Factors Causing Urban Heat Island ;

? *Jit Kumar Gupta

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The main cause of the UHI effect is from the modification of land surfaces while?waste heat?generated by energy usage remains the secondary contributor.Studies have revealed that? urban areas are known to record higher temperature when? compared to rural hinterland and rural settlements. The?temperature?difference is usually larger at night than during the day, and is most apparent when?winds?are weak, under?block?conditions, particularly during the?summer?and?winter. The difference in temperature recorded ?between an inner city and its surrounding suburbs in USA is frequently mentioned in weather reports, as in "68?°F (20?°C) downtown, 64?°F (18?°C) in the suburbs. In the United States, the difference during the day is between 0.6–3.9?°C (1–7?°F), while the difference during the night is 1.1–2.8?°C (2–5?°F). Besides impacting temperature, UHI is also known to? decreases?air quality?by increasing the production of pollutants such as?ozone, and decreases water quality as warmer waters flow into area streams and put stress on their?ecosystems.?The phenomenon of global warming remains more pronounced in the urban areas, for the reason cities house large concentration of people and activities in? a small/compact urban space.Densely-built?downtown?areas tend to be warmer than suburban residential areas or rural areas.. UHI not only raises urban temperatures but also increases ozone concentrations because ozone is a?greenhouse gas?whose formation will accelerate with the temperature. Tokyo, an example of an urban heat island. Normal temperatures of Tokyo go up higher than those of the surrounding area. However, it needs to be understood and appreciated that climate change?is not the cause of urban heat islands but it is causing more frequent and more intense?heat waves?which in turn amplify the urban heat island effect in cities. Major? reasons for ever growing global warming ?and climate change can be attributed to the; ?Nature and natural; Human-Driven;? population; Rapid Urbanisation; Irrational ?Urban planning; High? Density; Inefficient Transportation? ;Large ?generation/consumption of? fossil fuel based Energy; Unsustainable Buildings;? Polluting Industry & Manufacturing; Unsustainable Agriculture; Irrational ?Development; Large scale Deforestation; Lack of open spaces ?and individual life-choices; as briefed below;

Urbanization;? Urbanization,? as a process, has been recognized as the prime reason for ushering in an era of climate change and global warming. Urbanization? has been responsible for changing the? physical, social, economic , environmental structure, fabric, growth and? development trajectory of human settlements backed by concentration of activities, mechanization of system of production, large scale migration, ?production and use of large ?fossil fuel based energy and massive addition to built environment besides changing footprints? and typologies of buildings and altering and increasing mechanized mobility . All this led to creating ?urban heat island?(UHI) effect, as a distinct feature of urban growth, urban development and urabanisation;? making urban areas warmer than surrounding?rural areas.The urban heat island temperature difference is not only usually larger at night than during the day, but also larger in winter than in summer.

·?????? Industrialization;?? Having roots? in the invention of steam and fossil fuel based energy has been one of the major reasons for making urban areas hotter when compared with the surrounding rural hinterland. Burning of coal and generating large volume of CO2, Methane and other effluents has led to not only adding enormous heat in the environment besides trapping heat? close to the earth, raising the local temperatures. Cumulative effect of industrialization has been? to change the local climate and heating the urban settlements.

·?????? Population; ?Population at any place remains the major? determinant of urban heat island. ?As? population grows, human settlements for housing increased population,? tends to expand its area leading to increase? in average temperature. High density neighborhoods, as already stated, invariably are found to be hotter than low density development. ?

·?????? ?Density; Urban growth is invariably marked by the large concentration of people, vehicles, housing etc. in a limited area, giving rise to higher order of density and intensity of development. ?Density and urban heat island remain positively related. Higher the density / intensity? of growth and development in any city; higher will be the temperature of a place. Even? within the? city, different neighborhoods are known to record different temperatures. Neighborhood? housing high income groups, having low housing/population density, are less impacted by the heat island? when compared with neighborhoods having high density/low income housing.

·?????? Open Spaces; Dense urban living, without green spaces lead to a creating pronounced urban heat island effect Studies made have revealed that open spaces and heat island remain negatively/ inversely?? connected. Large proportion of? green spaces? with large number of trees,? invariably lead to lower temperature because of the capacity of trees to cool the air by trans-evaporation , provide shadow to the urban spaces and reduce the intensity of solar radiation on the land surface. Heat can be reduced by tree cover and green space, which act as sources of shade and promote evaporative cooling.

·?????? Built Environment;? Along with population, urban growth is marked by large scale construction of buildings ?and other structures necessary to accommodate growing populations. These surfaces absorb, rather than reflect the sun's heat, causing surface temperatures and overall ambient temperatures to rise .In addition, buildings? remain large consumers of energy and resources besides generators of large waste, Buildings are known to have energy implications in terms of embodied energy and operation and maintenanace energy because energy is used both during the construction for the production of materials and services used in the construction of buildings.? In addition, buildings continue to consume energy during its entire life cycle for heating, cooling, ventilation and lighting besides running large number of energy based appliances used within the building. Buildings, unless planned, designed and constructed as energy-efficient and sustainable buildings, are known to have major implications in terms of higher consumption of energy and resources. Ignoring orientation, cross ventilation, climate and? natural lighting ?in building design, invariably ?increases the inside temperature of building, necessitating the ?need? of? air-conditioning, leading to consumption of ?higher energy , promoting urban temperature, creating heat island and? propelling climate change. ?. Tall buildings within urban areas provide multiple surfaces for the reflection and absorption of sunlight, which by “urban canyon effect"? increases the efficiency with which urban areas are heated. Tall and cluster of buildings also block the flow of? wind, which inhibits cooling by?convection?and prevents pollutants from dissipating. Existence of inverse interactions between heat and built infrastructure is known to? increase the risk of causing enormous? heat stress? among urban residents.

·?????? Urban Planning; Unfortunately, role , relevance and importance of urban planning, in making cities warm and promoting climate change, has neither been understood nor appreciated, with the result cities are being planned, designed, developed and managed without understanding the implications of urban planning in making cities sustainable. Master Plans and Development Plans, prepared for the? rationalising, promoting the planned and sustainable development of cities, don’t space? the climate change as? a factor in the said plans. In the process cities go on increasing and expanding in length, breadth and contents, increasing distances between place of work and residence/leisure/ healthcare /education/ services, increasing trip length, travel demand and inducting more mechanized vehicles. In addition, urban planning negates the very concept of climate, orientation, solar movement and vegetation at local level , leading to formation of? heat canyons at the local level, resulting in considerable increase in temperatures at the local level ,making?? urban areas warmer. In fact, Master Plans/ Development Plans have accelerated the process of climate change and global warming.

·?????? Increasing Black Surfaces; Besides nature, large number of manmade factors have been enumerated as promoter of higher temperature in urban areas. Large scale metaling and paving of earth surfaces, by making roads, flyovers, footpaths, pavements, parking lots and cycle tracks, have led to reduction in the earth surface for absorbing the solar heat. In addition, large proportion of urban areas? is covered???? by the building footprints provided for shelter, offices, trade and commerce, industries , leisure, healthcare, education etc. Majority of ?these surfaces are of dark colours, because of the materials used in making them. Dark colours are known for their character and capacity of ?absorbing and storing majority of solar heat falling on them, instead of radiating. Accordingly, dark surfaces absorb significantly more?solar radiation, which causes urban concentrations of roads and buildings to heat more than suburban and rural areas during the day; Increase in vehicular population? coupled with increase in? population, have cumulatively led to increasing the volume? of such surfaces. Commutative? impact of? the dark surfaces lead to raising the temperature of? these surfaces and making cities warm and hot, particularly during the summer season because surfaces in the urban areas tend to warm faster than those of the surrounding rural areas. Considering high?heat capacities, urban surfaces act as a giant reservoir of heat energy. Concrete can hold roughly 2,000 times as much heat as an equivalent volume of air. As a result, the? daytime surface temperature of hard surfaces becomes very high . This warming? of surfaces also has the effect of generating?convective?winds within the urban areas. The urban heat island temperature difference is not only usually larger at night than during the day, but also larger in winter than in summer. The temperature difference between urban areas and the surrounding suburban or rural areas can be as much as 5?°C (9.0?°F). Nearly 40 percent of that increase is due to the prevalence of dark roofs, with the remainder coming from dark-colored pavement and the declining presence of vegetation

·?????? Deforestation; Large increase in population and? addition to urban limits,? invariably leads to? encroaching on the agricultural land and converting them for meeting the needs of urban areas. Large number of trees and existing flora and fauna, gets lost in the haze of? urban development. Trees are known for their capacity to modulate temperature by absorbing solar radiation, using solar energy for making food and growth of trees beside shading the area under the canopy of the tree. Loss of trees and green vegetation remains the major reason for promoting the local temperature and climate change. Another major reason is the lack of?evapotranspiration? caused by lack of vegetation in urban areas.?The U.S. Forest Service found in 2018 ; cities in the United States are losing 36 million trees each year. With? decreased amount of vegetation, cities also lose the shade and evaporative cooling effect of trees. Heat islands ?also becomes a reality? by proximity to different types of land cover. Proximity to barren land causes urban land to become hotter and proximity to vegetation makes it cooler

·?????? Traffic & Transportation; Traffic and transportation remains the two worst gifts of urabanisation, causing numerous problems of growth, development, environment and ecology in urban areas. Increase in travel demand and ever increasing trip length, caused by ?ever increasing travel distances between different land uses in urban areas and induction of large increase in mechanized fossil fuel-based vehicles to transport goods, services and human beings, have emerged as the major cause of city getting warmer by the emission of the Co2 and other gasses. Traffic and transportation invariably promote heat islands? because movement of vehicles requires ?vegetation to be replaced by asphalt and concrete?for roads. Pavements,?parking lots, roads or, more generally speaking?transport infrastructure, contribute significantly to the urban heat island effect. For example, pavement infrastructure is a main contributor to urban heat during summer afternoons in?Phoenix, United States. ?Vehicular traffic has been considered as one of the major reasons that increase the severity of UHI especially in mega cities, particularly in areas having high intensity of vehicular movement. Waste heat from automobiles, air conditioning, industry, and other sources also contributes to the UHI. High levels of pollution in urban areas can also increase the UHI, as many forms of pollution change the radiative properties of the atmosphere

·?????? Nature- led causes; Part of the problem pertaining to ?climate change can be attributed to the natural phenomena, which created the planet earth and ??under the shadow of which planet earth keeps its identity. . Over the period ?these ?natural phenomenon and natural forces coupled with earth’s existence,?volcanic eruptions, fluctuations in?solar radiation,?tectonic shifts, and even small?changes in our orbit?have all ?impacted ?planetary warming and cooling patterns to a large extent. However, data sourced on heat ?from different sources, ?have show that ?global warming Some amount of climate change can be attributed to natural phenomena. Over the course of Earth’s existence,?volcanic eruptions, fluctuations in?solar radiation,?tectonic shifts, and even small?changes in our orbit,?have considerable effects on planetary warming and cooling patterns.? But climate records are able to show that current ?global warming—particularly what has occurred since the start of the industrial revolution—is happening? quicker ?and ?much faster than ever before. According to?NASA, these natural causes are still in play today, but their influence is too small or they occur too slowly to explain the rapid warming seen in recent decades.” Records refute the?misinformation?that natural causes are the main reasons behind climate change. Fossil fuel; industry and ?built environment and transportation are the major source contributing to rise in temperature.

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