With their formation beginning with some tiny frozen embryos present in thunderstorm, hailstones can grow to at least 10 centimeters in diameter in just a few minutes, before dropping to the ground and damaging crops, vehicles and property.

Considered relevant globally and locally destructive, hailstorms cause major societal impacts, but despite this our level of understanding and ability to predict hailstorms remain limited.

With their formation beginning with some tiny frozen embryos present in thunderstorm, hailstones can grow to at least 10 centimetres in diameter in just a few minutes, before dropping to the ground thereby damaging crops, vehicles and property.

A recent research paper dubbed Review of Geophysics explains the processes through which hail is formed, available observations to confirm how frequent hail occurs, the limitations in predicting these events, how hail responds to climate change and variability and its known effects on society.

In this study, the authors have given an overview of the existing understanding of hail and provide an outline of research challenges and relevant data required to improve our level of understanding.

How widespread and frequent do hailstorms occur?

Hailstorms are basically local weather events that occur across the globe, with regular occurrence of the weather event on each continent except the Antarctica.

While taking into account indirect proxies, the latest research recommends that most of the mid-latitudes should experience at least one hailstorm that generates 2.5cm (1 inch) stones per 100 km2 every year.

Nevertheless, there’s really no good handle on how frequent the biggest hailstone sizes occur in different parts of the world. Hail of more than 12cm is regularly witnessed in the United States with the largest stone measuring 20cm. Argentina has also recorded 18cm ‘gargantuan’ hail with Germany receiving hailstones measuring 14cm in diameter.

Considering that historical records are incomplete, some evidence exists that prove that hail of this size has been witnessed in Australia, South Africa, China, Bangladesh and some other nations.

How damaging can hail be when compared to other climatic and atmospheric hazards?

Hail is one of the most impactful weather events when it comes to thunderstorm-related hazards, generating annual losses amounting to over US$13 billion in Europe and the United States.

By the time the research was published in summer of 2020, more than US$20 billion of the whopping US$27 billion of the global insurance losses had already been caused by severe storms in the United States, with hailstorm being the major contributor. In Europe alone, two hailstorms occurring in July 2013 led to economic losses amounting to US$4.3 billion.

While this may seem quite insignificant when compared to the losses related to hurricanes making landfalls, hail events happen each year and can affect anything from fields with crops to suburbs, causing serious injuries to people and animals.

The effects also vary based on the amount of hail falling at a particular time, and how big it is. When it comes to agriculture, smaller hail and large volumes of it often strips crops and trees while hail measuring at least 30cm (1.2 inches) causes damage to roofs, windows, cars and infrastructure.

Methods for measuring and observing hailstorms

Observing and categorising hail comes with a myriad of challenges. First and foremost, you will require someone to remain present to record whether there was hail or not. Again, the observers do not have training on how to measure hailstones, and therefore in most cases, the hail size is only estimated in comparison with another known object like a golf ball or better still the hail is measured using calipers or a ruler.

Hail size and the number falling differ over a distance of a few hundreds of meters—not forgetting that it melts, and this makes it even more challenging to observe.

Obviously there are many more scientific ways to measure hail such as deploying networks with impact sensors, utilising only trained weather observers, or gathering data from hailpads though these are rarely deployed for extended areas.

Remote sensing observations such as radar and satellite can also be used to report the presence of hail; however the information we receive does not carry the precision required to determine the true size of hail.

Environmental factors triggering hail occurrence, size, frequency and magnitude

The real drivers of hail formation haven’t been understood, simply because of observational limitations. Of course we know that powerful, extensive thunderstorms updrafts are required in order to move the small nuclei that produce each hailstone. However, large hailstones can’t be produced with this alone.

A change in the speed, direction and height of wind is required, which enables storms to arrange, live longer and more importantly rotate. This mid-level updraft rotation seems to be extremely crucial in hailstones that are suspended at almost constant altitude, thrashing earlier misconceptions that hail repeatedly move up and down within the storm’s updraft.

A recent model suggests that 7.5cm of hail only takes a few minutes to grow, and that the horizontal wind patterns within the updraft are quite important. Even though other factors like details of the microphysical processes are also considered crucial, we can’t put our finger to their true relation with hail growth.

How accurate can the frequency, severity and distribution of hail be forecasted?

Forecasting hail has always been challenging. We can rely on the conditions favouring storms that produce hail to forecast hailstones a day or two ahead, despite being with little if any idea about the size.

When the event occurs, we can engage high-resolution models to estimate hail size and growth, or new machine learning approaches to forecast probable swaths of hailstone and its size. Once a storm has been produced, the focus then turns to the radar products and possible satellite information that can approximate where hail is likely to fall. Techniques used to integrate these conditions with observations promise better skills over single sources.

Regardless of these recent innovations, our capacity to predict hail size and occurrence with specificity is quite low and there’s a lot more to be learnt.

How will the ongoing climate warming influence hail and its impact?

This is quite a challenge given our unreliable observations and partial understanding of hail forecasting, however an increasing number of studies point to a shift to larger-sized hail in a warming globe, but not uniformly.

For instance, we know that in North America, tiny hail seems to decrease compared to the southeastern parts of the United States as it melts away when it falls through the warmer, deeper layers, though the frequency of larger-sized hail is expected to grow in Southern Canada and in the Northern Plains.

Already, there’s solid evidence confirming increase in the frequency of larger hail in both Europe and North America, while in China the frequency of larger hailstones has seemingly decreased.

These uncertainties are worsened by the lack of knowledge regarding environmental controls on hailstone sizes as mentioned above.

Unresolved questions where additional data, research or modelling is required

Of all the hail impacts, there are several things that remain unknown about these storms including the lifecycle of both the hailstones and the storms, to embryos emanating from it eventually developing into destructive hailstones, to details of microphysical processes that occur along their paths in the cloud.

Other uncertainties can be seen on the surface, where a better understanding of hail size distributions, physical properties of hail are required alongside the understanding of how they fall and the impact of this on human lives. Without aerial observations in and around hailstorms, some of these questions can’t be answered.

More largely, we should first gain a better understanding of where, when and how hail occurs across the globe in order to accurately understand the expected hazards, and better prepare our properties, structures and farms to mitigate its effects in the current and future climate.


This material is partly based upon work supported by the National Science Foundation under Grant No. NSF-AGS1855054. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of Storm Assist.

The information contained in this article is of a general nature only. It does not take your specific needs, objectives or circumstances into consideration, and is not financial advice, legal advice or otherwise a recommendation to purchase any financial product or insurance policy. You should seek your own independent financial advice from a qualified financial and insurance adviser before making any financial decisions, and seek your own independent legal advice from a qualified solicitor before making any decisions of a legal nature.

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