The researchers Jordi Mazon and David Pino of the UPC's Department of Applied Physics and Mariano Barriendos of the UB's Department of Modern History propose the name flash heats to describe increases in temperature—usually accompanied by decreases in humidity—lasting more than two hours and less than two days. Though these anomalous temperature episodes are a recognised phenomenon, there is currently no standard scientific name for them.

The World Meteorological Organisation defines heat waves as periods of anomalously high temperature of at least 5°C above average that last between two days and several weeks. It defines heat bursts as similar episodes lasting between a few minutes and two hours.

Phenomena within the macroscale and the microscale are thus internationally recognised, but no name has yet been agreed for phenomena within the mesoscale. The researchers considered that little attention had been paid to the temporal and spatial resolution of these phenomena, which can affect health, agriculture, and the study of climate change and energy consumption. Their proposal was to call them flash heats because they cannot be considered heat waves or heat bursts.

The research used instrumental analysis and numerical simulation based on temperature and humidity data taken on two exceptional dates in Barcelona and in Heraklion, Crete, Greece. On 27 August 2010 the temperature in Barcelona reached 39.8°C at sea level: a record average maximum temperature in the 230 years since records began to be kept. The episode lasted less than 10 hours, and the central peak about 5 hours. The episode was defined as a 'small heat wave' in the absence of a specific name for this phenomenon. On 21 March 2008 the temperature in Heraklion reached 34°C during the early hours of the morning in an anomalous temperature rise lasting 12 hours.

Both cases involved a rise in average temperature of over 5°C and were caused by the rapid movement of a ridge of hot air from North Africa. A ridge is a region of the atmosphere in which the pressure is high relative to the surrounding region at the same level. It is represented on a synoptic chart by a system of nearly parallel isobars or contours which are concave towards an anticyclone. The emergence of the African ridge in the Iberian Peninsula involves the entry of hot air and consequently a rise in temperature.

Flash heats can be caused by a marked and persistent Foehn effect, which consists of a warming and drying of the air on the leeward side of a mountain range when all the moisture condenses on the windward side. Ridges and the Foehn effect often involve temperature rises with a continuous duration of less than 48 hours, and therefore cannot be classified as heat waves.

Distinguishing between heat waves and flash heats can help to refine the study of atmospheric behaviour in the analysis of the dynamics and evolution of global warming and climate change.

In their article the researchers also analyse the potential impacts on agriculture, energy consumption, environment and health. Rapid and sudden increases in temperature involve the real risk of starting wildfires or causing health problems such as heat strokes. However, if the episodes last no more than two days, they cannot be identified as heat waves, so fire prevention protocols are not activated and citizens are not informed of the precautions to be taken to avoid heat stroke.

Flash heats can also have effects on energy consumption and cause damage to crops, both of which could be prevented through the recognition and further study of these phenomena. In addition, because flash heats are not recognised by the meteorological authorities, the damage they cause is not usually covered by insurance policies. In most cases, only episodes qualified as heat waves are covered.

The research was published in the article Rapid and sudden advection of warm and dry air in the Mediterranean Basin in the February issue of the popular science journal Natural Hazards and Earth System Sciences. The authors will now propose a new definition to the World Meteorological Organisation through the Meteorological Service of Catalonia and the Spanish Meteorological Agency.