By Dr. Ed Brotak | Jun 6, 2017
On July 23, 2010, a hailstone fell out of a thunderstorm near Vivian, South Dakota. It had a diameter of 8 inches, a circumference of 18.62 inches, and weighed nearly 2 pounds. It was the largest hailstone ever recorded.
Hailstorms are far more common than tornadoes or hurricanes. In 2015, there were more than 5,000 reports of large (more than 1-inch-diameter) hail in the United States, and hail has occurred in all 50 states. Hail can cover the ground and actually form drifts up to 4 feet deep. Snow plows have been needed to clear roads. It is estimated that hail does $1 billion worth of damage to property and crops in the U.S. every year.
What Is Hail?
Hail is officially defined by the National Weather Service (NWS) as “showery precipitation in the form of irregular pellets or balls of ice more than 5 millimeters in diameter, falling from a cumulonimbus cloud.” The size of hailstones varies greatly, and precipitation often only lasts for a few minutes at most. Pea-size hail (about 1/4-inch) is fairly common and usually does little damage. Marble-size (about 1⁄2-inch) is typically the smallest size still capable of crop damage. One-inch hail (quarter-size) is considered the minimum for property damage to occur. Hail the size of tennis balls (2-1⁄2 inches) or even softballs (4-1⁄2 inches) is not uncommon. The giant hailstones, like the record holder described above, are probably aggregates of several stones that have frozen together. Hail smaller than 1 inch can damage tender vegetation. While small hail is not specifically forecasted, local NWS forecasts and meteorology reports typically will mention the possibility of any hail.
A couple of points to make here: Sleet and hail are not the same thing. Sleet, which only occurs in the winter when temperatures near the ground are below freezing, starts out as rain, then freezes into small ice pellets before reaching the ground. There is also something meteorologists call “graupel.” These are small ice pellets similar to sleet, but they start out frozen rather than liquid. They typically occur in showers during colder seasons (late fall and early spring, sometimes into summer). Graupel is also sometimes called snow pellets or soft hail.
How Does Hail Form?
Cumulonimbus clouds, or thunderheads, extend well up into the atmosphere, as high as 5 to 10 miles. At these heights, even in summer, temperatures are well below freezing. At these temperatures, you can still have liquid water (“super-cooled water”) as well as ice crystals. A small crystal of ice can become coated with super-cooled water, and as the water freezes around the ice particle, it grows. This process can continue until the weight of the hailstone causes it to fall out of the cloud. When this occurs depends on the strength (speed) of the updraft, which is the rising current of air that is the heart of the storm. An updraft of 25 mph can uphold pea-size hail. The strongest thunderstorms, known as “supercells,” can have updrafts of 100 mph and produce the largest hail. The stronger the updraft, the better chance of producing larger hail, because it will have longer to collect more water around the hailstone.
If hail is mixed with rain, it indicates that the thunderstorm updraft isn’t that strong, and smaller hail can be expected. If only hail is falling, this indicates a stronger updraft (the lighter rain drops are upheld in the cloud), and larger hail can be expected.
Since hail is produced by thunderstorms, the occurrence of hail roughly follows the pattern of thunderstorm occurrence. Hail is basically a warm-season phenomena, but there are exceptions. Along the Pacific Coast, the summers are dry. Hail is rare, but it can occur with big winter storms. In the Deep South, thunderstorms occur year-round, but freezing altitude is so high in summer, any hail produced melts before it reaches the ground. Infrequent hailstorms also occur in the cooler months.
“Hail alley” is found in the lee of the Rocky Mountains, the southern and central Plains extending from west Texas northward to southern South Dakota. This is where hail and large hail are most likely to occur. Stronger thunderstorms, lower freezing levels, and higher ground elevations all play a role. Wyoming, Colorado, and Nebraska — and the area where these three states meet — have the most hailstorms, averaging seven to nine hail days per year.
Although the U.S. sees more hailstorms than anywhere else in the world, hail also occurs in Canada, China, Russia, India, and even central Europe occasionally.
Season for Hail
Hail season extends from late spring through the summer, with a peak in June. Hail storms are more likely in the late afternoon and into the evening.
Predicting exactly when and where hail will fall is beyond the science of weather forecasting. However, the Storm Prediction Center (SPC) in Norman, Oklahoma, will give you as much information as possible. The SPC won’t specifically forecast damaging hail. Rather, they deal with the thunderstorm itself and the potential threats it may bring. The NWS defines a “severe thunderstorm” as one capable of “producing hail that is at least 1 inch in diameter or larger, and/or wind gusts of 58 mph or greater, and/or a tornado.” A “significant severe thunderstorm” is further defined as one capable of “producing hail that is at least 2 inches in diameter or larger, and/or wind gusts of 75 mph or greater, and/or a tornado that produces EF2 or greater damage.”
In forecasting such events, the meteorologists at the SPC look at low-level moisture, temperature lapse rates (temperature changes with height), lifting mechanisms, and wind shear (changes in wind speed and direction with height). The potential strength of the updraft and the height of the freezing level are critical items. The stronger the updraft and the lower the freezing level, the more likely hail will form.
The SPC will issue a “Severe Thunderstorm Watch” typically two to eight hours before actual thunderstorm development. This means that conditions are favorable for severe thunderstorms to form. Watches cover a large area, usually 25,000 square miles. Although some areas within the watch will likely see severe weather, the vast majority of people will not. For the general public, a watch means pay attention to your local weather and monitor your preferred weather-providing media. Severe thunderstorm warnings are issued by local NWS offices based on current conditions. Warnings can be issued either based on actual reports of severe weather by an official spotter or based on weather radar indications.
Prediction and Detection
There are a number of ways radar can detect hail. The hail itself reflects much of the radar signal and shows up as a very strong return on the radar screen. For color displays, hail can appear as purple (thunderstorms are typically red). In terms of storm structure, a return free region in the storm, called a “vault,” indicates a very strong updraft and a region of possible large hail formation. Any rotation, known as a “mesocyclone,” detected by the Doppler component of the radar, is also a good indication of a strong, hail-producing storm.
Warnings are typically issued only for a county or two, and typically last for an hour or less. Warnings will include where the storm is currently located, where it is headed, how fast it is moving, and what type of severe weather to expect. For the public, a warning means take cover immediately. A single severe thunderstorm is capable of producing hail, strong winds, and tornadoes, or it can produce just one of these damaging phenomena.
Of the $1 billion of damage in the U.S. annually due to hail, the majority is to property. Major cities like Dallas, Kansas City, and Phoenix have all had individual hailstorms that produced in excess of $2 billion in damage.
Quarter-size hail can damage roof shingles, larger hail can break windows and damage siding, and softball-size hail can actually go through the roof of a house. As for cars, 1-inch hail can put “dings” in the external sheet metal of a car, and baseball-size hail can break a car’s windshield. Imagine the damage in a parking lot or a car dealership. Aircrafts should also avoid hail. Although a crash due to hail alone is unlikely, damage to the plane’s exterior can be extensive.
Annual crop damage from hail is in the hundreds of millions of dollars. Unlucky gardeners and farmers can have just about everything wiped out. Keep in mind, even small, “non-severe” hail can do damage. Livestock are also at risk if they are outside when large hail occurs.
What can you do to protect yourself and your property from hail? You can actually “hail proof” your house to some extent. Hail-resistant roof shingles or even metal roofs are options. Hail-resistant siding is available, too, as are “impact-resistant” doors and windows (often sold as “hurricane-proof”). Obviously, there are cost factors, and you would need to weigh the costs versus the risk of hail damage in your area. As for your car or other vehicles, having a place for them under a good roof is the simplest and cheapest protection. There are “hailproof” car covers available for a more mobile solution. These are typically heavily padded blanket-type coverings.
As for your safety, keep track of the weather during the day if inclement weather is likely. If a warning is issued or hail starts to fall, you need to take shelter, at the very least inside a vehicle. There have only been three people reportedly killed by hail in the U.S. The latest was a pizza delivery man hit by a softball-size hailstone in Fort Worth, Texas, on March 28, 2000. Five years earlier at the Fort Worth Mayfest celebration, hundreds were injured, 60 seriously, when a severe hailstorm caught 10,000 people outside.
For the things you are growing outside, usually you just have to “let nature take its course.” Anti-hail net systems are available, though, and some farmers in hail-prone areas take out hail insurance.
Overall, know the risk of hail occurrence in your area. The NWS, and more specifically the SPC, have good records you can check. And when hail season arrives, always keep track of the local weather.
Hail is typically a warm-season occurrence, and the stronger the updraft within a thunderstorm, the better the chance of seeing larger hailstones. If your local weather station predicts the possibility of hail, take cover in a sturdy structure.
Hail occurrences, including size, are reported by trained “storm spotters.” If you are interested in becoming a spotter, contact your local NWS Office. There is also an organization of volunteer weather observers known by its acronym CoCoRaHS (Community Collaborative Rain, Hail & Snow Network). Besides rain and snow, they also report hail. If you are interested in becoming an observer, contact them.
For 30-plus years, Ed Brotak taught thousands of college students about the weather, and helped hundreds of them pursue careers in meteorology. He lives in Asheville, North Carolina, with his wife — also a meteorologist — and his two daughters, who vow never to be “weather weenies.” He still goes outside when he hears thunder.
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