What temperature must Barcelona reach to speak of a heatwave

Meteocat uses statistical criteria to define when a temperature is exceptional. It is not enough for the thermometer to show a high figure: a specific threshold must be exceeded, calculated for each point in Catalunya according to its own temperature records.

That threshold corresponds to the 98th percentile of the daily maximum temperature in June, July and August over the last 15 years. Put simply: only the hottest 2% of days in that period are above that value.

In Barcelona, the daytime threshold for strictly speaking of a heatwave stands this summer at 34.3 degrees. For the episode to fall into that category, that temperature must be reached or exceeded for three consecutive days or more.

The figure is not fixed. Meteocat updates the thresholds every year to reflect how the recent climate is evolving. That is why a temperature that would have been highly exceptional a few years ago may now be part of a new, warmer normality.

The change is particularly noticeable in cities such as Barcelona, where heat does not depend solely on the daytime maximum. Tropical nights, humidity, asphalt and the lack of ventilation in some homes can make an episode very harsh even when it does not technically fit the definition of a heatwave.

There may also be intense-heat warnings in some areas and not others. Catalunya does not operate as a single block: the coast, the interior, the Pyrenees and major urban areas have different thresholds. That is why an episode may strongly affect Lleida or places inland without Barcelona reaching its own official limit.

When the heat lasts less than three days, it is usually described as a heat spike or heat episode. If it is brief but very intense, some meteorologists use expressions such as “flaix de calor”, especially when a westerly wind pushes temperatures up suddenly in the city.

The Agencia Estatal de Meteorología uses a similar, though not identical, criterion. Aemet considers a heatwave to be an episode of at least three days in which at least 10% of its reference stations exceed the 95th percentile of July and August maximum temperatures in their historical series.

That difference explains why warnings or labels sometimes do not match exactly between bodies. Meteocat works with thresholds closely tailored to Catalan territory and a moving reference covering the last 15 summers; Aemet applies a national methodology to identify episodes of wider scope.

For people living in Barcelona, the important thing is not only to know whether a heatwave has been declared. It is also worth looking at intense-heat warnings, forecast maximums, overnight minimums and the duration of the episode. A single very harsh day can disrupt work, travel and rest.

Weather information thus becomes an everyday tool. It makes it possible to decide whether it is advisable to run errands earlier, avoid the middle hours of the day, seek climate shelters, reorganise outdoor tasks or pay closer attention to children, older people and people with health problems.

Barcelona's summer is no longer measured only by beaches or terraces. It is also measured by thresholds, warnings and nights when the city does not quite cool down. Understanding when heat is exceptional helps us not to minimise the risk, but also not to mistake every sweltering day for an official heatwave.