Forecast impossible Why do thunderstorms actually do what they want?
Even meteorologists don't know exactly when and where thunderstorms occur. But they can explain how farmers trigger thunderstorms and why you should never rely on weather apps.
It won't be really cold anywhere in Switzerland over the next few days. It will be mostly hot until Sunday, then it will be hot and humid. There will only be a few isolated thunderstorms to cool things down. They may occur more frequently over the next few days, the meteorologists predict.
However, they do not know where, when and how there will be lightning and thunder. It seems that forecasting thunderstorms is difficult, even for weather experts. Why is that? Why do thunderstorms seem to do what they want? Why can you only rely on weather apps to a limited extent? "blue News" asked Klaus Marquardt about this.
"A thunderstorm is a small-scale process," the meteorologist from Meteonews begins his explanation. "Heat storms in particular are dependent on many local factors." This is different from a cold front, for example, which extends over large parts of the continent and has a clear air mass boundary.
A farmer mowing his meadow ...
Thunderstorms, on the other hand, always occur locally. First of all, the set-up has to be right: Do the air masses even give rise to a thunderstorm? "Are the air masses unstable, is it warm enough? Is the humidity right?" says Klaus Marquardt, listing the questions that his guild asks itself.
"Then there has to be an updraft, a kind of initial spark that causes the warm air mass to rise." Sometimes it can make a difference whether a farmer mows his hay meadow, says Marquardt, explaining the effect with reference to paragliders and gliders.
"Thermal gliders in the lowlands in particular deliberately fly towards combine harvesters because the warm air that clings to the ground is released there. Thermals are created, then there is an updraft that transports the warm air upwards, and somewhere along the line a plume forms." A thunderstorm can then develop from this if the conditions are right.
Thunderstorms are like ping-pong balls
So far, so understandable. But why exactly don't meteorologists know where the thunderstorms will appear? "Thunderstorms develop a certain momentum of their own," says the weather expert. First, the formation of a thunderstorm cell is "nudged" and then everything happens on its own - depending on the given conditions and the fact that thunderstorm cells also influence each other.
Klaus Marquardt illustrates: "You can imagine it like this: If you throw a table tennis ball down a stairwell, it is clear that the ball will eventually reach the bottom. However, the actual path of the ball remains unpredictable: you simply don't know how it will bounce and where it will hit."
Meteorologists therefore work with probabilities when it comes to thunderstorms and only say that the risk of thunderstorms is increasing. "However, people have the idea that we can simulate everything from A to Z," says Marquardt. Which is simply not true when it comes to thunderstorms. "Everything somehow influences what happens: Relief, lakes, forests. That's why it's so exciting. And thank goodness not everything can be calculated down to the house number."
Hotspots in the Jura and the Alps
"In the mountains, for example, it depends on how sunny a slope is," explains the meteorologist. Typically, the Jura has a good orientation towards the sun, and there are also a few classic hotspots in the Alps where thunderstorms develop. It then depends on the high-altitude wind in which direction the thunderstorm cells move on, on the wind speed and on the wind direction at an altitude of 5500 meters, for example.
If there is no high-altitude wind, the thunderstorm cells remain stationary. "This is then problematic for all those who are there: because it's going full throttle all the time. A kilometer away, all you hear is a roll of thunder and at most you get a few drops."
At the moment, however, there is a high wind, especially from the southwest, says Marquardt. "It's also getting more rather than less. That means the thunderstorms are moving: They develop in the afternoon in the Jura, the Black Forest and the Alps - and can reach the Swiss Plateau from there." But it's impossible to predict whether they will hit Solothurn, Baden, Zurich or Schaffhausen.
"Better to use your common sense"
"No two thunderstorms are the same," says Marquardt, returning to the weather apps. "People still look at their smartphone and think: it's going to start at 5.30 pm and the first lightning will strike. That's complete nonsense, it has nothing to do with nature."
Especially as many free apps are based on weather models with a "rough mesh size of ten to 15 kilometers". Everything that happens in between is interpolated - i.e. mathematically extrapolated. "This means that the probability of thunderstorms in Cham is the same in the app as in Lucerne or Altdorf."
Of course, there are also apps with high-resolution weather models - but you shouldn't get too hung up on them either. "It's better to use your common sense," advises Klaus Marquardt. "If you can see in the sky that it's going to turn black with a south-westerly wind from the south-west, then you might as well not look at the app." No matter what it has predicted: In reality, a thunderstorm is on its way.
