The urban population is increasing worldwide and the urban expansion or the increase in urban density can affect both the global and the local climate having consequences, in turn, on human health. It is, therefore, important to increase the awareness of policy makers, urban decision makers and citizens about urban climate pathologies. In particular, I report in the following a brief and simplified explanation about the urban heat island effect, a very common urban climate phenomenon. The text in the following has been simplified in order to reach not only researchers or people already in the field, but, more importantly, citizens who might have no scientific background in such field, but that might be the most interested stakeholder in this theme. For this reason I decided to entitle this post “Urban Heat Island for Beginners”.
The description of the urban heat island phenomenon has been split into chapters that you will find published on this same blog in the coming months.
Enjoy the first part of the reading!
Introduction
At the beginning of the twentieth century, 15% of the world inhabitants lived in cities. Nowadays, about 50% of the world population lives in urban areas which are approximately 2.8% of the total land of our planet (Millennium Ecosystem Assessment, 2005). The rise of the urban inhabitants has led to urban sprawl, especially in developing countries (United Nations, 2004), and – as demonstrated in previous studies (e.g., Bacci & Maugeri, 1992) – urban sprawl is often correlated to the increase of the urban temperature compared to the rural surroundings, the so-called Urban Heat Island (UHI) effect (Landsberg, 1979; Frumkin, 2002).
The development of UHI mainly depends on the modification of the natural energy balance in urban areas. The modification of the natural energy balance is due to several factors such as urban canyons (Landsberg, 1981), substitution of natural materials with artificial ones featured by different thermal properties (Montavez, Rodriguez, & Jimenez, 2000), substitution of green areas with impervious surfaces which limit evapo-transpiration (Takebayashi & Moriyama, 2007; Imhoff, Zhang, Wolfe, & Bounoua, 2010; Lougeay, Brazel, & Hubble, 1996), and urban albedo decrease (Akbari & Konopacki, 2005). Another cause of the increase in urban temperature is the distribution of buildings that, in most cases, provokes an abatement of wind speed and a consequent reduction of heat dissipation (Morris & Simmonds, 2001).
The urban heat island is usually defined as the difference between the urban temperature and its rural surroundings, with temperatures recorded in the canopy layer [1] (equation 1), but often is also described through the difference of temperatures recorded in the boundary layer [2] for example through the use of towers, balloons or aircrafts.
UHI = ∆TU-R = TU - TR
Equation 1 UHI intensity general expression
Furthermore, other parameters, commonly used to depict UHI phenomenon, are the difference of surface temperatures (Imhoff, Zhang, Wolfe, & Bounoua, 2010) or vegetation index (Gallo, McNab, Karl, Brown, Hood, & Tarpley, 1993; Gallo & Owen, 1999; Weng, Dengsheng, & Jacquelyn, 2004).
It has been observed that UHI phenomenon consistently amplified over time for the enhancement of industrial activities and urbanization. Brunetti et al. (2000) investigated the historical series of temperatures in Italy, which shows that the increase of the UHI phenomenon in Italy (0.2°C/100 years) is higher than the global one (0.1°C/100 years).
Many other studies justified the rise of urban temperature not only with the climatic phenomenon, but also with the change in the urban structure (e.g., Bacci & Maugeri, 1992; Bonacquisti, Casale, Palmieri, & Siani, 2006). Gaffin et al. (2007), analyzed the historical series of temperatures recorded in New York City – from the beginning of the twentieth century to present – in order to study the temporal evolution of the UHI. The authors have reconnected the increase of the urban temperature during time to a significant drop of wind speed due to a change in the urban structure, in particular due to the increase of the height of buildings and the expansion of the city core.
Brief history of UHI
The UHI was detected and measured for the first time at the beginning of ninetieth century by Luke Howard. Luke Howard – known also as ‘the father of meteorology’ – was also a pioneer in urban climatology. From 1820 through 1833 he compared the temperatures he surveyed in at Plaistow, a village 6.4 Km far from London and at Tottenham with those recorded by the Royal Society at Somerset House and he recognized the urban heat island in London (Landsberg, 1981). Howard found a difference of temperatures between the ‘urban’ and ‘rural’ sites and he attributed the increase of the urban temperature to the high use of fuel and to the anthropogenic heat (Santamouris, 2006) [3]. About twenty years later, Renou detected the urban heat island in Paris. Renou mainly noted the difference of temperatures between urban and rural sites, especially during the afternoon, the increase of the urban temperatures also during winter and the heavily decrease in the wind speed in the urban context (Landsberg, 1981). After Howard and Renou a large number of important studies have been carried out and have contributed to decrypt and understand the urban heat island phenomenon. Tony Chandler recognized in 1959 the spatial characteristics of the UHI in London. He showed that the hot area in London occupied the built-up area in the city and increased its magnitude in the most densely urbanized areas while it was weaker in the greener areas generating the cool heat island. In ‘The Climate of London’, Howard underlined that in the period from 1794 through 1799, the difference of temperatures between urban and rural sites was about 3°C and that in the period from 1811 through 1816 that difference increased to 4.5°C (Howard, 1833). Notwithstanding the first observations were, in some cases, elementary, they constituted the first step in the detection of an increasing urban pathology and the roots for more refined further studies.
References
Akbari, H., & Konopacki, S. (2005). Calculating energy-saving potentials of heat-island reduction strategies. Energy Policy, 33, 721-756
Bacci, P., & Maugeri, M. (1992). The urban heat island of Milan. Il nuovo cimento, 15 (4)
Bonacquisti, V., Casale, G. R., Palmieri, S., & Siani, A. M. (2006). A canopy layer model and its application to Rome. Science of the Total Environment (364), 1-13
Brunetti, M., Mangianti, F., Maugeri, M., & Nanni, T. (2000). Urban heat island bias in Italian air temperature series. Nuovo Cimento, 23 (4)
Frumkin, H. (2002). Urban Sprawl and Public Health. Public Health Reports, 117
Gaffin, S. R., Rosenzweig, C., Khanbilvardi, R., Parshall, L., Mahani, S., Glickman, H., et al. (2007). Variations in New York city’s urban heat island strength over time and space. Theoretical and Applied Climatology
Gallo, K. P., & Owen, T. W. (1999). Satellite-based adjustments for the urban heat island temperature bias. Journal of Applied Meteorology, 38, 806-813
Gallo, K. P., McNab, A. L., Karl, T. R., Brown, J. F., Hood, J. J., & Tarpley, J. D. (1993). The use of NOAA AVHRR data for assessment of the Urban Heat Island effect. Journal of Applied Meteorology, 5 (32), 899-908
Howard, L. (1833). The climate of London, deduced from meteorological observations. London: Joseph Rickerby
Imhoff, M. L., Zhang, P., Wolfe, R. E., & Bounoua, L. (2010). Remote sensing of the urban heat island effect across biomes in the continental USA. Remote Sensing of Environment, 114, 504-513
Imhoff, M. L., Zhang, P., Wolfe, R. E., & Bounoua, L. (2010). Remote sensing of the urban heat island effect across biomes in the continental USA. Remote Sensing of Environment, 114, 504-513
Landsberg, H. E. (1979). Ampmospheric changes in a growing community (the Columbia, Maryland experience). Urban Ecology, 4, 53-81
Landsberg, H. E. (1981). The Urban Climate (Vol. 28). New York: International Geophysics Series
Lougeay, R., Brazel, A., & Hubble, M. (1996). Monitoring Intra-Urban Temperature Patterns and Associated Land Cover in Phoenix; Arizona Using Landsat Thermal Data; Geocarto International, 11, 79-98
Millennium Ecosystem Assessment. (2005). Millennium Ecosystem Assessment, Ecosystems and Human Well-being: Current State and TrendsAssessment. Washington, DC: Island Press
Montavez, J. P., Rodriguez, A., & Jimenez, J. I. (2000). A Study of the Urban Heat Island of Granada. International Journal of Climatology, 20, 899-911
Morris, C. J., & Simmonds, I. (2001). Quantification of the Influences of Wind and Cloud on the Nocturnal Urban Heat Island of a Large City. American Meteorological Society, 40, 169-182
Oke, T. R. (1982). The energetic basis of the urban heat island. Quarterly Journal of the Royal Meteorological Society, (108), 1-24
Santamouris, M. (2006). Environmental design of urban buildings. An integrated approach. London: Earthscan
Takebayashi, H., & Moriyama, M. (2007). Surface heat budget on green roof and high reflection roof for mitigation of urban heat island. Building and Environment (42), 2971-2979
United Nations. (2004). World Urbanization Prospects: The 2003 Revision. New York: United Nation Publication
Weng, Q., Dengsheng, L., & Jacquelyn, S. (2004). Estimation of land surface temperature–vegetation abundance relationship for urban heat island studies. Remote Sensing of Environment, 89, 467-483
Footnotes
[1] The canopy layer is defined by the mean urban roughness that is due to mean building height and urban vegetation height
[2] The boundary layer is a meso-scale internal layer determined by the urban characteristics (Oke, 1982)
[3] Although a limit in Howard’s measurements was the lack of simultaneity
* Rearranged text from:
Susca, T. (2011). Evaluation of the Surface Albedo in a LCA Multi-scale Approach. The Case Study of Green, White and Black Roofs in New York City. Ph.D. Thesis