What is an urban heat island?

miejska wyspa ciepła

Probably most of us observe how much the city where we live has developed in recent years. We also realize that inherent in this ongoing urbanization is the development of infrastructure. Currently, more than half of the world’s population lives in cities, but it is estimated that by 2050. It will be as high as 68%[1]. So we can easily imagine that the consequence of this process will be the growth of urban infrastructure, and thus an increasing share of artificial impervious areas. This is mostly at the expense of vegetation-covered areas, which provide cooling through evapotranspiration and shading.

The materials used to build urban surfaces (asphalt or concrete) have a high heat capacity and low albedo, so they heat up much faster than natural surfaces. Some of us may associate that pleasant warmth we feel, especially on a summer evening, as we enter from outlying areas to the residential areas. That’s what an urban heat island (MWC) is. The described phenomenon manifests itself in the occurrence of higher air temperature in the ground layer of the atmosphere in the city than outside it. The intensity of MWC depends on a number of factors, including astronomical (length of day), meteorological (cloudless and windless days), the geometry of urban urban structures (height and density of buildings) or demographic (emission of artificial heat into the atmosphere). MWC is described as a dynamic phenomenon, manifested by high seasonal and annual variability. It is usually identified with an island surrounded by a mass of cooler air, in fact, its shape depends mainly on the urban structure of the city, so in the case of polycentric metropolises it more resembles an archipelago than an island.

Ways to determine the urban heat island

By determining the MWC, we are determining the air temperature, which can only be studied by instrumental measurements, which are point information. Remote sensing provides the opportunity to determine the temperature of the active surface, providing data of a spatial nature so that we can determine the extent of the phenomenon. However, it should be remembered that air temperature and active surface temperature are two different indicators. At night they remain at a similar level, but during the day the temperature of the active surface is significantly higher. Hence, when we study air temperature, we speak of atmospheric MWC, while when we study active surface temperature, we speak of surface MWC (PMWC). These phenomena differ not only in terms of how they are identified. The most common measure of MWC intensity is the temperature difference between urban and non-urban areas. MWC is most intense at night, at dawn and during the winter season, reaching differences of 12°C, while during the day it may be little or nonexistent. PMWC, on the other hand, is most intense during the day and summer season, reaching differences of 10 – 15°C[2].

Remote sensing is the only way to study the MWC phenomenon in such detail at this point, but its results, without instrumental data on air temperature, will be strictly about the PMWC. Nevertheless, given that the temperature of the active surface significantly affects the temperature in the ground layer of the atmosphere (up to 800-1,000 meters in altitude during daytime in the summer season[3]), these studies are a valuable source of information about MWC. Besides, remote sensing data can more reliably identify heat islands in dense urban areas than instrumental data.

We already know that remote sensing allows us to study the spatial structure of PMWCs, but what exactly is it and how does it work? Remote sensing is a type of survey performed remotely thanks to electromagnetic radiation, which is used by the vast majority of remote sensing sensors as a carrier of information about the object under study. Accordingly, the basic prerequisite for using satellite imagery to determine the temperature of the active surface is a suitable sensor that records the electromagnetic radiation emitted by the Earth, i.e. Thermal infrared radiation (about 10 – 12μm).

Negative consequences of the urban heat island

While for most of the year we may indeed associate the MWC with pleasant warmth felt when crossing city limits, during the summer season, due to the occurrence of high temperatures, it is considered a dangerous phenomenon on a local scale. It negatively affects the health and lives of residents. Many researchers emphasize that under the influence of climate change, which is resulting in global warming, extreme heat waves will be more frequent and will further strengthen the MWC effect.

Studies confirm that heat waves are accompanied by an increase in mortality, and the possibility of hot days is three times more likely today than 150 years ago[4]. In Poland, a 19 to 22% increase in mortality has been observed during hot weather[5]. Of course, the greatest risk is for the elderly due to age-related diseases, including. cardiovascular problems. Besides, high temperatures and low humidity, which we increasingly experience in the city, reduce concentration in everyone, regardless of age. This causes a decline in productivity at work or in learning, and thus no longer negatively affects not only our health, but also the economy. In addition, the increasing prevalence of tropical nights means that our body will not regenerate even in sleep. During the highest temperatures, there is an increase in the number of traffic accidents and collisions, the causes of which may be related to a reduction in our concentration due to heat stress. The hot weather is hurting everyone, so more and more companies, but also private homes, are bailing out with air conditioners. This is a half-hearted solution, as the equipment cools the room and releases heat to the outside, which causes additional warming of the city, and higher electricity consumption results in increasedCO2 emissions into the atmosphere.

How to fight the urban heat island phenomenon?

Can anything be done to reduce the impact of the negative effects of the summer MWC? Yes, creating and implementing climate change adaptation and mitigation plans is key. The most important point is the cooling of the city, and the greatest ability to reduce the temperature is in vegetation and bodies of water. They constitute the areas of the so-called. surface cold islands (PWCH) – the coolest areas in the city. Studies confirm that up to 90% of the area of the cold islands is vegetated areas, with deciduous or mixed forests being the most effective[6]. By observing the structure of the cooling islands, we can identify the form of vegetation with the greatest cooling capacity and make more effective changes in other locations.

Plants are an unparalleled source of oxygen and are excellent air purifiers and noise dampeners, which is why plantings – especially trees – in urban areas, are one of the most desirable and widely used methods of climate change adaptation. In addition to the cooling effect of water and vegetation surfaces on their surroundings and their recreational functions, they are also important from the point of view of biodiversity in the city. Therefore, adaptation plans should protect old parks and gardens, ponds, wetlands, orchards and allotments.

Other ways to combat the negative effects of MWC are to reduce the creation of unnecessary artificial surfaces, such as by designing permeable parking lots overgrown with grass, painting roofs white, and covering easily heated materials like roofs and bus stops with vegetation to reduce the amount of solar radiation absorbed by these surfaces.

Given the above and the fact that Poland’s population is aging, recognizing the spatial structure of PMWCs and PWCH in the era of modern climate change is very important. The knowledge gained can contribute to the implementation of measures aimed at sustainable development of the city and improve the quality of life of its residents. Due to its cooling effect, green-blue infrastructure, which is a kind of oasis for urban residents, should be incorporated into urban spaces as a key factor in climate change adaptation.

In the article, I used, among others. From the works:

[1] United Nations, 2019. World Population Prospects: The 2018 Revision. United Nations, Department of Economic and Social Affairs, Population Division.

[2] Akbari H., Bell R., Brazel T., Cole D., Estes M., Heisler G., Hitchcock D., Johnson B., Lewis M., McPherson G., Oke T., Parker D., Perrin A., Rosenthal J., Sailor D., Samenow J., Taha H., Voogt J.A., Winner D., Wolf K., Zalph B., 2008. Reducing Urban Heat Islands: Compendium of Strategies. Urban heat island basics, Climate Protection Partnership Division in the U.S. Environmental Protection Agency’s Office of Atmospheric Program’s.

[3] Walczewski J., 2005. Meteorological and climatic conditions for the spread of air pollutants. Geophysical Review. 50 (3-4), 177-193.

[4] IPCC, 2021: Summary for Policymakers. In: Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [Masson-Delmotte V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M.I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu and B. Zhou (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 3-32.

[5] Kuchcik M., Degórski M., 2009. Heat and cold-related mortality in the north-east of Poland as an example of the socio-economic effects of extreme hydrometeorological events in the Polish lowland. Geogr P., Vol. 82, No. 1.

[6] Renc, A., Łupikasza, E., Błaszczyk, M., 2021. Spatial structure of the surface heat and cold islands in summer based on Landsat 8 imagery in southern Poland. Ecol. Indic., 142, 109181.

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