It was hot in Germany last week, very hot. But were the temperatures really “record-breaking”? Even before the heat wave had reached its apex, newspapers began to pose the question: how high, exactly, must temperatures climb before we can speak of “record heat”? To find an answer, we must know how high temperatures have climbed in the past – a task that can be surprisingly complicated.

Temperatures are recorded in specific locations, and international guidelines for the recording process are well established. To determine whether a new temperature record has been broken, one must measure the temperature in a given location over a long period of time under stable conditions. One example: in Münster, a city in the Northwest of Germany, the thermometer recorded a temperature of 37.2 degrees Celsius at the city zoo last week, higher than ever before. The problem: during the previous heat wave in the summer of 2003 – when many weather stations in Germany recorded record-breaking measurements – no recordings were taken at that location. The old weather station at the zoo had been closed several years before when a more modern station opened twenty kilometers away at the airport, and the new private weather station didn’t exist yet.