MADRID – In Valencia, the ‘heat island effect’ can make the city up to 11 degrees Celsius warmer than outside the city. Researchers looked at this phenomenon in the Spanish cities of Madrid, Barcelona, Valencia, Murcia and Málaga.
Heat islands are cities where the temperature is higher simply because of their urbanised character compared to less urbanised areas on the periphery. The effect can turn cities into real ‘furnaces’ in the Spanish summer.
However, not all ovens are equal and heat up in the same way, at the same time or with the same intensity. Therefore, researchers at the ISCIII decided to make a comparison between the phenomenon of heat islands in the cities mentioned.
In which city is the thermal island effect the greatest? And when does it occur? Is the effect greater in one city than in another? Does the phenomenon have health consequences and can it lead to hospitalizations and deaths?
The study was published a few days ago based on data collected between 2014 and 2018 during the summer months (June to September). The five cities were selected because they are the most densely populated cities. In addition, they have weather stations from the Spanish weather agency Aemet both inside and outside the city. Therefore, that makes a good comparison possible.
Heat island effect only occurs at night
The first conclusion of the study is that “the urban heat island effect only occurs at the minimum temperatures”, ie at night or rather early in the morning. Usually, the minimum temperature is recorded around 7.00 am.
This effect was observed more in Valencia than in Madrid and in the other cities studied. In Madrid, the temperature difference between the minimum temperature within the city (measured at the Retiro Observatory) and outside the city (in El Goloso) averaged 1.3 degrees. In other words, “on average, Madrid is 1.3 degrees warmer at night than outside the city”.
However, at some times and in some circumstances, the difference is greater. The study even measured a difference of 7.1 degrees between the minimum temperatures inside and outside the city. But according to the researchers, the same is not observed at the maximum temperatures.
Valencia is the city with the largest heat island effect
The city with the most pronounced temperature difference and therefore the largest heat island effect was Valencia. Here the researchers recorded an average temperature difference of 4.1 degrees between the nightly minimum temperatures in and outside the city. In this case, the city observatory is close to the sea, and the observatory outside the city is inland. According to the study, the temperature difference between the two points can be as much as 11.2 degrees.
Coastal cities are even more ‘ovens’
Something similar happens in Barcelona, where the urban heat island effect marks an average difference of 3.2 degrees in nighttime minimum temperatures.
In Málaga, there is not that much difference in the minimum temperatures, an average of 1.9 degrees between the city and the suburbs. However, in some circumstances that difference can be as much as 9.5 degrees.
In Murcia, the heat island effect is not very noticeable (an average difference of 1.2 degrees), but at times that difference increases to no less than 7.4 degrees.
“The effect of the heat island is much more pronounced in the coastal cities studied (Valencia, Barcelona and Málaga) than in the inland cities,” Díaz summarises.
Why is the urban heat island effect worse in coastal cities?
“In coastal cities, there is a lot of water vapour and the humidity prevents the temperature from dropping less at night.” But there are more factors that influence the intensity of the heat island effect: whether it is windy or not, whether it is cloudy or clear… Not all of these factors are meteorological in nature.
The researchers explain in Nius that “the city itself also plays a role: its layout, its size, the type of buildings, the number of green zones…”. The sun warms the buildings and streets during the day, and these contribute to the increase in temperatures at night. Then all that accumulated heat is released. In addition, the traffic plays a role, and the asphalt and the number of air conditioners. Moreover, these many factors together can turn cities into veritable ovens during the night hours.
The heat island effect and health
Another question the researchers wanted to answer is ‘how does the heat island phenomenon affect health?’ They already know it affects heat-related hospitalisations and deaths, but it’s not happening the same way in the cities studied.
Statistically, the study was unable to find a link between the heat island effect and death rates or hospitalisations. In Madrid, the daily maximum temperature appears to be most associated with heat-related deaths and hospitalisations. The same applies to Murcia.
Because it has also been concluded that the maximum temperatures are not affected by the urban heat island effect (only the minimum temperatures), so in both Madrid and Murcia this effect has “no influence on mortality rates or hospital admissions”. Residents may have trouble sleeping in the heat, but according to the ISCIII study, it has “no impact on health”.
Effect noticeable in Valencia and Málaga
In the cities of Valencia and Málaga, on the other hand, it does have an influence. “In those cities, the daily minimum temperature is the only one associated with heat-related deaths and hospitalisations.” The heat island effect is therefore crucial here. In Barcelona, the study shows that both maximum and minimum temperatures are important in explaining hospitalisations and deaths due to heat.
Adjustment must be done locally
The researchers conclude that local studies must be conducted to understand which variables influence mortality rates and hospitalisations. Furthermore, heat prevention plans must then be tailored to each specific city. So one cannot speak about the urban heat island effect in general.