Urban Heat Island

 Urban structures have largely replaced natural soil with impervious surfaces such as asphalt for road networks or concrete for buildings. This impacts upon the natural energy distribution of incoming radiation from the sun (global radiation, see Figure 1) on the Earth's surface. In natural settings a large fraction of the global radiation is used to evaporate water, which is displayed in Figure 1. Apart from the evaporation of water in lakes, rivers or wetlands, vegetation transpires water through their pores which is then evaporated. This combined effect of evapotranspiration leads to the displacement of heat away from the Earth's surface into the atmosphere as the water vapour moves upwards. The associated heat transfer is termed latent heat, latent as we cannot feel this heat on the Earth's surface. The energy used to evaporate water, about 1888 Watt hours (Wh) as a global mean, is released again when water vapour condensates as cloud droplets and warms the atmosphere. Hence, even on a sunny summer day a meadow will not feel hot due to latent heat. Evapotranspiration influences the amount of longwave thermal radiation by regulating surface temperatures. The warmer the surface the more thermal radiation will be generated. In the natural environment 38 percent of global radiation is directly converted to thermal radiation (Blanco et al, 2011). It can then be absorbed by clouds and greenhouse gases in the atmosphere.

Figure 1. Mean Global Radiation Balance on 1m² per day. Source: Blanco et al (2011)

Urban structures have altered this natural heat balance locally. Due to the sealing off of vegetated land, evapotranspiration is cut down substantially. Figure 2 shows the energy balance for an asphalt roof. Here, latent heat accounts for only 123 Wh. A significantly higher fraction (1827 Wh) of global radiation is converted into sensible heat, sensible as we can feel the heat on the Earth's surface. Most of this heat is stored in the impervious surfaces of the built environment. The subsequent release and distribution of this energy varies temporally. Hence, Zhou et al (2011) categorise a land surface temperature and an air temperature component.

Dark impervious surfaces such as asphalt, concrete or brick heat up when exposed to sunshine. As they do not store water, the energy is transformed to sensible heat. Subsequently, the rise in surface temperatures also increases thermal radiation (2923 Wh for an asphalt roof, see Figure 2). Remote sensing measures the longwave radiation of these surfaces through infrared analysis and deduces land surface temperatures (Website 3). Figure 3 displays the substantial temperature differences between these artificial materials and the surrounding vegetation for a summer day. Urban structures, such as highways and a high density city centre are clearly visible. Air which is in contact with these impervious surfaces is heated up. Moreover, high rise buildings in the city create a greater friction on wind. Air movement is therefore restricted in the so-called urban canyons. This leads to higher air temperatures in the city compared to its vegetated surroundings. However, an exception to this is formed by shadows of high buildings which block sunlight. They prevent streets or other impervious surfaces from being heated up as long as they are in the shade.

Figure 2: Asphalt roof, source: Blanco et al (2011) Figure 2. Asphalt roof as daily mean. Source: Blanco et al (2011)

Figure 3. Infrared Image of Atlanta’s heat island. Source: http://missionscience.nasa.gov/ems/03_behaviors.html

The air temperature component is caused by a more indirect effect of the sun’s energy. Here, the rural-urban temperature gradient is most pronounced at night. The large surface area of three-dimensional urban structures over- and underground, not only releases high amounts of thermal radiation but stores heat efficiently during hours of sunshine. Its high heat capacity produces a large temperature gradient in between the artificial surfaces and the soil. Subsequently, a steady heat flux is created. This energy flux into the soil is two to three times larger in urban settings than in the natural environment (Weischet et al, 2008). Energy is stored underground and released again when the energy gradient reverses. This happens after sun set, when no more energy is delivered by global radiation. The energy stored in the soil is gradually released and warms the air in urban spaces substantially. Weischet et al (2008) measured a maximum difference of 10°C between the warm city centre of Freiburg, Germany and its cooler surroundings for several summer evenings. The annual average maximum was 2 - 3°C. The urban-rural air temperature gradient is therefore greatest during a night following a day with intense sunshine.

The increased warming of impervious surfaces and air temperature in urban spaces compared to its rural surrounding is known as urban heat island (UHI).  

  The video below by the American Weather Channel (2009) illustrates some of the points made in this post. Moreover, it introduces techniques how the urban heat island may be counteracted. If these measures can be succesful will be the topic of posts to come...

To look out for
The increase in longwave radiation (compare Increase thermal radiation in Figures 1 & 2) in urban centres may play a significant role in enhancing the greenhouse effect globally. As urban centres spread rapidly, the influence of the urban heat island in the global climate system could increase dramatically.

Bibliography:
Blake, R., Grimm, A., Ichinose, T., Horton, R., Gaffin, S., Jiong, S., Bader, D., Cecil, L.D. (2011), Urban Climate: Processes, trends, and projections. Climate Change and Cities: First Assessment Report of the Urban Climate Change Research Network, Rosenzweig, C., Solecki, W.D., Hammer, S.A., Mehrotra, S., Eds., Cambridge University Press, UK, pp. 43-81

Blanco, H., McCarney, P., Parnell, S., Schmidt, M., Seot, K.C. (2011): The role of urban land in climate change. Climate Change and Cities: First Assessment Report of the Urban Climate Change Research Network, Rosenzweig, C., Solecki, W.D., Hammer, S.A., Mehrotra, S., Eds., Cambridge University Press, UK, pp. 217-248

Weischet, W. & Endlicher, W. (2008): Einführung in die Allgemeine Klimatologie, 7. Auflage. Gebr. Borntraeger: Berlin, Stuttgart

Zhou, W. et al (2011) Does spatial configuration matter? Understanding the effects of land cover pattern on land surface temperature in urban landscapes, Landscape and Urban Planning, Vol. 102, pp.54 - 63