Why are cities hotter than rural areas and what does this mean for heating & cooling?

P. Sismanidis and I. Keramitsoglou from the National Observatory of Athens explain the Urban Heat Island Effect, its causes and its consequences.


Urban areas are hotter than rural areas. This is known as the urban heat island effect and impacts cities and their demand for energy in various ways.


What is the Urban Heat Island effect and what is causing it?

The last decades have been marked by a population move from rural areas to cities, a phenomenon known as urbanisation. To satisfy the needs of the rising number of urban dwellers, urban areas vastly expanded and large portions of natural lands were converted to impervious surfaces, such as roads, pavements, buildings, squares etc. However impervious surface store more heat than natural lands and do not reflect as much solar radiation. The above in combination with a decrease in evapotranspiration and the flow of air through the cities (the building act as obstacles for the wind) and an increase in anthropogenic heat emissions (e.g. car emissions) and air pollution led to a raise in city ambient temperatures. This relative warmth of urban areas in respect to rural areas is known as the Urban Heat Island (UHI) effect.


Does the intensity of the UHI effect vary within a city?

The UHI effect strongly depends on the land cover and land use. This implies that its intensity varies within a city. For instance, green spaces like parks and playgrounds have lower temperatures than built-up areas, while high-density residential districts have higher temperatures than low-density districts where trees and plants are more abundant. Moreover, areas that are close to locations with increased emissions, such as industrial sites or highways, are more affected than areas far away. Besides spatial variations, UHIs also vary with time and season. In particular the time where the UHI effect is most intense is usually during the night, where city temperatures can be as high as +10°C in respect to rural temperatures.  Because UHIs vary rapidly in space and time their study has special requirements in data characteristics. A prominent data source is satellite remote sensing, which can facilitate the process of generating city temperature maps very frequently. PLANHEAT takes advantage of this technology and uses satellite-derived data to support the mapping of heating and cooling demand.


How do UHIs affect cities and their demand for energy?

The study of UHIs is one of the most important topics in urban science and has received considerable attention in recent years. In general the UHI effect has been related to a range of issues, such as energy demand, environmental degradation, thermal discomfort, and human health. UHIs also affect the magnitude and duration of heatwaves within cities, which can cause many excess deaths. Regarding energy demand, the UHI effect increases the demand for energy during summer months so as to satisfy the need for cooling. This is particularly evident in Mediterranean Europe and during heatwaves the demand can be so high that blackouts may occur. During winter months, UHI can have a positive impact. In particular it can lower the demand for heating because the ambient temperatures of cities are increased. It is important to emphasize again that UHI effects primarily impact the urban areas, i.e. the areas with the greatest concentration of people, and thus their impact on human welfare and health is exacerbated.