Hail – a dangerous weather phenomenon

It’s been more than five years since the day the record was broken for the size of a hailstorm created by a storm cell over Poland. The previous record of 10 centimeters was broken by 2 centimeters by a supercell storm on June 11, 2019. passed over Gorzow Wielkopolski. This was a rarity on a nationwide and even European scale, as hail larger than 10 cm had not been recorded before. Unfortunately, as a result of the changing climate, we are seeing more and more giant hailstones falling across Europe, and the record from Gorzow Wielkopolski was broken by another 1.5 cm two years later in Tomaszow Mazowiecki.

For several years now, pictures of hail balls a few centimeters in size have been circulating the media every now and then, surprising no one, and meteorological warnings have included the constant phrase “possible hail storms.” However, are there really more such phenomena? Is it just that, thanks to social media, we’re learning about them more often? Or perhaps the Internet portals, which outdo each other with ever more catchy headlines, frighten us too often and ultimately reduce the audience’s alertness to dangerous phenomena?

What is hail?

Hail is precipitation of ice nuggets with a diameter of more than 5 mm. The hailstones are out of shape, and alternating layers of transparent ice and opaque snow may be visible in cross-section. It is formed in Cumulonimbus storm clouds or strongly extended Cumulus congestus clouds. Hail precipitation generally occurs in the warm season, with temperatures at the ground surface well above 0°C, but episodically it is also observed in cooler times. The maximum incidence occurs in May and remains high until August. In Poland, hail precipitation is most often observed in hot and humid tropical air masses or during the passage of cool atmospheric fronts, pushing out the lingering tropical air.

The peculiar terrain and surface coverage result in hailfall occurring more frequently in some areas of Poland than in others. Thanks to years of observation, back in the 1990s. In the 1970s. hail trails have been mapped in Poland. It was determined that the most frequent hail storms occurred in southeastern Poland – in a belt from Krakow to Chelm and in the Kashubian Lake District. In the experience of meteorologists, storm clouds are most dangerous between 3pm and 7pm, then the temperature begins to drop and the likelihood of hail formation fades.

The size and shape of hailstones varies, but all can cause loss and damage. Rushing at speeds of up to 160 kilometers per hour, a lump of ice 2 centimeters in size can cause damage to agriculture, breaking twigs, bruising fruit or causing plants to freeze. Larger hailstones (2-3 cm) dent car bodies, damage windshields, and, at 3-5 cm in diameter, break windows and roof tiles and injure people and animals. The fallout, reaching a size of 5-7 cm, can pierce car bodies, seriously injure people and kill animals, and above 7 cm it can kill people and cause damage to building structures and infrastructure.

How is hail formed?

In order for a hailstorm to form, conditions are necessary that favor the formation of high-elevation storm clouds – a large energy resource in the form of hot and humid air, and conditions that allow its rapid ascent. The hail forms in the center of the cloud, generally around condensation nuclei, which can take the form of a free-floating snow crumb or ice grain in the cloud. Once it hits the ascending current, it travels upward, traversing alternating layers of dry and moist air. Once the barrier of -15°C is exceeded, the hornwort seed collects droplets of overcooled water, which momentarily freeze when they come into contact with it.

As it travels further in the ascending current, it accumulates ice crystals and water droplets, increasing its weight and diameter. When it is too heavy for the force of the ascending current to defy gravity, it begins to descend, forming a strong descending current along with other hail and often rainfall. Along the way, it continues to collect ice crystals and droplets of overcooled water, further expanding until it crosses the positive temperature limit. The size of the hail balls depends largely on the humidity and temperature distribution of the storm cloud. The higher the humidity and greater the thermal contrasts, the larger the hailstones can form.

Although small hail can form in any thunderstorm, most cases larger than 4 cm in diameter are associated with supercell thunderstorms, i.e. thunderstorms that form in a very humid, hot and potential energy-rich environment and are characterized by a deep and persistent vortex in the core of the storm cloud. Thanks to the swirling current in the storm supercell and the separation of ascending and descending currents, hailstones can remain for a long time within moist and overcooled air layers, increasing in size. The lifespan of such a storm cell is calculated in hours – compared to tens of minutes for less intense storms. It has a characteristic comma or hook shape and can travel several hundred kilometers in its lifetime.

Is hail precipitation on the rise?

Comparing historical data on the incidence of hail with current reports from media outlets or databases of extreme weather incidents, there has been an exponential increase in the number of hail storms. However, it should be taken into account that climatological data on hail precipitation published by IMGW-PIB are still based on observations from synoptic ground stations, the number of which has decreased, and observer work has sometimes been replaced by automatic instruments. This does not now, and has not in the past, allowed detection of all cases of hail storms in Poland or even in the immediate vicinity of the observation station.

Nowadays, almost everywhere we have a meteorological observer with a camera who, within minutes, posts a photo of the hailstorm, the size of the hailstones and the damage caused on social media or on the publicly accessible pages of the European Storm Surge Database(ESWD). In service since 2010. reports are collected and verified, among others. About the big grads in Poland. With verification, this base becomes reliable. In addition, thanks to weather radar images, it is possible to track the development of potential storm clouds in real time and monitor the phenomena associated with them.

However, it is a fact that the increase in the number of hail cases and the size of hailstones in Poland is not only influenced by the development of technology to track the phenomenon. The changing climate and the increasingly frequent influx of hot and humid air masses over Poland, which are conducive to the formation of hail, are also undoubtedly contributing to this.

Records

As already mentioned, the largest hailstone in Poland fell near Tomaszów Mazowiecki in 2021. and was 13.5 cm in diameter. By contrast, in Italy in 2023. A nineteen-centimeter nugget was observed, which is now an unofficial European record. They make way for world records from Bangladesh, where in the 1980s. In the 1970s. A frozen cluster of hail weighing more than 4 kg was recorded, as well as the record hail of 2010. from the US measuring 20 cm in diameter and weighing almost 1 kg. Unfortunately, observations show that the size of hailstones is increasing, so more records can be expected to be broken in Europe and around the world.

Warnings and accidents

Hailstorms can be a devastating and deadly phenomenon. In 2023. In Germany, hailstorms led to the death of more than a dozen deer, and in Italy, hail the size of tennis balls injured more than 100 people. Human fatalities are quite rare, but historically have been recorded in the Polish territory.

The hail also contributes to massive construction losses and damages cars. It can also be the cause of aviation accidents. A hailstorm combined with heavy rain contributed 1988. to the failure of both engines of a TACA Airlines Boeing 737 and an emergency landing on a flood barrier in the southern US. In 2018. American Airlines’ A319 made an emergency landing in Texas due to hail damage. The latest high-profile aviation incident occurred when an Austrian Airlines Airbus A320 lost the front part of its fuselage and the glass protecting the cockpit after it flew into a superstorm cell.

As you can see, even these days hailstorms can be dangerous and carry devastating consequences. So it’s a good idea to keep up to date with weather alerts, meteorological warnings or follow the recommendations in text messages from the Government Security Center. And every now and then look up at the sky to see the black shaft of the most dangerous storm clouds in time.

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