Hail, No!

Hail is among the sometimes-dangerous threats posed by severe storms. The Upper Midwest is expected to experience episodes of baseball-sized (!) projectiles falling from the sky. in the next few days. A rain-laden low pressure system will pull in moisture from the High Plains and Northern Rockies into the Dakotas, bringing with it severe thunderstorms carrying hail and high winds. When hail reaches a significant size, it can cause major damage to structures and crops.

WHAT IS HAIL?

Hail is a type of solid precipitation. It shouldn’t be confused with sleet, smaller pellets which generally fall in cold weather. Hail consists of irregular lumps of ice called “hailstones.” Hailstones usually measure between 5 mm (0.2 in) and 15 cm (6 in) in size and can weigh as much as a pound. They can occur during, but are not as closely associated with, cold weather.

Hail can be seen in just about any thunderstorm. In the mid-latitudes, hail forms in the interior, while in the tropics, it tends to be confined to high elevations.

A thunderstorm which produces hail is known as a “hailstorm.” Unlike sleet, hailstones are often layered and can clump together. Hail is made up of mostly transparent ice which is formed as it travels through the cloud, remaining suspended aloft for up to 30 minutes by air updrafts until its weight causes it to fall to the ground miles below. The more intense the updraft, the larger the hail. It should be noted that conditions that produce hail are also favorable for the formation of tornadoes.

HOW HAIL FORMS

Like other precipitation, hail begins as water droplets. As the droplets rise and the temperature goes below freezing, they become supercooled water and freeze. A cross-section through a large hailstone shows an onion-like structure. This means that the hailstone has thick and translucent layers alternating with ones that are thin, white, and opaque. Classic theory suggested that hailstones were subjected to multiple ups and downs, falling into a zone of humidity and refreezing as they lifted. This theory, however, is being questioned in modern times.

Generally speaking, larger hailstones will form some distance from the stronger updraft, where they can spend more time growing. As the hailstone grows, its exterior stays in a liquid phase, keeping it “sticky” and allowing growth by collision with other hailstones.

Hail can also develop via “dry growth”, in which the exterior is not in a liquid state. Hail forming in this manner appears opaque due to tiny air bubbles that become trapped in the stone during rapid freezing. These bubbles can escape, making the hailstone clearer in appearance.

RISK FACTORS FOR HAILSTORMS

Hail formation is more likely when the freezing level is below 11,000 ft, usually in interior areas of mid-latitude land masses. Movement of dry air into strong thunderstorms can increase the frequency of hail by promoting cooling, giving hail a larger area to grow in. Conversely, hail is less common in the tropics despite a much higher frequency of thunderstorms than in the mid-latitudes. This is because the atmosphere tends to be warmer at a much greater altitude. It is not impossible, however; hail in the tropics can occur at the higher elevations.

HAIL ALLEY

This year, scientists in the United States became engaged in a project studying hail processes in thunderstorms in “Hail Alley.” This area encompasses the Great Plains and the Colorado and Wyoming’s Front Range of the Rocky Mountains. Hailstorms are most active from May to September, and the research of “hail hunters” may help us prevent some of the damage.

Aside: Canada has its own “Hail Alley” in South/Central Alberta.

HOW HAIL CAUSES DAMAGE

Hailstones greater than two centimeters (3/4 inch or so) in size are considered large enough to cause damage to agriculture. Wheat, corn, soybeans, and tobacco are the most vulnerable crops, especially early in their growth.

Damage is proportional to the velocity of falling hail. Speed varies by size and other factors. It’s thought that a 1 centimeter hailstone (0.39 in) falls at a rate of 20 miles per hour (9 meters/second), while larger stones the size of 8 cm (3.1 in) in diameter fall at a rate of 110 mph (48 meters/second). Hailstone velocity is dependent on the size of the stone, its drag coefficient, the motion of wind it is falling through, collisions with raindrops or other hailstones, and melting as the stones fall through a warmer atmosphere. It should be admitted that, since hailstones are not perfect spheres, it’s not easy to accurately calculate their speed.

HAIL DAMAGE EVENTS

Hail can cause serious damage, mostly to crops but also to aircraft, automobiles, glass-roofed structures, and livestock. Non-glass roof damage is also common, but often goes unnoticed until leaks and cracks in the ceiling are identified. Metal roofs are fairly resistant, but cosmetic deformities such as dents are seen. In 2023, hailstorms cost the United States $46 billion in damage to cars, roofs, and crops.

When hailstones exceed 0.5 inch (13 mm), they also present a hazard to landing aircraft. Hail is a common nuisance to car owners too, denting automobiles and cracking or even shattering windshields and windows unless covered.

Although rare in the modern era, large hailstones have been known to cause serious or even fatal head trauma to people caught outside without shelter. On April 30, 1888, a severe hailstorm killed more than 200 people in the Moradabad district of India. In the U.S., however, only three people are on record ever to be killed by falling hail.

Hailstorms normally last from a few minutes up to 15 minutes in duration, but can cause major damage to infrastructure during that time.  Accumulating hail can blanket the ground, cause loss of power, and bring down trees. Flash flooding due to blocked drains and mudslides within areas of steep terrain can also occur.

On rare occasions, a thunderstorm can become nearly stationary while producing hail and significant accumulation. On July 29, 2010, a foot of hail accumulated in Boulder County, Colorado. On June 5, 2015, hail up to four feet deep fell on one city block in Denver, Colorado.

HAIL MEASUREMENTS

The National Weather Service often refers to hailstones based on their similarity to everyday objects:

• 0.375 in. Pea
• 0.50 in.  Small Marble
• 0.75 in.  Penny
• 0.88 in.  Nickel
• 1.00 in.  Quarter
• 1.25 in.  Half-Dollar
• 1.50 in. Walnut/Ping Pong Ball
• 1.75 in.  Golf Ball
• 2.00 in. Lime
• 2.50 in. Tennis Ball
• 2.75 in. Baseball
• 3.00 in. Large Apple
• 4.00 in. Softball
• 4.50 in. Grapefruit

Records exist for the heaviest hail and largest: 2.2 pounds (1 kilogram) in Bangladesh in 1986 and 20 centimeters in diameter (8 inches) in South Dakota in 2010.

HAIL SAFETY

In the event you find yourself caught in a hailstorm:

• If you’re caught outside, seek shelter immediately. If you can’t get under some kind of cover, at least to protect your head. Seeking shelter under trees, however, are a last resort. Hail can bring down large branches right on top of you.
• If you’re in a building, stay inside and away from the windows until the hail stops.
• If you’re in an automobile, get under a highway underpass or other cover. Pull completely off the road and don’t leave the vehicle. Windshields can shatter, so get face down with your back to the glass.

HAIL-PROOFING A HOME

To prepare your home for a hailstorm: Let’s assume your roof is in good shape. If it’s a flat roof, it’s at most risk for damage. Fiber-cement is superior to vinyl or aluminum siding, which is much more resilient against hail.

It’s especially important to protect windows, doors, and skylights. Close drapes and blinds to form a barrier against broken glass. Of course, in Hail Alley, storm shutters are a great investment and also help protect against high wind conditions. Consider an upgrade to impact-resistant windows.

Bring all your patio furniture, barbecues, and potted plants either under sturdy cover or inside. For larger items that can’t be moved, cover with a tarp or other protective barrier.

Joe Alton MD