How quickly can the weather change

Why is there no 100 percent forecast? | That is how probable are weather forecasts

"It is difficult to predict, especially when it comes to the future," Mark Twain (1835-1910), American writer

All over the world, supercomputers calculate how our weather will be. Meteorologists feed them with data from weather satellites, weather balloons, measuring buoys, weather stations, airplanes and and and - and then our barbecue party falls into the water because suddenly an unpredicted thunderstorm approaches.

Why is there no 100 percent forecast?

"Even from the very start, nature doesn't like to look at the cards," says meteorologist Dominik Jung (36) from "The atmosphere is a chaotic system."

To calculate the weather forecast, meteorologists always need the current state of the atmosphere. In plain language: as much measurement data as possible from weather stations all over the world.

Dominik Jung: “The catch is that two thirds of the world is covered with water, and the measuring network looks very thin. So we have no way of knowing exactly how the weather is everywhere and so we have a big mistake in the starting position right from the start. "

Predicting small-scale thunderstorm cells in particular is a headache for meteorologists, as these tend to slide through the grid of the computer models.And so it is never enough for a 100 percent forecast.

What can we expect from a good weather forecast?

According to the German Weather Service (DWD), a temperature forecast for local climatic conditions is considered “good” if it deviates by less than 2.5 degrees from the actual temperature. If the deviation is greater than 4.5 degrees, this is assessed as a "gross error".

Two-day predictions of the daily mean temperature are today on average about 1.3 degrees off. In the case of six-day forecasts, the deviation increases to just under 2.5 degrees.

The hit rates of the forecasts

The weather portal has examined how high the hit rate actually is in its own forecasts.

Nowcasting (period: 2 to 3 hours from now): The weather expert looks at the current radar image - and can see, for example, how heavily it is raining, whether it may even be hailing, and estimate where the whole thing is going in the coming hours.

Hit rate: 98 percent

Short-term forecast (3-day forecast): We know them from the daily newspapers or the TV. There the weather for the current day and the next two days is usually presented.

Hit rate: around 90 percent

Medium-term forecast (4 to 10 days): This is where things get trickier. Meteorologist Dominik Jung: “If you want to know exactly how the weather will be on Sunday, you should only look at the weather forecast from Friday. Before that, it can happen to him that the forecast changes wildly several times. "

Hit rate: Up to day 5 at 70 percent. On the 10th day only a good 50 percent - that is, just over the "chance".

Long-term trend (longer than 10 days): Here meteorologists try to estimate whole months and even seasons. Dominik Jung from “But from such a distant period you can really only give tendencies: Is it getting warmer, is it getting colder, is there a high or will the lows stay with us? You can then no longer really seriously say that it will rain in the morning of the 14th day and then sunny in the afternoon. That will not do!"

Whether long-term forecasts are reliable at all is hotly debated among weather experts themselves.

Hit rate: 50 percent - but it's not about precise predictions, only a relative assessment.

Difficult season

In addition, the quality of weather forecasts fluctuates over the course of a year.

In summer the weather conditions are more stable than in winter, and the forecasts are correspondingly more reliable.

It can be difficult to make predictions in the transitional seasons, for example in the typically changeable April weather. Or like this year with the normally not quite so changeable May weather.

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