45 What is Urban Heat Island

General Introduction: What is Urban Heat Island ?

The concept of urban heat island is defined as a zone of warmer temperatures in the center of a city produced by enhanced absorption of solar radiation by urban surfaces and by waste heat release.

What is Urban Heat Island?

We call the central area an urban heat island because it has a significantly elevated temperature. Such a large quantity of heat is stored in the ground during the daytime hours that the heat island remains warmer than its surroundings during the night, too.

The difference between rural and urban surfaces creates an urban heat island effect that makes cities warmer than surrounding areas. Waste heat from urban activities also significantly warms cities.

Heat islands are urbanized areas that experience higher temperatures than outlying areas.

Structures such as buildings, roads, and other infrastructure absorb and re-emit the sun’s heat more than natural landscapes such as forests and water bodies.

Urban areas, where these structures are highly concentrated and greenery is limited, become “islands” of higher temperatures relative to outlying areas. Daytime temperatures in urban areas are about 1–7°F higher than temperatures in outlying areas and nighttime temperatures are about 2-5°F higher.

Heat Island Impacts

The elevated temperatures of Heat Islands can affect the environment and the quality of life of a community in multiple ways.

The urban heat island effect has significant economic impacts. In the summer, higher temperatures consume more air conditioning and electrical electricity.

The fossil fuel used to create this power emits CO2 and air pollutants into the atmosphere.

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Figure 1: – Air conditioning systems release waste heat into the atmosphere such that their widespread use can inadvertently elevate city air temperatures.

Increased temperatures may cause smog, which is both harmful and destructive to materials.

The heat island effect is not always applicable to cities in arid regions. The evapotranspiration of the city’s irrigated vegetation may actually keep the city cooler than the neighboring barren area in the desert.

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Figure 2: – This image, taken at night in May, over downtown Atlanta, Georgia, shows the urban heat island. The red and yellow part of the imagery is warmer as compared with the green and blue outer part of the core city.
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Urban heat islands may be associated with reduced air quality and other health hazards and often afflict less affluent neighborhoods with proportionately less access to parks and other forms of cooling green space.

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Figure 3: – This graphical representation of the diagram shows how air temperatures might vary across the urban and rural areas during the late afternoon.

Elevated Emissions of Air Pollutants and Greenhouse Gases

  • As described above, heat islands increase the demand for electricity in the summer.
  • Electricity supply companies often rely on fossil fuel power plants to meet this demand, leading to increased air pollution and greenhouse gas emissions.
  • These pollutants are harmful to human health and also contribute to complex air quality problems, such as the build-up of ground-level ozone (smog), fine particulate matter, and acid rain.
  • The increased use of plants that run on fossil fuels also increases emissions of greenhouse gases such as carbon dioxide, which contribute to global climate change.
  • In addition to their effect on energy-related emissions, elevated temperatures can directly increase the rate of ground-level ozone formation.
  • Ground-level ozone is formed when nitrogen oxides and volatile organic compounds react in the presence of sunlight and warm weather.
  • If all other variables are equal, such as past emissions levels into the air and wind speed and direction, then more ozone will accumulate at ground level as the environment becomes sunnier and warmer.
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Compromised Human Health and Comfort

1. Heat islands contribute to higher daytime temperatures, less coolness at night, and higher levels of air pollution. In turn, these contribute to heat-related deaths and heat-related illnesses such as malaise, breathing difficulties, heat cramps, heat exhaustion, and non-fatal heat stroke.

2. Heat islands can also amplify the effects of naturally occurring heat waves, which are periods of unusually hot and often humid weather. Vulnerable populations, such as children, older adults, and people with existing health problems, are at particular risk during these events.

3. Extreme heat events, or sudden and dramatic increases in temperature, are particularly dangerous and can result in higher average death rates. From 2004 to 2018, the Centers for Disease Control and Prevention recorded 10,527 heat-related deaths in the United States, an average of 702 per year. These numbers include deaths where heat was the underlying cause and deaths where heat was a contributing cause.


Impaired Water Quality

  • Higher temperatures from pavement and roof surfaces can heat stormwater runoff, which flows into storm sewers and raises the temperature of the water as it is released into streams, rivers, ponds and lakes.
  • Water temperature affects all aspects of aquatic life, particularly the metabolism and reproduction of many aquatic species.
  • Rapid temperature changes in aquatic ecosystems resulting from warm stormwater runoff can be particularly stressful and even fatal to aquatic life. One study found that urban streams are on average warmer than streams in forested areas and that temperatures in urban streams rose by more than 7 °F during small storms due to warm runoff from urban materials.
  • Green infrastructure is an alternative to cooling stormwater runoff and improving water quality. Downspout disconnects may include the use of rain gardens, planters, bioswales, permeable sidewalks, green streets and alleys, green parking lots and green roofs; as well as land conservation efforts.

Heat Island Compendium

  • Urban areas are generally warmer than their rural surroundings, a phenomenon known as the “heat island effect.” As cities grow, more vegetation is destroyed and more surfaces are paved or covered with buildings.
  • Changes in ground cover reduce shade and humidity to keep urban areas cool. Built-up areas also tend to evaporate less water, contributing to elevated surface and air temperatures. The properties of urban materials, in particular solar reflectivity, thermal emission, and heat capacity, also influence the development of urban heat islands, as they determine how solar energy is reflected, emitted, and absorbed.
  • The extremes of summer can affect Heat Island communities by increasing energy demand, air conditioning costs, air pollution, and greenhouse gas emissions, heat-related illness and mortality, and the quality of the water.
  • Reduction of urban heat islands: Collection of strategies (October 2008) describes the causes and effects of urban heat islands in summer.
  • Promotes strategies to reduce temperatures in communities. Provides an overview of heat islands, how they form and their effects, and a detailed description of the main urban heat island reduction strategies. It also describes the voluntary and political efforts of state and local governments to reduce urban heat islands.

Measuring Heat Islands

Designing an Approach for Assessing Your City’s Heat Island

Is your city an urban heat island? Where are the “hot spots” and best areas within the city? Do vulnerable populations, such as older adults or people with low incomes, live in warm neighborhoods? The information on this page can help you answer these questions. Guides you through the steps to design an approach to determine if the heat island effect is affecting your city and, if so, how. Start by clarifying your objectives, which will help you focus your efforts on finding and analyzing information that meets your needs.

You can then move on to designing a general approach, which includes defining the geographic area you will study (where), identifying relevant times of day and weather (when), and considering whether What type of data is appropriate. Once you have drawn up your approach and are ready to begin the actual assessment, you can use this method to find references to existing data sources and detailed protocols and methodologies for collecting and analyzing temperature and other information. You can access the information at the bottom of the page.

This page provides a simple decision framework and important considerations to keep in mind when designing a heat island impact assessment for your city. It does not present a detailed “how-to” for completing a complete heat island analysis, as the specific steps you take will depend on your goals, the type of data you use, and your available resources.

There are many reasons to undertake a heat island assessment, but the two most common are:

  • Understand the effects of energy:

Higher urban temperatures increase the demand for air conditioning, leading to higher energy bills during the warmer months of the year. Analyzing how the temperature in an urban area differs from that of the surrounding area will help you measure energy impacts.

  • Understanding the risks to public health:

Heat islands can contribute to poor air quality, exacerbate the effects of extreme heat events, and put people’s health at greater risk. Identifying hot spots within a city can help focus interventions where they are most needed during heat waves.

Identifying Data Needs

After clarifying the objectives, you are ready to determine the geographic coverage of your data collection effort, the type of data needed (air or surface temperature, or both), and where you can find useful sources of existing temperature data. Your goals will determine your needs and data sources.

Geographic coverage (where)

  • Assessments focused primarily on the energy-related impacts of heat islands generally compare the temperature in the general urban area with the temperature in the surrounding rural area to determine how much additional energy demand is caused by the urban heat island.
  • Assessments focused on the health-related impacts of heat islands generally focus on assessing differences in air temperature between different locations within the city (ie, identifying hot spots).


Types of Temperature Data and Technologies for Capturing Data

The next step after deciding where to measure is choosing whether to collect data on air temperatures, surface temperatures, or a combination of both.

1. Air temperatures are important for estimating heat islands, they are found within an urban canopy, from ground level to the tops of trees and buildings. They are most useful for a study aimed at reducing public health risks, as they are the best indicator of the conditions that people actually experience.

Air temperature can be measured directly using standard weather stations and other monitoring devices and/or mobile cruises (cars with sensors that record temperature along a fixed-line). However, because monitoring networks and crossings typically cover only a portion of a city area, they may not provide a representative picture of citywide temperatures. Urban climate models can be used in conjunction with observed data to estimate temperatures in locations where field data is not available.

2. Surface temperature represents the thermal energy emitted by the earth, buildings, and other surfaces. Technologies that measure surface temperatures, such as instruments on satellites and aircraft, may provide better geographic coverage than instruments used to record air temperature. They can reveal temperature differences on very fine scales: for example, between roofs, sidewalks, and green areas. However, satellite data has several limitations (see “Considerations for measuring surface temperature” below). A combination of satellite data for surface temperature and data from monitoring stations or traverses for air temperature provides the most complete picture of a city’s heat island.

3. Daily and seasonal temperature patterns are another data item to consider. For example, nighttime temperatures may pose a greater health risk than daytime temperatures, and heat islands may exist in seasons other than summer.


Considerations for Measuring Air Temperatures

1. Measure and map the locations of existing data monitors (such as standard weather stations and other monitoring networks) to identify city areas with relevant information gaps for your specific analysis.

2. Select data collection sites that represent your environment in terms of surface material, geometry, and human activity.

3. Establish an appropriate number of monitoring sites to avoid bias (for example, comparing data from an urban site to data from a rural site may not accurately represent a city heat island).

4. Follow a consistent protocol for monitor placement, sensor height and direction, protection from sunlight, and other important factors that affect recorded temperature. For example, the ceiling is a popular place to monitor air temperature, but recorded temperatures can be inaccurate if the monitors are too close to ceiling heating, ventilation, and air conditioning equipment.

5. Clearly document study methodology and metadata (time period, spatial region).

Considerations for Measuring Surface Temperatures

The surface temperature can be measured indirectly using satellites, aircraft, and ground-based instruments. While it is possible to estimate air temperature from surface temperature data, estimates are less reliable than direct measurements.

1. Satellites provide broad geographic coverage, but may not represent the finest detail of hot spots within a neighborhood. Trees or tall buildings can prevent satellites from accurately capturing surface temperatures at ground level. Data is collected only when a satellite passes over a city and is available only for clear weather conditions.

2. Surface temperature data from aircraft instruments provides higher resolution than satellite data, as aircraft fly at lower altitudes, but aircraft data is more expensive and provides less coverage.

3. Use of ground-based thermal sensing (eg, the use of hand-held devices pointed at surfaces to measure their temperature) for specific urban features (eg, parking lots versus city parks) or for different types of surfaces can be used to obtain surface temperature data. . Like a light-colored ceiling versus a dark-colored ceiling.

Heat Island Cooling Strategies

Strategies and Technologies:

  • Trees and vegetation:

Trees and growing vegetation reduce surface and air temperatures by providing shade and evaporative cooling. Trees and vegetation can also reduce stormwater runoff and protect against erosion. 

  • Green roofs:

the growth of a topsoil (plants, shrubs, grass and / or trees) on the roof reduces the temperature of the roof surface and the surrounding air and improves stormwater management. Also called “roof gardens” or “green roofs,” green roofs achieve these benefits by providing shade and removing heat from the air through evaporation.

  • Cold roofs –

Installing a cold roof, one made of materials or sheathing that significantly reflects sunlight and heat away from the building, reduces roof temperature, increases occupant comfort, and reduces energy demand.

  • Quieter sidewalks –

Using paving materials on sidewalks, parking lots and streets that remain cooler than traditional sidewalks (by reflecting more solar energy and increasing water evaporation) not only cools the pavement surface and the surrounding air, It can also reduce storm water runoff and improve night time. visibility.

  • Smart development:

These practices include a variety of development and conservation strategies that help protect the natural environment and make our communities more attractive, financially stronger, and more livable.

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