After a very hot and dry summer months, a cold front caused thunderstorms and cooling to the lowlands. 20 to 30 liters were used on a large scale, in many places even about 50 liters were measured, elsewhere there was almost no rain. So how was the drought softened?
The rainfall totals are from Thursday, August 23, 2018 (Tue to midnight). Image: Meteonews
"Well, 50 liters is better than nothing," explains meteorologist Klaus Marquardt of Meteonews. Rain quantities between 10 and 30 liters are normal during thunderstorms. By comparison, the summer months are usually the wettest time in Switzerland because the hot air can hold more water. On the northern side of the Alps, 130 to 160 liters of rain per month would be normal. "But we only had about half of this time each summer," says Marquardt.
But such intense local thunderstorms are a poor replacement for widespread rainy days for two main reasons: the uneven distribution and poor water absorption. "If you look at the distribution, it looks like a leopard skin," says Marquardt. There are huge differences in a small space: some places get a lot of rain in a short time, others get less rain. "That was a small, poisonous stuff," Marquardt describes the thunderstorms of yesterday's Thursday night.
The thunderstorms brought about 20 to 30 liters: a picture of Emmen LU – in Lucerne 26 liters fell in the last 24 hours. (August 23, 2018) Image: Tamedia / Readers Reporter
Even in places with heavy rainfall, the "dry dry" soils can not absorb much of the rainfall. They run away, land in the gully or evaporate on warm ground – "a waste", the meteorologist sums up.
Lake Zug settled his historic low
The hydrologist Edith Oosenbrug of the federal agency for the environment explains the situation in the same way: "It is like a dry flowerpot," says Oosenbrug. The dried floors need a certain amount of time to get damp and thus restore their absorbency.
The fact that the ground does not like to hold the water is also demonstrated by the current water levels. After storms the water levels of the rivers increased again and again – especially for smaller water basins, this effect lasted only for a day before the levels returned to the level before the storm.
The runoff values of the rivers show thunderstorms as a short peak – here, for example, the measurement data of the Kleine Emme, other examples can be found at the Federal Office for the Environment. (August 24, 2018) Image: Bafu
The drought in Switzerland shows quite impressively on the water levels of the lakes. The level of Lake Constance is currently about one meter lower than would be normal in August. The lake normally only reaches a low level in December. Many lakes have already reached their winning level, explains Oosenbrug. Lake Zug even matched the historic low of 1949.
The water level of the Zugmeer. (August 24, 2018) Image: Federal Office for the Environment
In addition to the drought of the past few months, there is another reason for this, according to Oosenbrug. The warm spring had caused a strong snow, so now in the summer months less meltwater in the lakes of the flat land.
Created: 24.08.2018, 12:06 clock