Day and night heat waves in the city of Barcelona. 1971-2020.

There is no universal definition of a heatwave, but extreme events associated with particularly hot, sustained temperatures have been known to have a notable impact on human mortality, regional economies and ecosystems. In this paper, we use the concept of heatwave applied by the Spanish Meteorological Agency (AEMET). In this definition, a heatwave is considered an episode of at least three consecutive days in which the stations that are considered register maximums above the 95% percentile of the series of maximum daily temperatures for the months of July and August from the period 1971 to 2000. However, this definition has a major limitation: it refers only to maximum temperatures, not minimum ones. Maximum temperatures can have serious consequences, especially on heat stroke. However, the health effects are more pronounced in the case of night heat, where the inability to rest (especially in homes without air conditioning, as is generally the case in in Spain) can cause significant worsening of respiratory and cardiovascular diseases that produce a high proportion of premature deaths. For this reason, in this study we differentiate between heatwaves during the day (DHW) and at night (NHW), paying special attention to the latter.

The research aims to study extreme heat events in the city of Barcelona between 1971 and 2020. Since the urban climate presents a marked spatial variation, taking into account the geographical characteristics of the territory, as well as the spatial distribution of the island of urban heat, the research is carried out based on the information provided by four representative meteorological stations of the study area: Fabra Observatory, CMT, Raval and Barcelona Airport.

The maximum average temperatures at the Fabra Observatory, and for the last 50 years, increased 2.88 degrees, which represents an annual increase of 0.058 degrees/year. The minimum average temperature increased 2.38 degrees, 0.048 degrees/year. However, the increases differ very significantly depending on the spatial location of the meteorological station. The results are quite different for other observatories, as Barcelona airport. At the airport, the increase in maximum temperature was less prominent (2 °C in the last 50 years, 10.3%). However, the minimum temperatures increased by 35.8%; 3.62 °C between 1971 and 2020. An OLS model, with the maximum and minimum daily temperatures of the last 50 years from various stations (Fabra, Airport, Raval and CMT), and using the year, the month and the calendar day (cd *) as explanatory variables, generally confirmed the warming process in the Barcelona area.

Therefore, global warming is a clear reality in the Mediterranean area in which Barcelona is located. The work shows a marked difference in extreme heat events between different urban locations. The proximity to the sea, the altitude, the different urban density and the quantity and quality of urban greenery have a determining effect on daytime and nighttime heat waves.