Heatwave and city, adaptation and resilience in the face of the new summer nightmare

The summer of 2021 will be remembered, among other things, for its high temperatures. These records, moreover, have been set in countries unaccustomed to the summer heatwave. These include Canada and the United Kingdom, where the National Met Office activated an amber alert for extreme heat for the first time in 167 years.

Is it a foretaste of the normality we are going to experience from now on? Is the lasting heatwave going to become the new summer companion?

Summer in the city, from pleasant scenery to canicular nightmare

If you frequent social media, you may have seen comments along the lines of “it’s always hot in summer”. But one thing is hot and another is HEAT, in capital letters. And we believe that the close to 50 ºC that some urban areas have reached this summer is closer to the second meaning.

Urban areas usually have a higher temperature than surrounding areas. This is known as a “heat island“. This is a meteorological phenomenon that arises as a consequence of the artificialisation of surfaces. The change in land use, with a transformation in which asphalt materials replace vegetation, facilitates the absorption and retention of heat. And the result, as explained by Masson, Lemonsu, Hidalgo and Voogt (1), leads to:

  • Higher temperatures, which in larger cities can reach up to 10°C.
  • Alteration of wind conditions, inducing, in light wind conditions, a sea breeze-like circulation that can contribute to the recirculation of pollutant emissions.
  • Modification of precipitation and cloud cover regimes. Although this point is still under debate, recent studies (2) suggest that heat islands do indeed destabilise the local atmosphere and can give rise to extreme precipitation, often leading to flash floods.

But what happens when a heat wave is added to this situation?

Heat island and heatwave, a combination that does not bode well

Heat waves are a threat to human health. In the United States, they are the most deadly extreme weather event. Add to this the fact that cities are highly vulnerable to these extreme temperatures and that forecasts point to an increase in high temperature episodes, and the combination of factors does not bode well for the future.

Some studies (3,4) suggest, in fact, that the combination of urban heat island and heat wave generates synergies. In other words, the consequences of their simultaneous occurrence are greater than the sum of their parts. Thus, for example, heat waves not only increase ambient temperatures. They also intensify the difference between urban and suburban temperatures. Consequently, the added heat stress in cities will be even greater than the sum of the background urban heat island effect and the heat wave effect.

Health implications

A body exposed to a high temperature is at risk of heat stress. And the consequences are aggravated if the ambient humidity is high. The evaporation process of perspiration, the cooling mechanism used by humans, is hindered if the atmosphere is saturated with humidity.

But other factors that increase the risk can also be distinguished (5):

  • Intrinsic factors, such as age, the existence of chronic illnesses, pregnant women, etc.
  • Extrinsic factors, including socio-economic aspects, educational level, the urban structure of the different neighbourhoods of the city and the materials used in the construction of housing, the existence of green areas, etc.

Air quality also suffers during a heat wave. The main reason is usually an increase in tropospheric ozone levels, a secondary pollutant that forms in the presence of sunlight and heat. High temperatures also often lead to forest fires. In these cases, distance does not mean greater safety, as it has been shown that smoke can travel hundreds or even thousands of kilometres, affecting pollution levels in urban areas.

What benefits can a network of meteorological sensors bring?

It is therefore clear that one of the first steps to be taken is to monitor weather conditions.

There is no doubt that satellite-based early warning systems have improved the response to heat waves. These tools are, in fact, evolving to adapt to seasonal variations.

Complex urban patterns, however, require high-resolution atmospheric models. This is where sensor-based monitoring networks show their usefulness.

Traditional weather observation networks are designed to collect synoptic data, leaving out intra-urban analysis (6). But solutions such as our SmartyMeteo station, which are small in size and wirelessly transmit quality weather data, open up a new range of possibilities.

If we take into account the size of some metropolises (the urban area of New York, for example, is similar in size to the entire province of Zaragoza), this approach allows the creation of hyperlocal forecasts for the different neighbourhoods of the city (7).

But the massive use of these devices also allows for other applications.

Meteorological information for delimiting ventilation corridors

Understanding a city’s temperature variations helps to design adaptation strategies in the face of climate change. And how detailed the information is depends largely on the density of the monitoring network (no, digital urban thermometers do not count).

Sometimes it is sufficient to equip the urban area with low-cost devices, as was done in Bern (8). In this Swiss city, a pilot project tested the effectiveness of a series of LCD thermometers protected by solar radiation shields. The data obtained were compared with reference equipment (the team members emphasise the need for this process in view of the differences found, especially during the day) and showed that this approach allows a deeper understanding of urban climate dynamics.

How can these data be used? One possible application is to support modelling for the identification of ventilation corridors that favour air flow and green spaces that contribute to reducing temperature (9, 10), helping to reduce the heat island effect. This is an approach with enormous potential from an urban planning point of view and has already been implemented in cities such as Stuttgart.


The Earth is becoming a warmer planet. This is not an unknown fact, but the latest IPCC report published in August 2021 has reminded us of it. And everything seems to indicate that this circumstance is also going to be felt in the temperature of urban environments.

Monitoring technologies alone will not stop global warming. But they will allow us to put numbers, to put it bluntly, to the problem we face. And the more and more detailed the data, the better, because it will allow us to know in depth the particularities of each habitat and to take the appropriate measures.

What we measure today may represent our chances tomorrow. Remember this the next time you see a thermometer reflecting a scorching temperature.

Sources consulted

  1. Masson, V., Lemonsu, A., Hidalgo, J., & Voogt, J. (2020). Urban Climates and Climate Change. Annual Review Of Environment And Resources, 45(1), 411-444. https://doi.org/10.1146/annurev-environ-012320-083623
  2. Li, Y., Fowler, H., Argüeso, D., Blenkinsop, S., Evans, J., & Lenderink, G. et al. (2020). Strong Intensification of Hourly Rainfall Extremes by Urbanization. Geophysical Research Letters, 47(14). https://doi.org/10.1029/2020gl088758
  3. Ao, X., Wang, L., Zhi, X., Gu, W., Yang, H., & Li, D. (2019). Observed synergies between urban heat islands and heat waves and their controlling factors in Shanghai, China. Journal Of Applied Meteorology And Climatology, 58(9), 1955-1972. https://doi.org/10.1175/jamc-d-19-0073.1
  4. Li, D., & Bou-Zeid, E. (2013). Synergistic Interactions between Urban Heat Islands and Heat Waves: The Impact in Cities Is Larger than the Sum of Its Parts. Journal Of Applied Meteorology And Climatology, 52(9), 2051-2064. https://doi.org/10.1175/JAMC-D-13-02.1
  5. Fernandez Milan, B., & Creutzig, F. (2015). Reducing urban heat wave risk in the 21st century. Current Opinion In Environmental Sustainability, 14, 221-231. http://doi.org/10.1016/j.cosust.2015.08.002
  6. Meier, F., Fenner, D., Grassmann, T., Otto, M., & Scherer, D. (2017). Crowdsourcing air temperature from citizen weather stations for urban climate research. Urban Climate, 19, 170-191. https://doi.org/10.1016/j.uclim.2017.01.006
  7. Skarbit, N., Stewart, I. D., Unger, J., & Gál, T. (2017). Employing an urban meteorological network to monitor air temperature conditions in the “local climate zones” of Szeged, Hungary. International Journal of Climatology, 37, 582–596. https://doi.org/10.1002/joc.5023
  8. Gubler, M., Christen, A., Remund, J., & Brönnimann, S. (2021). Evaluation and application of a low-cost measurement network to study intra-urban temperature differences during summer 2018 in Bern, Switzerland. Urban Climate, 37, 100817. https://doi.org/10.1016/j.uclim.2021.100817
  9. Gu, K., Fang, Y., Qian, Z., Sun, Z., & Wang, A. (2020). Spatial planning for urban ventilation corridors by urban climatology. Ecosystem Health And Sustainability, 6(1), 1747946. https://doi.org/10.1080/20964129.2020.1747946
  10. Tomasi, M.; Favargiotti, S.; van Lierop, M.; Giovannini, L.; Zonato, A. Verona Adapt. Modelling as a Planning Instrument: Applying a Climate-Responsive Approach in Verona, Italy. Sustainability 2021, 13, 6851. https://doi.org/10.3390/su13126851

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