slowly. A large number of supercooled water droplets and a warmer hail surface result in rapid growth. The hailstone’s surface melts and is actively joined by supercooled droplets from the cloud. At this time, a transparent ice layer is formed. This is the stage of wet growth.

When an updraft loses its power and when hailstones get heavy enough, they fall to the ground, and thus a hail storm occurs. The grains of ice can be several inches thick. Sometimes they even get as big as tennis balls or soccer balls.

Hail formation correct diagram Image source: DTN

Hail Size, Explained

Why does hail sometimes reach gigantic proportions, threatening people and all living beings? Abnormal heat causes the relief of the foothill areas to warm up to varying degrees. As a result, thunderstorm updrafts appear that can send steam up to 6 miles. At this altitude, the temperature can drop to -104°F. These are the optimal conditions for the formation of huge hailstones, which develop a speed of about 100 mph when falling to the ground. The consequences can be terrifying: injured people who did not have time to find shelter, damaged infrastructure, houses and cars, as well as destroyed crops and plants.

In the table below, you can find details about the hail damage potential depending on the hailstone diameter and its approximate traveling speed. How fast hailstones are when they hit the ground depends on many factors, for example, the shape and density of the hailstones. A chunk of ice with many air pockets and bulges offers greater air resistance and therefore stays in the air longer than a compact ball of ice with high density. What is certain, however, is that the final speed increases on average with the weight of the hailstone. Huge hail can fall at speeds of more than 124 miles per hour, and the damage it can cause to the ground is correspondingly large.

DiameterSpeedDamage Potential
>1/4 inch (pea)9-25 mphDamage to crops, fruits, and trees
>1 inch (quarter)25-40 mphRisk of injury to small animals, breakage of clay bricks and window panes, visible dents on vehicles
>2 inches (tennis ball)~44 mphIrreparable damage to vehicles, destruction of tile and shingle roofs, breakage of metal window frames
>2 3/4 inches (baseball)~72 mphSerious risk of injury to people, cracking of concrete bricks, tearing apart of small tree trunks
>3 inches (tea cup, softball, grapefruit)~87 mphHuman life threat

When and Where Does Hail Occur More Often?

Have you ever seen hail in winter or at night? Experts agree that the chances to see hail after sunset are virtually zero. Meteorologists note that ice precipitation typically occurs only during the bright part of the day. They explain it by the absence of strong gusts of wind at night as well as the cooling of the Earth’s surface, when the steam just cannot reach the upper atmosphere.

We can observe an identical situation in winter. Sometimes freezing rain or sleet is mistaken for hail, but this weather phenomenon has a number of differences. The main thing is that hail can be of different structures, and an icy rain is characterized by uniform transparency.

Therefore, tropical regions and polar latitudes are not the best places for hail to form. But this is a frequent phenomenon for temperate latitudes. Hail forms less often over the sea, because strong updrafts of air are more common above the surface of the earth. A perfect location is close to the seashore.

Very often hail happens in Senegal, West Africa. Sometimes a layer of ice fragments even appears on the ground, and its thickness can reach at least one inch. Northern India also often suffers from this dangerous precipitation during the summer monsoons. Statistics confirm that the size of each fourth hailstone here is about one inch in diameter.

More than 0.7 inches of hail Image source: Twitter

In the United States, there is a place called “Hail Alley”, where hail occurs most frequently - between 7 and 9 days a year. The alley is located at the border of three states - Colorado, Nebraska, and Wyoming. Other areas with regular hail storms are Australia, Canada, China, and northern Italy. By the way, in northern Italy, there is the maximum number of hail days in Europe. Every year, this area experiences four days with more than 0.7 inches of hail, and every two years - about 2 inches.

How Can You Protect Yourself During Hail?

A hail threat is not a joke - this severe weather phenomenon is extremely dangerous. It can damage aircraft, homes and cars, break windows and kill people, so it is crucial to seek shelter if you suddenly get caught in a hail storm.

If you could not get to the shelter, you should cover your head, thus protecting yourself from the fall of dangerous hailstones. Do not hide under the trees, because large branches can fall on you. While indoors, it’s better to move away from windows and turn off all electrical appliances, because hail often results from severe thunderstorms and lightning. To avoid hail damage while driving, pull over near the curb and, if possible, hide in a garage or drive under a bridge. Stay away from car windows because breaking glass can cause injuries.

With the RainViewer app, you can get timely notifications about upcoming precipitation, including hail. In settings, you can specify the intensity of precipitation for which you want to receive notifications. You can also turn on the severe weather alerts option. This will help you remain up-to-date with the latest weather and stay safe in case of an emergency.

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