Sudden freezing rain

The year 2013 will never be forgotten by climate researcher Michael Kunz from the Karlsruhe Institute of Technology. Because his specialty is hail. And hail the size of a grapefruit then fell in southwestern Germany. At Reutlingen, a weather expert picked up a lump of ice with a 13 diameter – a new record . According to the insurance companies, the damage afterwards amounted to over a billion euros.

Modern buildings in particular are poorly equipped against the ice levels. Slat structures and blinds shatter easily. The hailstones hit deep craters in facades when they are insulated with polystyrene panels from the outside. The sensitive surfaces of solar cells and solar collectors on the roofs are also at risk.

“Because modern buildings are less hailproof, the damage to the infrastructure has now far exceeded that of agriculture,” says Kunz. “In some regions of Central Europe, hail is the most damaging natural event before flooding.” And that, although the freezing rain is often only on a strip from a few hundred meters to at most 20 kilometers wide. After a few minutes, the clouds have already moved on.

Working on a hail forecast

But weather services are still unable to predict hail. “If we could warn at least ten minutes in advance, people could drive their car into the garage and pull up the blinds. That would prevent the worst, ”says Ulrich Blahak from the Research and Development department of the German Weather Service.

[Wenn Sie die wichtigsten Nachrichten aus Berlin, Deutschland und der Welt live auf Ihr Handy haben wollen, empfehlen wir Ihnen unsere runderneuerte App, die Sie hier für Apple- und Android-Geräte herunterladen können.]

“However, there are no other weather phenomena that we understand as badly as hail and tornadoes,” says Kunz. The ignorance starts at the weather stations. Because hailstorms are so small, they do not systematically record the icy precipitation and are therefore not equipped with suitable instruments.

Die Größe der Hagelkörner könnte im Zuge des Klimawandels künftig zunehmen, vermuten Forscher.

The size of the hailstones could increase in the course of climate change, researchers suspect. Photo: picture alliance / dpa

Researchers and weather services are now working on a hail forecast for the first time. Switzerland is currently setting up a hail measurement network for this purpose. The German Weather Service is working on improving the weather simulations in order to better predict small-scale and fast phenomena such as hail.

Lightning signals the hail

A new approach to predicting is to measure the lightning from the storm cells. According to international research, if there is a sudden increase in lightning intensity, this indicates an imminent hailstorm. Because the growth of the ice balls in the cloud causes an increased charge separation and thus more lightning. “We are currently researching whether this is reliable enough,” says Blahak.

Even if lightning should reveal the hail, accurate prediction should remain the free choice of meteorology. So far, the computers of the German Weather Service spit out a new simulation of the weather situation over Germany every three hours. But hail occurs in periods of minutes.

“We are working on shortening this cycle and moving more towards real time,” explains Blahak. But that alone is not enough. The forecast only provides results every two kilometers. This mesh size is too coarse for small-scale hailstorms.

17 Radar sensors from the German Weather Service already detect from the ground when hail reflects the radar radiation at a height of a few kilometers. “If we see hail up there, we warn – so that's basically when it's hailing,” says Blahak. It is not a forecast, so to speak, but only a weather report that can be wrong. When the grain of ice melts in the air, rain sometimes arrives on the ground.

Die Erderwärmung führt zu wärmeren und feuchteren Luftmassen über dem Hagelhotspot Zentraleuropa. Dadurch nimmt die Wahrscheinlichkeit schwerer Gewitter zu.

Global warming leads to warmer and wetter air masses above the hail hotspot Central Europe. This takes the … Photo: DPA

In order to better understand hail, the German Weather Service will in future also call on citizens to report, Blahak announces. MeteoSwiss has been practicing this successfully since 2016. Confederates report when it is hailing and can tell the approximate size of the grains.

The Swiss colleagues are now also driving a hail forecast. Measuring stations for hail on the ground are currently being built in Switzerland 80. They carry a membrane that causes hail to vibrate. A microphone records the drumming of the ice grains and transmits it to MeteoSwiss. The data can be used to determine the size of the grains and the force of their impact, among other things.

Extreme hail could become more common

Common come the ice projectiles with 100 to 200 kilometers per hour as fast as a motorway driver. The duration of the hail and its frequency can also be derived from the noise.

The call for more hail prevention is also becoming louder because climate researchers fear that extreme hail will occur more frequently in the future. “It is clear that global warming leads to warmer and wetter air masses above the hail hotspot in Central Europe. This at least increases the likelihood of severe thunderstorms, ”says Kunz. But does that mean more hail?

While researchers at Munich Reinsurance are already expecting an increase, the Karlsruhe climate researcher is reluctant. “At the moment we cannot say that for sure.”

The grains are getting bigger and bigger

Hail sensors are detected in neighboring countries, France and Spain since the 80 the rare weather event. There the colleagues around José Luis Sanchez from the University of Leon came to the conclusion: hail itself is not becoming more common. But the grains are getting bigger. Italian researchers also provide such clues.

The last word is still not spoken. The formation of hail in the cumulus clouds is too complicated for that. This can only occur if the right number of ice nuclei is present, explains Kunz. Pure, distilled water would only freeze at around minus 38 degrees Celsius. However, if there are some bacteria in the cloud, the water freezes at around zero degrees.

Microbes are a real hail catalyst. Fine dust in the atmosphere produces hail from minus 10 degrees Celsius, pollen at minus 15 degrees Celsius. So what falls to the ground is also a question of atmospheric pollution.