How do you forecast rain

Weather report Why is rain so difficult to predict?


Precipitation - for meteorologists it is one of the most complex issues. And if they are pretty much committed to temperatures, then they only trust themselves to be probable when it rains. Let's take an example. The forecast is: Zeitz, Wednesday, 10:30 a.m., 30 percent probability of rain. But what does that actually mean? Reinhold Hess, data expert at the German Weather Service (DWD) has the answer:

It is said that in 100 comparable situations it has rained 30 times in the past.

Reinhold Hess, DWD

In other words. This statement about the weather is not a statement about the future, but a statement about how it was in the past with comparable weather constellations. To do this, the experts at DWD use the full capacity of their 40 million euro computer system.

It is not enough to look at the clouds for precipitation, because “Precipitation is one of the most complicated processes in the atmosphere,” says Detlev Majewski, head of the DWD's Meteorological Analysis and Modeling department.

Thanks for the subjunctive

Let's start with the clouds. The droplets that make them up are much smaller than the raindrops. In order for a raindrop to fall to the ground, many cloud droplets must first cluster together. Dust particles in the air can make them do that. This also applies to snowflakes. Or there are just a lot of drops so that they also come together to form raindrops. But a lot can still happen on the way down. The meteorologists have to calculate the wind, which can carry the drops far away. And of course the temperatures too. Is it so warm downstairs that the drops evaporate again? Or do they all make it down?

“The smaller the area, the less likely it is that the forecast will be correct,” says statistics expert Hess. If the area is larger, the forecast is better, "but then it is of no use to anyone". Anyone who wants to know whether he can go for a walk on the banks of the Elbe in Radebeul without an umbrella today does not use a forecast for the “Dresden area”. But in spite of gigantic computing power, nature always pushes us to our limits here. There are simply too many factors, Antje Khamis from the MDR Wetterstudio knows them from her work and has learned to live with probabilities. "We have to say thank you every day that the subjunctive is there."