Today’s rain won’t fall tomorrow. But when will it?

It’s a well-known Dutch expression: The rain that falls today, won’t fall tomorrow. A hopeful idea on a rainy day, especially if you’ve got something planned for the next day. But what if you’re planning to go on holiday to a place where it’s still raining cats and dogs? Can you also say: The rain that falls today, won’t fall next week? We asked Aarnout van Delden, associate professor in meteorology.

“In the summer, the atmosphere over the Netherlands contains about 24 kg of water per square metre of land surface,” Van Delden explains. “If all that water rains out, it produces 24 mm of precipitation. If that precipitation falls in half an hour, we call it a ‘downpour’, which can cause flooding. If it falls in a day, we call that a ‘rainy day’. Good for the plants, but no big trouble. The atmosphere is then completely rained out.”

Nine days

“Through the evaporation of surface water and evapotranspiration by plants, the water supply in the atmosphere is replenished. This takes energy and therefore time. On a global and annual average, the daily water evaporation rate is approximately 2.7 kg per square metre. So it takes almost nine days to replenish the water supply in the atmosphere to 24 kg per square metre,” Van Delden calculates. “That means that on average, water in the atmosphere stays there for about nine days.”

Water from the North Sea

Does that mean you could say: The rain that falls today, won’t fall in the next nine days? That’s putting it too simply, says Van Delden. “Replenishing the water supply over the Netherlands does not only happen through evaporation. The atmosphere above the Netherlands also receives a lot of water from the North Sea. As a result, the water supply in the atmosphere above the Netherlands can be replenished in a much shorter time.”

This accelerated supply of water in the atmosphere can have disastrous consequences. “That’s what happened in July in the Eiffel, the Ardennes and the Vosges. The water in the atmosphere above these mountain areas was replenished by the circulation from the North Sea and also from the Baltic Sea at a very fast rate. The huge precipitation amounts of 200 mm in two or three days over a very large area were the result of a steadfast circulation pattern: a stationary cyclone over Germany and its surroundings.”