Causes And Effects Of Uhi

Published Date: 02 Nov 2017

-Krupa Desai secc 0512

URBAN HEAT ISLAND

Definition

The term "heat island" refers to urban air and surface temperatures that are higher than nearby rural areas. Many U.S. cities and suburbs have air temperatures up to10°F (5.6°C) warmer than the surrounding natural land cover.

The graph of heat island shows a city's heat or temperature profile. It demonstrates how urban temperatures are typically lower at the urban-rural border than in dense downtown areas. The graphic also reveals how parks, open land, and water bodies can create cooler areas.

How Do Heat Islands form?

UHI’s are primarily nocturnal features that arise as a result of differences in the cooling rates between urban and rural areas. Heat islands form as cities convert natural land cover to pavement, buildings, and other infrastructure. These changes contribute to higher urban temperatures in a number of ways: Displacing trees and vegetation minimizes the natural cooling effects of shading and evaporation of water from soil and trees (evapo-transpiration). Tall buildings and narrow streets can restrict the circulation of air. This results into entrapment of heat and rise in temperature. Waste heat from vehicles, factories, and air conditioners may add warmth to their surroundings, further exacerbating the heat island effect.

In addition to these factors, heat island intensities depend on an area's weather and Climate, proximity to water bodies, and topography. Measuring heat islands can help determine how these factors influence the heat island effect.

When Do Heat Islands Form?

Heat islands can occur year-round during the day and night. Urban-rural temperature differences are often largest during calm, clear evenings. This is because rural areas cool off faster at night than cities, which retain much of the heat stored in roads, buildings, and other structures. As a result, the largest urban-rural temperature difference, or maximum heat island effect, is often occurs three to five hours after sunset.

Causes of UHI

Reduced vegetation in

urban regions

Reduces the natural cooling effect from shade and evapotranspiration.

Properties of urban materials

Contribute to absorption of solar energy, causing surfaces, and the air above them, to be warmer in urban areas than those in rural surroundings.

Urban geometry

The height and spacing of buildings affects the amount of radiation received and emitted by the buildings

Anthropogenic heat emissions

Contribute additional warmth to the air.

Weather

Certain conditions, such as clear skies and calm winds, can foster urban heat island formation.

Geographic location

Proximity to large water bodies and mountainous terrain can influence local wind pattern and urban heat island formation

Why Do We Care about Urban Heat Islands?

Elevated temperatures from urban heat islands, particularly during the summer, can affect a community’s environment and quality of life. Communities may benefit from the wintertime warming effect of urban heat islands. Warmer temperatures can reduce heating energy needs and help to melt snow and ice on roads. Though some heat island impacts seems to be positive, most impacts are negative and include:

Increased energy consumption

Elevated emissions of air pollutants and

greenhouse gases

Compromised human health and comfort

Impaired water quality

Causes and effects of UHI

Urban Heat Islands, Climate Change, and Global Warming

The warming that results from urban heat islands over small areas such as cities is an example of local climate change. Local climate changes resulting from urban heat islands fundamentally differ from global climate changes in that their effects are limited to the local scale and decrease with distance from their source. Global climate changes, such as those caused by increases in the sun’s intensity or greenhouse gas concentrations, are not locally or regionally confined.

While they are distinct phenomena, it can be argued that summertime heat islands may contribute to global warming by increasing demand for air conditioning, which results in additional power plant emissions of heat-trapping greenhouse gases. Strategies to reduce heat islands, therefore, can also reduce the emissions that contribute to global warming.

Mitigation strategies followed by different countries

Although urban climatologists have been studying urban heat islands for decades, community interest and concern regarding them has been more recent. This increased attention to heat-related environment and health issues has helped to advance the development of heat island reduction strategies.

US mitigation strategies

Demonstration projects

provide the data and publicity needed to develop larger initia­tives, promote new technologies and help get them to market, and sometimes even

encourage local economic development.

Incentive programs

Gathering research on local beneits, establishing a resource for the community, educating New Yorkers on the value of green roofs, and advocating sustainable building practices.

Urban forestry programs

Increasing tree cover.

Outreach and education

Awareness among people and school children’s

Awards.

High light innovation and promote solutions to mitigate heat islands across the public and private sectors.

Policy Efforts

Procurement

The city revised its general bid criteria to ensure that materials used are equivalent to those on the ENERGY STAR Roofing Products list.

Resolutions

A city or county council, or organizations, issue resolutions. It can be the first step in getting an initiative started.

Tree and landscape ordinances

Ensure public safety, protect trees or views,

and provide shade. Three types of ordi­

nances, in particular, are most useful from

a heat island perspective: tree protection,

street trees, and parking lot shade

Comprehensive plans and design guidelines

Comprehensive plans, sometimes called general plans in California and other states, are adopted by a legislative body of a local government, and set forth policies, goals, and objectives to direct development and conservation that occurs within its planning jurisdiction.

Zoning codes

These regulations generally dictate function for an area, building height and bulk, population density, and parking requirements.

Green building standards

Many local, state, and federal governments have adopted green building programs, or standards, that capture heat island reduction strategies.

Building codes

Some cities and states have begun including cool roofing in their building codes because of

its potential to save energy, particularly

during peak loads.

Canada mitigation measures

Pilot garden project on the roof of the the Télé-Université (TÉLUQ) building –Project carried out by the group Alternatives.

Urban agriculture.

Greening of the Milton Park neighbourhood.

Planting of 70 trees, 260 shrubs, 86 climbing

plants and more than 200 perennials.

Tree, Reforestation and Environment

Enhancement (TREE) Program.

Planting of 100,000 trees between 2007 and

2010.

Green Roof Strategy

Green roof incentive program in the form of a

pilot project

Green Roof By-law.

First major city in Canada to make green roofs

mandatory on new buildings:

- 50 to 75 new green roofs per year;

- The roof of the city hall will be converted to a

garden in the fall of 2009.

60 cm retention ditches along city streets.

Reduced municipal taxes.

Reduced costs related to management of the

stormwater drainage system

O’Train: light rain transit – 8,000 trips

per day

Improved air quality by reducing polluting emissions from

vehicles.

Reduced anthropogenic heat.

Novo climat certification:

- Optimized ventilation system.

- Upgraded insulation.

Improved building energy performance (minimum 25%).

Treatment wetland in Vincent-d’Indy

Park.

Natural filtration of runoff.

Recreation: trails and skating rinks in winter.

Conclusion

The extent to which urban areas can benefit from heat island reduction strategies depends on a number of factors—some within and some outside of a community's control. Although prevailing weather patterns, climate, geography, and topography are beyond the influence of local policy, decision makers can select a range of energy-saving strategies that will generate multiple benefits, including vegetation, landscaping, and land use design projects, and improvements to building and road materials.

An urban heat island mitigation strategy must be based on an integrated and multidisciplinary approach to urban development and requires the participation of various actors, particularly from the community concerned (members of the public), as well as various sectors, for example public health, urban planning, architecture, transportation and natural resources.