Extreme events such as heat and cold waves are of great importance for life on earth and also cause high economic damage. They also play a major role in the discussion of climate change, as climate model calculations predict a change in frequency, duration and intensity of such events.

In the years 2010 and 2011, there were a number of events in Europe that occurred as extreme temperature and rainfall anomalies. Examples include the cold winter of 2009/2010 and December 2010, particularly in Northern and Western Europe, the severe precipitation events in Eastern Central Europe in May, August and September 2010, the heat wave in Eastern Europe in summer 2010 and the drought in much of Europe in February until May 2011. All these events occurred over relatively large areas and continuously. They therefore had a more or less pronounced effect on the annual mean of the relevant climate elements in 2010, in some cases even on the global average for 2010.

For climate monitoring, it is important to record the characteristics of these events (in particular duration, intensity, spatial extent) in order to be able to compare them with other events and to make reliable assessments of the past and possibly future occurrence of such events.

Appropriate criteria are needed to identify a heat or cold wave. In a climatically heterogeneous area such as Europe, it is a major challenge to find a uniform definition which can be applied to the entire continent. In practice, a number of very different definitions of extreme events are used from country to country, depending on the local climatic conditions and the special user requirements.

For the climatological assessment of a heat / cold wave, it is important to compare current heat and cold waves with earlier corresponding events. Therefore, a representation was chosen which displays not only current, but also historical heat waves and their properties (duration, intensity, spatial extent) on a diagram. Of interest here are above all large-scale heat and cold waves, which extend over several countries within Europe.

This representation was taken over from the French weather service Météo France, which had originally applied it to France. It has been adapted for the area of WMO Region VI, Europe.