Role of Urban Heat Island in local climate change


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Role of Urban Heat Island in local climate change:

Amit Kumar Centre for Environmental Sciences and Technology School of Environment and Earth Sciences Central University of Punjab, Bathinda E-mail: Role of Urban Heat Island in local climate change 09/01/2014 1


Contents Introduction Types of UHI Factors modifying the UHI Methodology for measuring UHI Impacts of UHI Mitigation of UHI Conclusion 09/01/2014 2


Introduction Urban Heat Island (UHI) : I ncreased surface and/or air temperatures observed in an urban environment compared to the rural surroundings. Fig 1: Schematic diagram of UHI (USEPA,2012) Luke Howard was one of the first scientists to observe these temperature differences in 1833 for the city of London ( Oke , 1991). 09/01/2014 3

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09/01/2014 4 Characteristic contributing to heat island formation Effect on the energy balance Lack of vegetation Reduces evaporation Widespread use of impermeable surfaces Reduces evaporation Increased thermal diffusivity of urban materials Increases heat storage Low solar reflectance of urban materials Increases net radiation Urban geometries that trap heat Increases net radiation Urban geometries that slow wind speeds Reduces convection Increased levels of air pollution Increases net radiation Increased energy use Increases anthropogenic heat Table 1: Urban characteristic important to Heat I sland formation ( Gartland , 2011 )

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09/01/2014 5 Table 2: U rban H eat I sland intensity (UHII) of some cities around the world ( Gartland , 2011). City UHII ˚ C Mexico 7.8 New York 8.0 Alaska 2.2 Singapore 4.0 Shanghai 7.4 Beijing 3.3

Types of UHI:

Types of UHI Surface UHI (strong during day time). Atmospheric UHI Canopy layer UHI: Exist in the air of where people live Boundary layer UHI: Start from the rooftop and treetop level and extend up to the point where urban landscapes no longer influence the atmosphere (up to 1 mile). 09/01/2014 6

Factors affecting the Urban Heat Island phenomenon:

Factors affecting the Urban Heat Island phenomenon Population size/density ( Oke , 1973 ) Land/Building Material ( B Offerle , 2006 ) Anthropogenic Heat Sources (cars, air conditioning, industrial processes, power plants, etc.) ( Qihao Weng , 2004 ) UHI intensity decreases with increasing wind speed and cloud cover ( Ackerman, 1985 ) Season 09/01/2014 7

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09/01/2014 8 Fig 2: Different Urban Environment Albedos (USEPA, 2012)

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Source: Morris and Simmonds, 2000. FIG 3: Summer and winter air temperatures in the Central Business District (Urban) and Airport (Rural) of Melbourne, Australia 09/01/2014 9

Methodology for measuring UHI:

Methodology for measuring UHI Fixed Stations Energy balances Mobile traverses Remote sensing Vertical sensing 09/01/2014 10

Fixed Stations:

Fixed Stations Comparing the obtained data from a single pair of urban and rural weather stations. Analysing the data obtained from multiple weather stations to find regional, two-dimensional impacts Analysing a huge set of historical data to examine Urban Heat Island trends over time as a region develops. 09/01/2014 11

Energy balances :

Energy balances Energy balance Equation for UHI Convection + Evaporation + Heat storage = Anthropogenic heat + Net radiation Net Radiation = Incoming solar –Reflected solar + Atmospheric radiation –Surface radiation M easured by albedometer (incoming and reflected solar radiation), radiometer (atmospheric radiation) and pyrgeometre (surface radiation). 09/01/2014 12

Mobile traverses :

Mobile traverses A mobile traverse is carried out on a predetermined path throughout a region, stopping at representative locations to take readings. Drawback: Inability to record simultaneous measurements at different locations. 09/01/2014 13

Remote Sensing :

Remote Sensing Remote sensing can be used to find temperatures by measuring the energy reflected and emitted from them. Power to visualize temperatures over large areas. 09/01/2014 14

Vertical Sensing :

Vertical Sensing These methods include: Sending instrumented balloons aloft, Installing monitoring equipment on radio towers or, Flying at different altitudes in an instrumented helicopter or aero plane. It gives clear cut view of boundary layers. 09/01/2014 15

Impacts of UHI:

Impacts of UHI Differences in Urban- Rural Temperature: Arises due to the interaction of building structure, land cover and land use changes Use of different types of material for urban development ( Oke , 1976; Wang et al., 1990; Arnfield , 2003) . Air Quality (USEPA,2012) Energy Use CO2 Emissions 09/01/2014 16

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Change in rainfall pattern and intensity: Increased level of air pollution may be the cause of reduction in precipitation. Studies reveal that urbanization affects rainfall pattern and rainfall intensity, but how this happens is very poorly understood (Lowry, 1998). Alters humidity, surface wetness and soil moisture level Health Impacts 09/01/2014 17

Mitigation measures:

Mitigation measures Cool roofing materials Cool paving materials Cooling with trees and vegetation 09/01/2014 18


Conclusion Urban heat island play a major role in urban environment. In Indian context it is a challenging task for environmentalist, policy maker urban development planner due to rapid increase of population and its shifting towards urban areas. 09/01/2014 19


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09/01/2014 24 Thank You

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