“The persistence and extent of the high, which also extended over the North Sea, making room, as it were, for the large volume of water flowing out of the Baltic Sea, is responsible for the scale of the phenomenon,” he says. “We are talking about 275 cubic kilometres of water! This exceptional situation did not happen in isolation from the large-scale processes we observe in the Earth’s atmosphere. The most important of these in this context is the disintegration of the polar vortex, a circulation of air in the upper layers of the atmosphere (10-50km) which, colloquially speaking, is responsible for keeping the Arctic cold. This vortex is linked to the jet stream, the rapidity of which and the course of this current are responsible for the migration of lows and highs. Hence, deviations such as blocked highs, Arctic frost waves or heat waves in the north are the result of disturbances to this jet stream, and hence the warming of the Arctic.”