Weather conditions change over the years, so after decades, so does the climate. An investigation carried out by the Spanish and French has focused on the study of the precipitations in the form of hail registered in the south of France in the last decades.
The study, published in the journal Atmospheric Research, examined hail records from 1948 to 2015. What results did it obtain, and what is their significance?
Hail precipitation
The study observes a shift towards an atmospheric environment that favors an increase in this meteorological phenomenon, but researchers believe other factors could mitigate it, meaning that only storms with larger hailstones are actually increasing in frequency , while weaker hailstorms are decreasing. This is also related to the study of hail events in Spain.
Because the meteorological conditions that form hail are quite unstable and irregular in space and time, it is quite complex to have a complete database in order to study its evolution and trends.
This study was carried out by the Atmospheric Physics Group of the Institute of Environment of the University of León, together with the Complutense University of Madrid and Anelfa, a research center in Toulouse.
For the aforementioned reason, the study focused on French regions where continuous and uninterrupted data exist for more than 25 years. Anelfa has more than 1.000 stations that measure hailfall. From there, statistical techniques widely used in climatological studies were used to calculate trends. Furthermore, it has been integral to understanding the context of these precipitations.
Records and data
In the Pyrenees region, the frequency of hailstorms has increased over the last 25 years, according to the areas analyzed. This data cannot be extrapolated to other nearby regions because the conditions for hail formation are highly irregular. If the precipitation were in the form of rain, it would be possible to determine the rainfall patterns of the regions near those analyzed.
To try to reach some more solid and generalizable conclusions, given that Spain lacks such continuous data and records on hailstorms, the aim has been to find relationships between atmospheric fields and hailfall. This is also related to the phenomenon of hail and its characteristics.
Thus, the study analyzed the trends in atmospheric conditions that are more prone to and conducive to hail formation. The results show a significant trend over the last 60 years toward environments more favorable for hailstorms. This is also related to the measurement of hail and its characteristics.
However, this trend should not be interpreted as an increase in the frequency of hail registered on the ground, since there are many more factors to take into account, such as the melting of the hail as it falls from the cloud. Many precipitation events in the form of hail do not end up reaching the ground because they return to a liquid state before falling to the ground.
Due to global warming, environments and conditions more conducive to hailstorms are becoming increasingly frequent. It's also important to note that, with global warming, the snow and freezing levels are rising. This level is known as the zero-degree Celsius isometer, the altitude at which temperatures reach zero degrees Celsius and above which hail begins to melt. This is relevant to hail forecasting and how to prepare.
This results in a greater number of storms with the potential for hail, but in many of them the hail melts before reaching the ground, and only the most severe storms with the largest hailstones actually reach the surface. If you're interested in this phenomenon, you can read about the differences between hail and snow.
Hail and global warming
The uncertainty of the frequency of hail is difficult to transfer to global warming scenarios, since it is difficult to predict with models the reliable trends of this phenomenon.
In a warmer atmosphere, there is more energy available for deep convection, which favors the formation of storms with the potential for hail. However, at the same time, the rise in the freezing level promotes the melting of hailstones, making them less likely to reach the ground. It is difficult to determine which of these two factors has a greater influence on hail formation.
