Hailstorms rank among the costliest natural catastrophes in Europe and have become a growing concern for insurers and society alike, with record-breaking events, such as the July 2023 storms in Northern Italy, causing over $3 billion in damage.
While the global rise in hail-related insured losses is primarily driven by increasing exposure and economic growth, the severity and frequency of recent events have amplified concerns about how climate change may be influencing hail risk. Some studies suggest that hailstorms may produce larger hail more frequently in a warming climate, but the science remains far from settled.
A global review by Raupach et al. (2021) summarized the state of knowledge: while small hail may become less common due to increased melting, large hail is expected to become more frequent, driven by stronger updrafts and greater atmospheric moisture. However, Raupach also highlighted that the trends are highly uncertain because changes in other factors influencing hail potential were less understood, such as storm types, storm frequencies and aerosol concentrations.
Proxies reveal important trends despite limited hail observations
One major challenge in understanding hail trends is that no single data set captures all the key aspects of changing hail risk. Hail reports and insurance claims can offer some of the most direct measures of impact, but these are often sparse in rural areas or influenced by changes in exposure and reporting practices. As a result, researchers typically rely on proxy data to infer trends in hail occurrence and severity.
For example, lightning activity over the last two decades shows negligible or even negative trends over large parts of central Europe. However, less lightning does not necessarily mean lower hail risk: if individual thunderstorms become more intense, they could produce larger and more damaging hailstones even as storm frequency declines.
Indeed, the high-density networks of hailpads (a scientific instrument used to measure the size, density, and kinetic energy of hailstones during a storm) in Northern Italy and Western France indicate a slight shift towards fewer but larger hailstones over a similar period.
Another proxy indicator is the atmospheric environment of thunderstorms, which has become more supportive for large hail because a warmer atmosphere holds more moisture and generates stronger convective updrafts, both favoring larger hailstone formation. While this thermodynamic relationship is well understood, proxies cannot fully represent the complex evolution of hailstorms, so additional approaches are needed to develop a robust understanding.
Models point to stronger hailstorms
The development of models with horizontal resolution of a few kilometers enables simulations of individual thunderstorm updrafts, a major step forward in hail research. However, this "convection-permitting" resolution is not yet fine enough to fully resolve the complex structure of storms, which is important because the shape and peak intensities of thunderstorm updrafts are critical for large hail formation. In addition, most models do not simulate the detailed microphysical processes involved in hailstone growth and evolution.
Keeping these caveats in mind, simulation studies focused on specific events or thunderstorm episodes consistently suggest that future hailstorms will feature stronger updrafts, greater water content and thus produce larger hail on average.
Extended-period model simulations, although computationally expensive, are also more recently being conducted by several research groups. Preliminary results for Europe using sophisticated hail diagnostics show regionally varying trends, with some areas indicating an increase and others a decrease in hail risk. A similar pattern has been found in the United States.
This apparent divergence reflects the fact that hail risk is influenced by several competing factors. While stronger updrafts in a warmer climate favor larger hailstone growth within individual storms, changes in storm frequency, storm type and melting processes can vary significantly across regions. Studies disagree on what factors are most important in what region. Moreover, while high-resolution models represent a major advance in hail research, they still have important limitations.
Nevertheless, what these models do consistently show is that thunderstorm updrafts tend to support larger hailstones, especially in already hail-prone regions in Europe such as around the Alps.
