Abstract
Urban transition is an unstoppable process. Globally, several planning measures are taken by the city and country administration to control the sprawling process. Despite all the planning, most of the cities experience appreciable impact of urbanization on the localized weather parameters. This chapter summarizes the understanding relating to urban modification of localized weather, that is, changes in precipitation, temperature, and wind speed in the form of increase or decrease, their spatio-temportal distribution, urban heat island (UHI), and urban wind island (UWI). The impacts of the urbanization are primarily because of changes in land-surface characteristics due to the alteration of land use in a city. The urbanization effects on local or mesoscale weather could be studied both through observations and/or numerical modeling. The purpose of this chapter is to provide a review of most of the relevant studies carried out globally and with a special emphasis on India.
TopIntroduction
Urban transition is broadly the shift from rural to urban land use (LU) and economically from agricultural to industrial, commercial, or service employment (Pannel, 1995). According to the United Nations (UN), globally, 55% percent of the population currently resides in cities, and this figure projected to be two-thirds of the world population by 2050 as the growing population concentrates in cities and towns in search of better livelihoods and opportunities (DESA, 2018). This process of urbanization gives rise to the expansion of existing cities to form mega cities and evolution of new cities, which accelerates the changes in existing land use and land cover (LULC). Studies suggest that the major causes of land cover (LC) changes are neither population nor poverty alone; rather, it is people’s response to economic opportunities. Mostly, global factors are the main determinants of LU change as they are weakened by the local factors (Lambin et al., 2001). Besides these causes of urban transition, the development of an area as urban land takes place only when the post-development value of the land and the percentage growth rate of rent increases. Urban transition in the outskirts of core cities is directly affected by the emerging economy and due to foreign investments.
Figure 1. Global trends and rate of change of urbanization.
The global trends and rate of urbanization over different parts of the world illustrated in figure 1. The rate of urbanization found to be either stagnant or declining in other parts of the world except for Asia and Africa, where the rural to urban transition mostly occurs. However, the urbanization in African countries is driven by global historical process like population dynamics and rural-urban migration, both of which are stimulated by technological and institutional changes. In Asia, the southeastern region, including India and China, are in a continuous state of development. Chinese cities are proceeding toward rapid urbanization with a dense population and a structural shift in employment. Economic reforms and development-friendly policies by the local governments are primarily the causes for the rapid shift (Pannell, 1995; Han, 2010). In contrast to popular beliefs, cities in China are becoming more and more like their first-world counterparts.
The policymakers exclude rapidly growing areas outside the boundary of large urban agglomerations, which often misleads their conclusions regarding the growth rate (Zheng et al., 2009). Thus the focus of the urban related studies need to be diverted toward the developing countries like India, where the diversified low density development in the peri-urban area is seen. In India, both Gibrat’s Law and Zipf’s Law failed to explain the spatial equilibrium of urbanization (Chauvin et al., 2017). Urbanization in India started close to ports during the British colonization era. There are similarities between urban developments in India and in European countries, which deteriorated severely because they lacked proper expansion plans and-passed the focus over other smaller cities after independence (Harari, 2016). India also witnessed the development of corporate urbanism in forms of company towns to industrial cities to the present-day urban agglomerations (Sood, 2015). Growth of Indian cities is found to be driven by the initial population, capital city status, and proximity to larger cities (Abhishek et al., 2017).
Although the process of urbanization is relatively localized, it has regional and global impacts as well. Human activities influence the fluxes, and the amount of gases present between the land surface and atmosphere (Guo et al., 2016). Anthropogenic emissions from urban areas also alter the chemical composition of the atmosphere and the radiative balance in the earth (Lindberg et al., 2013). The resulting feedback of land-atmospheric interaction consequently influences the hydrological cycle (Brubaker & Entekhabi, 1996). Urbanization can influence the rainfall occurrence and distribution over a city and its neighboring regions (Li et al., 2016) too.
Key Terms in this Chapter
Tier-I/II: Indian cities are classified into different tiers based on their population according to the 2011 census. Tier-I cities generally consist of metropolitan areas like New Delhi, Mumbai, Bengaluru, Chennai, Kolkata, and Hyderabad with developed infrastructure and high cost of living. Tier-II includes relatively smaller cities and state capitals, which are still more developed than tier-III (all other cities). Tier-I, II, and III are also known as X, Y, and Z categories of cities respectively.
Upwind/Downwind: In meteorology, a wind direction is a direction, the wind is coming from. ‘Upwind’ is in the direction towards the source of the wind, and ‘Downwind’ is in the direction away from the source of the wind.
UHI/UWI: UHI or ‘Urban Heat Island’ refers to a part or whole of the urban area, which is significantly warmer than the surrounding rural counterpart due to human activities. Similarly, UWI or ‘Urban Wind Island’ refers to the scenario when the wind speed is noticeably higher over urban areas.
Urbanization: ‘Urbanization’ is the act or fact of taking on the characteristics of a city.
Thunderstorms: ‘Thunderstorms’ are generally accompanied by lightning and thunder with rain and gusty winds, sometimes with hail or snow, produced by cumulonimbus clouds.
Boundary-Layer: Also known as the ‘Atmospheric Boundary Layer’, is the lowest part of the atmosphere near the ground that is directly influenced by heat, moisture, or momentum transfer to or from the surface.
Micrometeorology: ‘Micrometeorology’ is the study of small-scale atmospheric phenomena ranging up to several kilometers in diameter and confined to the lower troposphere.
LCZ: LCZ or ‘Local Climate Zones’ is a systematic classification scheme for urban land use and land cover, which is used for zoning and categorization of the internal structure of urban areas. The main applications include studies related to urban climatology.
Hydrological Cycle: ‘Hydrological cycle’ or ‘water cycle’ is the continuous movement of water within the earth-atmosphere system. The movement involves various processes, including evaporation, transpiration, condensation, precipitation, and runoff. The total amount of water within the cycle remains constant, while phases and distribution of water change during these processes.