Abstract
The formation and occurrence of urban heat island (UHI) is a result of rapid urbanization and associated concretization. Due to intensification of heat combined with high pollution levels, urban areas expose humans to unexpected health risks. In this context, the study aims at comparing the UHI in the two largest metropolitan cities of India, i.e., Delhi and Mumbai. The presence of surface UHI is analyzed using the Landsat 5 TM image of 5 May 2010 for Delhi and the 17 April 2010 image for Mumbai. The validation of the heat island is done in relation to the Normalized Difference Vegetation Index (NDVI) patterns. The study reveals that built-up and fallow lands record high temperatures, whereas the vegetated areas and water bodies exhibit lower temperatures. Delhi, an inland city, possesses mixed land use and the presence of substantial tree cover along roads; the Delhi Ridge forests and River Yamuna cutting across the city have a high influence in moderating the surface temperatures. The temperature reaches a maximum of 35 °C in West Delhi and a minimum of 24 °C in the east at the River Yamuna. Maximum temperature in East Delhi goes to 30 °C, except the border areas. North, Central and south Delhi have low temperatures (28 °C–31 °C), but the peripheral areas have high temperatures (36 °C–37 °C). The UHI is not very prominent in the case of Delhi. This is proven by the correlations of surface temperature with NDVI. South Delhi, New Delhi and areas close to River Yamuna have high NDVI and, therefore, record low temperatures. Mumbai, on the other hand, is a coastal city with lower tree cover than Delhi. The Borivilli National Park (BNP) is in the midst of dense horizontal and vertical growth of buildings. The UHI is much stronger where the heat is trapped that is, the built-up zones. There are four small rivers in Mumbai, which have low carrying capacity. In Mumbai suburban district, the areas adjoining the creeks, sea and the lakes act as heat sinks. The coastal areas in South Mumbai record temperatures of 28 °C–31 °C; the Bandra-Kurla Complex has a high range of temperature i.e., 31 °C–36 °C. The temperature witnessed at Chattrapati Shivaji International Airport is as high as 38 °C. The temperature is nearly 37 °C–38 °C in the Dorai region in the Mumbai suburban district. The BNP has varied vegetation density, and therefore, the temperature ranges from 27 °C–31 °C. Powai Lake, Tulsi Lake and other water bodies record the lowest temperatures (24 °C–26 °C). There exists a strong negative correlation between NDVI and UHI of Mumbai, owing to less coverage of green and vegetation areas.
Highlights
Urban areas, due to intense built-up areas, concrete zones and high concentrations of anthropogenic activities, have led to the development of urban microclimates (UMCs)
atmospheric urban heat islands (AUHIs) can be subdivided as urban canopy layer (UCL) and urban boundary layer (UBL)
The land surface temperature (LST) of Delhi ranges from 23 °C to 40 °C, whereas the LST of Mumbai ranges from 22 °C to 38 °C
Summary
Due to intense built-up areas, concrete zones and high concentrations of anthropogenic activities, have led to the development of urban microclimates (UMCs). The development of UMCs is associated with the urban heat island (UHI) phenomenon. Extensive urbanization transforms the land use/cover (LULC), thereby modifying the energy balance, making cities warmer than their hinterland and surroundings. The growth of industries and vehicles add to greenhouse gases in the atmosphere that absorb the outgoing terrestrial long wave radiation and contribute to increasing temperatures in the city. The city center is heated up much more than the periphery, leading to the creation of UHI. UHI can be of two types—surface urban heat islands (SUHIs) and atmospheric urban heat islands (AUHIs) [1,2]. AUHIs can be subdivided as urban canopy layer (UCL) and urban boundary layer (UBL). There is an abundance of research on SUHIs largely due to the dependence of data on remotely-sensed images that are freely available [3]
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