Reducing Urban Heat Islands: Urban Heat Island Basics

8/9/2019 Reducing Urban Heat Islands: Urban Heat Island Basics

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Reducing Urban Heat IslandsCompendium of Strategies

Urban Heat Island Basics


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Acknowledgements

Reducing Urban Heat Islands: Compendium of Strategiesdescribes the

causes and impacts o summertime urban heat islands and promotes

strategies or lowering temperatures in U.S. communities. This compendium

was developed by the Climate Protection Partnership Division in the U.S.

Environmental Protection Agencys Oce o Atmospheric Programs. Eva

Wong managed its overall development. Kathleen Hogan, Julie Rosenberg,

and Andrea Denny provided editorial support. Numerous EPA sta in

oces throughout the Agency contributed content and provided reviews.

Subject area experts rom other organizations around the United States and

Canada also committed their time to provide technical eedback.

Under contracts 68-W-02-029 and EP-C-06-003, Perrin Quarles Associates,

Inc. provided technical and administrative support or the entire

compendium, and Eastern Research Group, Inc. provided graphics and

production services.

PositvEnergy provided support in preparing the Trees and Vegetation, Cool

Roos, and UHI Activities chapters under contract PO #2W-0361-SATX.

Experts who helped shape this chapter include:

Hashem Akbari, Ryan Bell, Tony Brazel, David Cole, Maury Estes, Gordon

Heisler, David Hitchcock, Brenda Johnson, Megan Lewis, Greg McPherson,

Tim Oke, Danny Parker, Alan Perrin, Joyce Rosenthal, David Sailor, Jason

Samenow, Haider Taha, James Voogt, Darrell Winner, Kathy Wol, and Barry

Zalph.


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Contents

Urban Heat Island Basics 1

1. What Are Urban Heat Islands? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1

1.1 Surace Urban Heat Islands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2

1.2 Atmospheric Urban Heat Islands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3

2. How Do Urban Heat Islands Form? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7

2.1 R educed Vegetation in Urban Areas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7

2.2 Properties o Urban Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8

2.3 Urban Geometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10

2.4 Anthropogenic Heat . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12

2.5 Additional Factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12

3. Why Do We Care about Urban Heat Islands? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13

3.1 Energy Consumption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13

3.2 Air Quality and Greenhouse Gases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14

3.3 Human Health and Comort . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14

3.4 Water Quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15

4. Strategies to Reduce Urban Heat Islands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16

5. Additional Resources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16

Endnotes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .18


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URBAN HeAt IslANd BAsIcs dRAFt 1

As urban areas develop, changes occur in the landscape. Buildings, roads, and other

inrastructure replace open land and vegetation. Suraces that were once perme-

able and moist generally become impermeable and dry.* This development leads to

the ormation o urban heat islandsthe phenomenon whereby urban regions experience

warmer temperatures than their rural surroundings.

This chapter provides an overview o dierent types o urban heat islands, methods or

identiying them, and actors that contribute to their development. It introduces key con-

cepts that are important to understanding and mitigating this phenomenon, as well as ad-

ditional sources o inormation. It discusses:

General eatures o urban heat islands

Surace versus atmospheric heat islands

Causes o urban heat island ormation

Urban heat island impacts on energy consumption, environmental quality, and human health

Resources or urther inormation.

1 What Are Urban Heat Islands?

Many urban and suburban areas experience elevated temperatures compared to their out-lying rural surroundings; this dierence in temperature is what constitutes an urban heat

island. The annual mean air temperature o a city with one million or more people can

be 1.8 to 5.4F (1 to 3C) warmer than its surroundings,1 and on a clear, calm night, this

temperature dierence can be as much as 22F (12C).2 Even smaller cities and towns will

produce heat islands, though the eect oten decreases as city size decreases.3

This chapter ocuses onsurfaceand atmosphericurban heat islands. These two heat island

types dier in the ways they are ormed, the techniques used to identiy and measure

them, their impacts, and to some degree, the methods available to mitigate them. Table 1

summarizes the basic characteristics o each type o heat island. These eatures are de-

scribed in more detail in the ollowing sections o this chapter.

Urban Heat Island Basics

* This change in landscape may dier in regions such as deserts, where moisture may increase in urban areas i development introduces grass lawns and

other irrigated vegetation.


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2 RedUcING URBAN HeAt IslANds dRAFt

1.1 Surace Urban Heat Islands

On a hot, sunny summer day, the sun can

heat dry, exposed urban suraces, like roos

and pavement, to temperatures 50 to 90F

(27 to 50C) hotter than the air,5 while

shaded or moist suracesoten in more

rural surroundingsremain close to air

temperatures. Surace urban heat islands

are typically present day and night, but

tend to be strongest during the day when

the sun is shining.

On average, the dierence in daytime sur-

ace temperatures between developed and

rural areas is 18 to 27F (10 to 15C); the

dierence in nighttime surace tempera-

tures is typically smaller, at 9 to 18F (5 to

10C).6

The magnitude o surace urban heat is-

lands varies with seasons, due to changes

in the suns intensity as well as groundcover and weather. As a result o such

variation, surace urban heat islands are

typically largest in the summer.7

To identiy urban heat islands, scientists

use direct and indirect methods, numerical

modeling, and estimates based on empiri-

cal models. Researchers oten use remote

sensing, an indirect measurement tech-

nique, to estimate surace temperatures.

They use the data collected to produce

thermal images, such as that shown in

Figure 1.

Feature Surace UHI Atmospheric UHI

Temporal Development Present at all times o the day and

night

Most intense during the day and in

the summer

May be small or non-existent during

the day

Most intense at night or predawn and

in the winter

Peak Intensity

(Most intense UHI

conditions)

More spatial and temporal variation:

n Day: 18 to 27F (10 to 15C)

n Night: 9 to 18F (5 to 10C)

Less variation:

n Day: -1.8 to 5.4F (-1 to 3C)

n Night: 12.6 to 21.6F (7 to 12C)

Typical Identifcation

Method

Indirect measurement:

n Remote sensing

Direct measurement:

n Fixed weather stations

n Mobile traverses

Typical Depiction Thermal image Isotherm map

Temperature graph

Table 1: Basic Characteristics o Surace and Atmospheric Urban Heat Islands (UHIs)4

How Weather Inuences

Urban Heat Islands

Summertime urban heat islands are

most intense when the sky is clear

and winds are calm. Heavy cloud

cover blocks solar radiation, reducing

daytime warming in cities. Strong

winds increase atmospheric mixing,lowering the urban-rural temperature

dierence. This document,Reducing

Urban Heat Islands: Compendium

of Strategies, ocuses on mitigating

summertime heat islands through

strategies that have maximum impact

under clear, calm conditions.


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1.2 Atmospheric Urban Heat

Islands

Warmer air in urban areas compared to

cooler air in nearby rural surroundings

denes atmospheric urban heat islands.

Experts oten divide these heat islands into

two dierent types:

Canopy layer urban heat islands exist

in the layer o air where people live,rom the ground to below the tops o

trees and roos.

Boundary layer urban heat islands

start rom the rootop and treetop

level and extend up to the point where

urban landscapes no longer infuence

the atmosphere. This region typically

extends no more than one mile (1.5

km) rom the surace.8

Canopy layer urban heat islands are the

most commonly observed o the two

types and are oten the ones reerred to in

discussions o urban heat islands. For this

reason, this chapter and compendium use

the more general term atmospheric urban

heat islandsto reer to canopy layer urban

heat islands.

Atmospheric urban heat islands are oten

weak during the late morning and through-

out the day and become more pronounced

ater sunset due to the slow release o heat

rom urban inrastructure. The timing o

this peak, however, depends on the proper-

ties o urban and rural suraces, the season,

and prevailing weather conditions.

Figure 1: Thermal Image Depicting a

Surace Urban Heat Island

This image, taken rom an aircrat, depicts a

midday surace urban heat island in Salt Lake

City, Utah, on July 13, 1998. White areas are

around 160F (70C), while dark blue areas are

near 85F (30C). Note the warmer urban surace

temperatures (let side o image) and cooler

suraces in the neighboring oothills (on the right).

NASA/MarshallSpaceFlightCenter


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Surace and Air Temperatures: How Are They Related?

Surace temperatures have an indirect, but signicant, infuence on air temperatures,

especially in the canopy layer, which is closest to the surace. For example, parksand vegetated areas, which typically have cooler surace temperatures, contribute to

cooler air temperatures. Dense, built-up areas, on the other hand, typically lead to

warmer air temperatures. Because air mixes within the atmosphere, though, the rela-

tionship between surace and air temperatures is not constant, and air temperatures

typically vary less than surace temperatures across an area (see Figure 2).

Surface Temperature (Day)Air Temperature (Day)

Surface Temperature (Night)Air Temperature (Night)

Urban

Residential

Suburban

Temperature

DAY

NIGHT

Warehouse

or Industrial

Urban

Residential

Downtown Park RuralPondRural Suburban

Surace and atmospheric temperatures vary over dierent land use areas. Surace

temperatures vary more than air temperatures during the day, but they both are airly similar

at night. The dip and spike in surace temperatures over the pond show how water maintains

a airly constant temperature day and night, due to its high heat capacity.

* Note: The temperatures displayed above do not represent absolute temperature values or

any one particular measured heat island. Temperatures will uctuate based on actors such as

seasons, weather conditions, sun intensity, and ground cover.

Figure 2: Variations o Surace and Atmospheric Temperatures

ModifedromVoogt,2000


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Atmospheric heat islands vary much less in

intensity than surace heat islands. On an

annual mean basis, air temperatures in large

cities might be 1.8 to 5.4F (1 to 3C) warm-

er than those o their rural surroundings.9

Researchers typically measure air tem-

peratures through a dense network o

sampling points rom xed stations or

mobile traverses, which are both direct

measurement methods. Figure 3 illustrates

a conceptual isotherm map that depicts an

atmospheric urban heat island. The center

o the gure, which is the hottest area, is

the urban core. A simple graph o tempera-

ture dierences, as shown in Figure 4, is

another way to show the results.

+1+3

+2

+4+3

+2

+5

+1

+2

+2

Park

+4+1+5

+6

+2

+3

+3

+1

+3

+1+3

+2

+4+3

+2

+5

+1

+2

+2

Wind

Park+4+1

+5

+6

+2

+3

+3

+1

+3

Figure 3: Isotherm Map Depicting an Atmospheric

Nighttime Urban Heat Island

This conceptual map with overlaid isotherms (lines o equal air temperature)

exhibits a ully developed nighttime atmospheric urban heat island. Thedotted red line indicates a traverse along which measurements are taken.

ModifedromVoogt,2000

Sunset

(a)

2C

(b)

Sunrise

AirTemperature

HeatIslandIntensity

Time

T = CLUHIUR

12 18 24 06 12

Urban

Rural

Figure 4: Conceptual Drawing o the Diurnal Evolution o the Urban Heat

Island during Calm and Clear Conditions

Atmospheric urban heat islands

primarily result rom dierent cooling

rates between urban areas and their

surrounding rural or non-urban

surroundings (section (a) o Figure

5). The dierential cooling rates are

most pronounced on clear and calm

nights and days when rural areas cancool more quickly than urban areas.

The heat island intensity (section

(b)) typically grows rom mid- to late

aternoon to a maximum a ew hours

ater sunset. In some cases, a heat

island might not reach peak intensity

until ater sunrise.

ModifedromOke,1

982,andRunnalls

andOke,2

000


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Urban heat islands reer to the elevated tempera-

tures in developed areas compared to more rural

surroundings. Urban heat islands are caused bydevelopment and the changes in radiative and

thermal properties o urban inrastructure as well

as the impacts buildings can have on the local

micro-climateor example tall buildings can

slow the rate at which cities cool o at night.

Heat islands are infuenced by a citys geographic

location and by local weather patterns, and their

intensity changes on a daily and seasonal basis.

The warming that results rom urban heat islands

over small areas such as cities is an exampleo local climate change. Local climate changes

resulting rom urban heat islands undamentally

dier rom global climate changes in that their

eects are limited to the local scale and decrease

with distance rom their source. Global climate

changes, such as those caused by increases in

the suns intensity or greenhouse gas concentra-

tions, are not locally or regionally conned.

Climate change, broadly speaking, reers to

any signicant change in measures o climate(such as temperature, precipitation, or wind)

lasting or an extended period (decades or

longer). Climate change may result rom:

Natural actors, such as changes in the

suns intensity or slow changes in the

Earths orbit around the sun

Natural processes within the climate sys-

tem (e.g. changes in ocean circulation)

Human activities that change the atmo-

spheres composition (e.g. burning ossil

uels) and the land surace (e.g. deoresta-

tion, reorestation, or urbanization).

The term climate change is oten used inter-

changeably with the term global warming, but

according to the National Academy o Sci-

ences, the phrase climate change is growing

in preerred use to global warming because

it helps convey that there are [other] changes

in addition to rising temperatures.

Global warming is an average increase in

the temperature o the atmosphere near the

Earths surace and in the lowest layer o the

atmosphere, which can contribute to changes

in global climate patterns. Global warming

can occur rom a variety o causes, both natu-

ral and human induced. In common usage,

global warming oten reers to the warming

that can occur as a result o increased emis-

sions o greenhouse gases rom human activi-ties. Global warming can be considered part

o global climate change along with changes

in precipitation, sea level, etc.

The impacts rom urban heat islands and

global climate change (or global warm-

ing) are oten similar. For example, some

communities may experience longer grow-

ing seasons due to either or both phenom-

ena. Urban heat islands and global climate

change can both also increase energy de-mand, particularly summertime air condition-

ing demand, and associated air pollution and

greenhouse gas emissions, depending on the

electric system power uel mix.

Strategies to reduce urban heat islandsthe

ocus o this document,Reducing Urban

Heat Islands: Compendium of Strategies

produce multiple benets including lower-

ing surace and air temperatures, energy

demand, air pollution and greenhouse gasemissions. Thus, advancing measures to

mitigate urban heat islands also helps to ad-

dress global climate change.

For more inormation on global warming see

EPAs Climate Change website, .

Urban Heat Islands, Climate Change, and Global Warming


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2 How Do Urban Heat Islands

Form?

While many actors contribute to urban

heat island ormation (see Table 2), this

chapter ocuses on vegetative cover and

surace properties because communities

can directly address these actors with

available technologies. See the Trees and

Vegetation, Green Roos, Cool Roos,

and Cool Pavement chapters or detailed

inormation on these strategies.

2.1 Reduced Vegetation in Urban Areas

In rural areas, vegetation and open land

typically dominate the landscape. Trees

and vegetation provide shade, which helpslower surace temperatures. They also help

reduce air temperatures through a process

called evapotranspiration, in which plants

release water to the surrounding air, dis-

sipating ambient heat. In contrast, urban

areas are characterized by dry, impervious

suraces, such as conventional roos, side-

walks, roads, and parking lots. As cities

develop, more vegetation is lost, and more

suraces are paved or covered with build-

ings. The change in ground cover results

in less shade and moisture to keep urban

areas cool. Built up areas evaporate less

water (see Figure 5), which contributes to

elevated surace and air temperatures.

40% evapotranspiration30% evapotranspiration

10% runoff

25% shallow

infiltration 25% deep

infiltration

10% shallow

infiltration5% deep

infiltration

55% runoff

Figure 5: Impervious Suraces and Reduced Evapotranspiration

Highly developed urban areas (right), which are characterized by 75%-100% impervious suraces, have less surace

moisture available or evapotranspiration than natural ground cover, which has less than 10% impervious cover (let).

This characteristic contributes to higher surace and air temperatures in urban areas.

ModifedromtheFederalInteragencyStreamRestorationWorkingGroup(FISRWG)


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2.2 Properties o Urban Materials

Properties o urban materials, in particular

solar refectance, thermal emissivity, and

heat capacity, also infuence urban heat

island development, as they determine how

the suns energy is refected, emitted, andabsorbed.

Figure 6 shows the typical solar energy that

reaches the Earths surace on a clear sum-

mer day. Solar energy is composed o ultra-

violet (UV) rays, visible light, and inrared

energy, each reaching the Earth in dierent

percentages: ve percent o solar energy is

in the UV spectrum, including the type o

rays responsible or sunburn; 43 percent o

solar energy is visible light, in colors rang-ing rom violet to red; and the remaining

52 percent o solar energy is inrared, elt

as heat. Energy in all o these wavelengths

contributes to urban heat island ormation.

Solar refectance, or albedo, is the percent-

age o solar energy refected by a surace.

Much o the suns energy is ound in the

visible wavelengths (see Figure 6); thus,

solar refectance is correlated with a mate-

rials color. Darker suraces tend to have

lower solar refectance values than lighter

suraces. Researchers are studying and

developing cool colored materials, though,

that use specially engineered pigments that

refect well in the inrared wavelengths.

These products can be dark in color but

have a solar refectance close to that o a

white or light-colored material. (See the

Cool Roos chapter or urther discussion

o cool colored roo products.)

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600

NormalizedSolarIntensity

ultraviolet visible infrared

Figure 6: Solar Energy versus Wavelength Reaching Earths Surace

Solar energy intensity varies over wavelengths rom about 250 to 2500 nanometers.


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Urban areas typically have surace materi-

als, such as roong and paving, which have

a lower albedo than those in rural settings.

As a result, built up communities gener-

ally refect less and absorb more o the

suns energy. This absorbed heat increases

surace temperatures and contributes to

the ormation o surace and atmospheric

urban heat islands.

Although solar refectance is the main

determinant o a materials surace tem-

perature, thermal emittance, or emissivity,

also plays a role. Thermal emittance is a

measure o a suraces ability to shed heat,

or emit long-wave (inrared) radiation. All

things equal, suraces with high emittance

values will stay cooler, because they will

release heat more readily. Most construc-

tion materials, with the exception o metal,

have high thermal emittance values. Thus,

this property is mainly o interest to those

installing cool roos, which can be metallic.

See the Cool Roos chapter o the com-

pendium or more inormation.

Another important property that infuences

heat island development is a materials heat

capacity, which reers to its ability to store

heat. Many building materials, such as steel

and stone, have higher heat capacities than

rural materials, such as dry soil and sand.

As a result, cities are typically more e-

ective at storing the suns energy as heat

within their inrastructure. Downtown met-

ropolitan areas can absorb and store twice

the amount o heat compared to their rural

surroundings during the daytime.10

Radiative and Thermal

PropertiesCool Roosand Cool Pavements

Albedo and emissivity are considered

radiative properties. Heat capacity,

on the other hand, is one o several

thermal properties a material can

possess. For thin materials like roo-

ing, which is typically placed over

insulation, refectance and emittance

are the main properties to consider,

as the heat capacity o a well insu-

lated roo is low. For pavements,

which are thicker than roongproducts and are placed on top o

the ground, which has its own set o

thermal characteristics, designers and

researchers need to consider a more

complex set o actors that include

radiative and thermal properties

such as heat capacity, thermal con-

ductivity, and density.


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2.3 Urban Geometry

An additional actor that infuences urban

heat island development, particularly at

night, is urban geometry, which reers to

the dimensions and spacing o buildings

within a city. Urban geometry infuenceswind fow, energy absorption, and a given

suraces ability to emit long-wave radiation

back to space. In developed areas, suraces

and structures are oten at least partially

obstructed by objects, such as neighbor-

ing buildings, and become large thermal

masses that cannot release their heat very

readily because o these obstructions. Espe-

cially at night, the air above urban centers

is typically warmer than air over rural ar-

eas. Nighttime atmospheric heat islands canhave serious health implications or urban

residents during heat waves (see textbox

in Section 3.3, Factors in Heat-Related Ill-

nesses and Death.)

Researchers oten ocus on an aspect o

urban geometry called urban canyons,

which can be illustrated by a relatively nar-

row street lined by tall buildings. During

the day, urban canyons can have compet-

ing eects. On the one hand, tall buildings

can create shade, reducing surace and air

temperatures. On the other, when sunlight

reaches suraces in the canyon, the suns

energy is refected and absorbed by build-

ing walls, which urther lowers the citys

overall albedothe net refectance rom

surace albedo plus urban geometry

and can increase temperatures.11 At night,

urban canyons generally impede cooling,

as buildings and structures can obstruct

the heat that is being released rom urbaninrastructure.

Table 2: Factors that Create Urban Heat Islands

Factors Communities are Focusing On

Reduced vegetation in urban regions: Reduces the natural cooling eect rom shade and evapotranspiration.

Properties o urban materials: Contribute to absorption o solar energy, causing suraces, and the air above

them, to be warmer in urban areas than those in rural surroundings.

Future Factors to Consider

Urban geometry: The height and spacing o buildings aects the amount o radiation received and emitted by

urban inrastructure.

Anthropogenic heat emissions: Contribute additional warmth to the air.*

Additional Factors

Weather: Certain conditions, such as clear skies and calm winds, can oster urban heat island ormation.

Geographic location: Proximity to large water bodies and mountainous terrain can inuence local wind patterns

and urban heat island ormation.

* Although communities currently can lower anthropogenic heat emissions through energy efciency technologies

in the building and vehicle sectors, this compendium ocuses on modiying vegetative cover and surace properties

o urban materials, as they have long been regarded as urban heat island reduction strategies. An emerging body

o literature on the role waste heat plays in urban heat island ormation, though, may lead communities to ocus on

anthropogenic heat in the near uture.


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The Urban Surace Energy Budget

An energy budget provides an equation that quanties the balance o incoming

and outgoing energy fows, or fuxes (see Figure 7). The surace energy budgets ourban areas and their more rural surroundings will dier because o dierences in

land cover, surace characteristics, and level o human activity. Such dierences can

aect the generation and transer o heat, which can lead to dierent surace and air

temperatures in urban versus rural areas. Various elements o the budget include:

Short-wave radiation is ultraviolet, visible light, and near-inrared radiation rom

the sun that reaches the Earth (see Figure 6). This energy is a key driver o urban

heat islands. Urban suraces, compared to vegetation and other natural ground

cover, refect less radiation back to the atmosphere. They instead absorb and store

more o it, which raises the areas temperature.

Thermal storage

increases in cities in part due to the lower solar refectance ourban suraces, but it is also infuenced by the thermal properties o construction

materials and urban geometry. Urban geometry can cause some short-wave radia-

tionparticularly within an urban canyonto be refected on nearby suraces, such

as building walls, where it is absorbed rather than escaping into the atmosphere.

Short-wave radiation

Latent heat

Long-wave radiationAnthropogenic heat

Sensible heat

Thermal storage

Figure 7: Urban Surace Energy Budget

Adaptedro

mDavidSailor

Continued on next page


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The eects o urban geometry on urban heat

islands are oten described through the sky

view actor (SVF), which is the visible area

o the sky rom a given point on a surace.

For example, an open parking lot or eld that

has ew obstructions would have a large SVF

value (closer to 1). Conversely, an urban can-

yon in a downtown area that is surrounded

by closely spaced, tall buildings, would have a

low SVF value (closer to zero), as there wouldonly be a small visible area o the sky.

2.4 Anthropogenic Heat

Anthropogenic heat contributes to atmo-

spheric heat islands and reers to heat

produced by human activities. It can come

rom a variety o sources and is estimated

by totaling all the energy used or heating

and cooling, running appliances, transpor-

tation, and industrial processes. Anthro-

pogenic heat varies by urban activity and

inrastructure, with more energy-intensive

buildings and transportation producing

more heat.12 Anthropogenic heat typically

is not a concern in rural areas and during

the summer. In the winter, though, and

year round in dense, urban areas, anthro-pogenic heat can signicantly contribute to

heat island ormation.

2.5 Additional Factors

Weather and location strongly infuence

urban heat island ormation. While commu-

nities have little control over these actors,

The Urban Surace Energy Budget (continued)

Similarly, urban geometry can impede the release o long-wave, or infrared,

radiation into the atmosphere. When buildings or other objects absorb incom-ing short-wave radiation, they can re-radiate that energy as long-wave energy, or

heat. However, at night, due to the dense inrastructure in some developed areas

that have low sky view actors (see section 2.3), urban areas cannot easily release

long-wave radiation to the cooler, open sky, and this trapped heat contributes to

the urban heat island.

Evapotranspiration describes the transer o latent heat, what we eel as humid-

ity, rom the Earths surace to the air via evaporating water. Urban areas tend to

have less evapotranspiration relative to natural landscapes, because cities retain

little moisture. This reduced moisture in built up areas leads to dry, impervious

urban inrastructure reaching very high surace temperatures, which contribute to

higher air temperatures.*Convection describes the transer o sensible heat,what we eel as temperature,

between the surace and air when there is a dierence in temperature between

them. High urban surace temperatures warm the air above, which then circulates

upwards via convection.

Anthropogenic heat reers to the heat generated by cars, air conditioners, indus-

trial acilities, and a variety o other manmade sources, which contributes to the

urban energy budget, particularly in the winter.

* This change in landscape may dier in regions such as deserts, where moisture may increase in

urban areas i development introduces grass lawns and other irrigated vegetation.


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residents can benet rom understanding

the role they play.

Weather. Two primary weather char-

acteristics aect urban heat island

development: wind and cloud cover. In

general, urban heat islands orm during

periods o calm winds and clear skies,

because these conditions maximize the

amount o solar energy reaching urban

suraces and minimize the amount o

heat that can be convected away. Con-

versely, strong winds and cloud cover

suppress urban heat islands.

Geographic location. Climate and

topography, which are in part deter-

mined by a citys geographic location,infuence urban heat island ormation.

For example, large bodies o water

moderate temperatures and can gener-

ate winds that convect heat away rom

cities. Nearby mountain ranges can ei-

ther block wind rom reaching a city, or

create wind patterns that pass through

a city. Local terrain has a greater signi-

cance or heat island ormation when

larger-scale eects, such as prevailing

wind patterns, are relatively weak.

3 Why Do We Care about Urban

Heat Islands?

Elevated temperatures rom urban heat

islands, particularly during the summer,

can aect a communitys environment

and quality o lie. While some heat island

impacts seem positive, such as lengthening

the plant-growing season, most impacts are

negative and include:

Increased energy consumption

Elevated emissions o air pollutants and

greenhouse gases

Compromised human health and comort

Impaired water quality.

3.1 Energy Consumption

Elevated summertime temperatures in cities

increase energy demand or cooling and

add pressure to the electricity grid during

peak periods o demand, which generally

occur on hot, summer weekday aternoons,

when oces and homes are running cool-

ing systems, lights, and appliances (see

Figure 8). This peak urban electric demandincreases 1.5 to 2 percent or every 1F

(0.6C) increase in summertime tempera-

ture. Steadily increasing downtown temper-

atures over the last several decades mean

that 5 to 10 percent o community-wide de-

mand or electricity is used to compensate

or the heat island eect.13 During extreme

heat events, which are exacerbated by ur-

ban heat islands, the resulting demand or

cooling can overload systems and require a

utility to institute controlled, rolling brown-outs or blackouts to avoid power outages.

Wintertime Benefts o

Urban Heat Islands

Communities may benet rom thewintertime warming eect o urban

heat islands. Warmer temperatures

can reduce heating energy needs and

help to melt snow and ice on roads.

Fortunately, urban heat island mitiga-

tion strategiesor example, trees and

vegetation and green roosgenerally

provide year-round benets, or their

winter penalty, such as that rom cool

roos, is much smaller than their sum-

mertime benets.


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3.2 Air Quality and Greenhouse Gases

As discussed in Section 3.1, higher tempera-

tures can increases energy demand, which

generally causes higher levels o air pollu-

tion and greenhouse gas emissions. Cur-

rently, most electricity in the United States is

produced rom combusting ossil uel. Thus,

pollutants rom most power plants include

sulur dioxide (SO2), nitrogen oxides (NOx),

particulate matter (PM), carbon monoxide

(CO), and mercury (Hg). These pollutants

are harmul to human health and contrib-

ute to complex air quality problems such as

acid rain. Further, ossil-uel-powered plants

emit greenhouse gases, particularly carbon

dioxide (CO2), which contribute to globalclimate change.

In addition to increases in air emissions,

elevated air temperatures increase the rate

o ground-level ozone ormation, which

is produced when NOx and volatile or-

ganic compounds (VOCs) react in the

presence o sunlight. I all other variables

are equalsuch as the level o precursor

emissions or wind speed and direction

ground-level ozone emissions will be

higher in sunnier and hotter weather.

3.3 Human Health and Comort

Increased daytime surace temperatures,

reduced nighttime cooling, and higher

air pollution levels associated with urban

heat islands can aect human health by

contributing to general discomort, respira-

tory diculties, heat cramps and exhaus-

tion, non-atal heat stroke, and heat-related

mortality.

Urban heat islands can also exacerbate the

impact o heat waves, which are periods o

abnormally hot, and oten humid, weather.

Sensitive populations, such as children,

older adults, and those with existing health

conditions, are at particular risk rom these

events. For example, in 1995, a mid-July

heat wave in the Midwest caused more

than 1,000 deaths.15 While it is rare or a

0

200

400

600

800

1000

1200

0

(-18)

20

(-7)

40

(4)

60

(16)

80

(27)

100

(38)

120

(49)

Maximum Daily Temp F (C)

AverageElectricLoad

(MWh)

Figure 8: Increasing Power Loads with Temperature Increases14

As shown in this example rom New Orleans, electrical load can increase steadily once

temperatures begin to exceed about 68 to 77F (20 to 25C). Other areas o the country show

similar demand curves as temperature increases.

Sailor,2006,withdatacourtesyoEntergy


18/22


heat wave to be so destructive, heat-related

mortality is not uncommon. The Centers or

Disease Control estimates that rom 1979 to

1999, excessive heat exposure contributed

to more than 8,000 premature deaths in

the United States.18 This gure exceeds the

number o mortalities resulting rom hur-

ricanes, lightning, tornadoes, foods, and

earthquakes combined.

3.4 Water Quality

Surace urban heat islands degrade waterquality, mainly by thermal pollution. Pave-

ment and rootop suraces that reach tem-

peratures 50 to 90F (27 to 50C) higher

than air temperatures transer this excess

heat to stormwater. Field measurements

rom one study showed that runo rom

urban areas was about 20-30F (11-17C)

hotter than runo rom a nearby rural

area on summer days when pavement

temperatures at midday were 20-35F

(11-19C) above air temperature. When

the rain came beore the pavement had

a chance to heat up, runo temperatures

rom the rural and urban areas diered by

less than 4F (2C).19 This heated storm-

water generally drains into storm sewers

(see Figure 5) and raises water tempera-

tures as it is released into streams, rivers,

ponds, and lakes. A study in Arlington,Virginia, recorded temperature increases

in surace waters as high as 8F (4C) in

40 minutes ater heavy summer rains.20

Water temperature aects all aspects o

aquatic lie, especially the metabolism

and reproduction o many aquatic spe-

cies. Rapid temperature changes in aquatic

Factors in Heat-Related Illnesses and Death

Low income elderly people who live in row homes are at a particular risk or heat-

related health incidents. Living on the upper foor o a typical row home, with a darkroo, brick construction, and windows on only two sides, could contribute to the risk

o heat-related illness or death during heat waves, as temperatures in these homes

can be extreme.16 These homes oten lack air conditioning, especially in areas un-

accustomed to high temperatures. Further, even when air conditioning is available,

residents may not use it or ear o high utility bills.

Social isolation and physical health also contribute to ones vulnerability. Elderly

people, especially, may not have amily or riends nearby, may not report to work

regularly, and may lack neighbors who can check on them, leaving them stranded

during extreme heat events. The elderly may also ail to hear news or other warnings

o impending heat waves and recommendations on how to cope. Finally, their bod-ies may be less able to handle heat stress.

The lack o nighttime relie in air temperatures is strongly correlated with increased

mortality during heat waves. Some studies suggest that these oppressive nighttime

temperatures may be more signicant than high maximum daytime temperatures.17

For more inormation on heat-related health incidents and ways to respond, see the EPA

Excessive Heat Events Guidebook


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ecosystems resulting rom warm storm-

water runo can be particularly stress-

ul. Brook trout, or example, experience

thermal stress and shock when the water

temperature changes more than 2 to 4F (1-

2C) in 24 hours.21

4 Strategies to Reduce Urban

Heat Islands

Although urban climatologists have been

studying urban heat islands or 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 o heat island reduction strat-

egies, mainly trees and vegetation, green

roos, and cool roos. Interest in cool pave-

ments has been growing, and an emerg-

ing body o research and pilot projects are

helping scientists, engineers, and practitio-

ners to better understand the interactions

between pavements and the urban climate.

This compendiumReducing Urban Heat

Islands: Compendium of Strategiespro-

vides details about how these strategies

work, their benets and costs, actors

to consider when selecting them, and

additional resources or communities to

urther explore. It presents the multiple

benetsbeyond temperature reduction

that a community can accrue rom advanc-

ing heat island reduction strategies. It also

gives examples o how communities have

implemented these strategies through

voluntary and policy eorts in the Heat

Island Reduction Activities chapter. Com-

munities can use this compendium as a

oundation and starting point or under-

standing the nuts and bolts o existing

urban heat island reduction strategies that

communities are currently advancing.

Future policy eorts may ocus on en-

couraging strategies to modiy urban

geometry and anthropogenic heat in

communities to reduce urban heat is-

lands. Research in this area is on-going,

and there is a growing awareness o the

importance o these actors.

5 Additional Resources

The table on the next page provides ad-

ditional resources on urban heat island

ormation, measurement, and impacts.


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Name Description Web Link

General Inormation

EPAs Heat Island Website Through this website, EPA provides background in-

ormation, publications, reports, access to nationalwebcasts, a database o urban heat island activities,

and links to other resources to help communities

reduce urban heat islands.

International Association

or Urban Climate (IAUC)

This international website is the main orum in which

urban climatologists communicate. Urban climate

resources, including a bimonthly newsletter, and in-

ormation on upcoming meetings can be ound here.

Lawrence Berkeley

National Laboratory

(LBNL) Heat Island Group

LBNL provides background inormation on urban

heat islands and their impacts through this website.

It also presents some o the impacts heat island re-

duction strategies can have on temperature, energyconsumption, and air quality.

National Center o

Excellence - SMART

Innovations or Urban

Climate and Energy

Arizona State Universitys National Center o Excellence

collaborates with industry and government to research

and develop technologies to reduce urban heat islands,

especially in desert climates. Its website provides back-

ground inormation on urban heat islands.

Urban Heat Islands:

Hotter Cities

This article explains urban heat islands and presents

solutions to mitigate them.

Measuring Heat Islands and Their Impacts

National Aeronautics andSpace Administration

(NASA) and the U.S.

Geological Survey Landsat

Program

The Landsat program is a series o Earth-observingsatellites used to acquire images o the Earths land

surace and surrounding coastal regions. These

images provide inormation rom which research-

ers can derive surace temperatures and evaluate

urban heat islands.

National Weather Service The National Weather Service is a source or air

temperature measurements, climate and weather

models, and past and uture climate predictions.

The site also has links to excessive heat outlooks,

atality statistics, historic data on major heat waves,

drought inormation, and advice on how to mini-mize the health risks o heat waves.

EPAs Excessive Heat

Events Guidebook

This document is designed to help community ofcials,

emergency managers, meteorologists, and others plan

or and respond to excessive heat events by highlight-

ing best practices that have been employed to save

lives during excessive heat events in dierent urban

areas. It provides a menu o options that ofcials can

use to respond to these events in their communities.

Table 3: Urban Heat Island Resources
http://www.asusmart.com/urbanclimate.phphttp://www.asusmart.com/urbanclimate.phphttp://www.actionbioscience.org/environment/voogt.htmlhttp://www.actionbioscience.org/environment/voogt.htmlhttp://www.epa.gov/hiri/about/heatguidebook.htmlhttp://www.epa.gov/hiri/about/heatguidebook.htmlhttp://www.epa.gov/hiri/about/heatguidebook.htmlhttp://www.actionbioscience.org/environment/voogt.htmlhttp://www.asusmart.com/urbanclimate.php


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Endnotes

1 Oke, T.R. 1997. Urban Climates and Global Environmental Change. In: Thompson, R.D. and

A. Perry (eds.) Applied Climatology: Principles & Practices. New York, NY: Routledge. pp.

273-287.

2 Oke. T.R. 1987. Boundary Layer Climates. New York, Routledge.

3 Oke, T.R. 1982. The Energetic Basis o the Urban Heat Island. Quarterly Journal o the Royal

Meteorological Society. 108:1-24. The threshold city population or heat islands o the size

2-5F may be closer to 100,000 inhabitants in some cases. See also Aniello, C., K. Morgan, A.

Busbey, and L. Newland. 1995. Mapping Micro-Urban Heat Islands Using Landsat TM and a

GIS. Computers and Geosciences 21(8):965-69.

4 From: 1) Oke, T.R. 1997. Urban Climates and Global Environmental Change. In: Thompson,

R.D. and A. Perry (eds.) Applied Climatology: Principles & Practices. New York, NY: Rout-

ledge. pp. 273-287. 2) Oke. T.R. 1987. Boundary Layer Climates. New York, Routledge. 3)

Voogt, J.A. and T.R. Oke. 2003. Thermal Remote Sensing o Urban Areas. Remote Sensing o

Environment. 86. (Special issue on Urban Areas): 370-384. 4) Roth, M., T. R. Oke, and W. J.

Emery. 1989. Satellite-derived Urban Heat Islands rom Three Coastal Cities and the Utilization

o Such Data in Urban Climatology. Int. J. Remote Sensing. 10:1699-1720.

5 Berdahl P. and S. Bretz. 1997. Preliminary Survey o the Solar Refectance o Cool Roong Ma-

terials. Energy and Buildings 25:149-158.

6 Numbers rom Voogt, J.A. and T.R. Oke. 2003. Thermal Remote Sensing o Urban Areas. Remote

Sensing o Environment. 86. (Special issue on Urban Areas): 370-384. Roth, M., T. R. Oke, and

W. J. Emery. 1989. Satellite-derived Urban Heat Islands rom Three Coastal Cities and the Utili-

zation o Such Data in Urban Climatology. Int. J. Remote Sensing. 10:1699-1720.


Meteorological Society. 108:1-24.


Meteorological Society. 108:1-24.9 Oke, T.R. 1997. Urban Climates and Global Environmental Change. In: Thompson, R.D. and

A. Perry (eds.) Applied Climatology: Principles & Practices. New York, NY: Routledge. pp.

273-287.

10 Christen, A. and R. Vogt. 2004. Energy and Radiation Balance o a Central European City. Inter-

national Journal o Climatology. 24(11):1395-1421.

11 Sailor, D.J., and H. Fan. 2002. Modeling the Diurnal Variability o Eective Albedo or Cities.

Atmospheric Environment. 36(4): 713-725.

12 Voogt, J. 2002. Urban Heat Island. In Munn, T. (ed.) Encyclopedia o Global Environmental

Change, Vol. 3. Chichester: John Wiley and Sons.

13

Akbari, H. 2005. Energy Saving Potentials and Air Quality Benets o Urban Heat Island Miti-gation. Retrieved 2 Jul. 2008 rom .

14 Sailor, D. J. 2002. Urban Heat Islands, Opportunities and Challenges or Mitigation and Adap-

tation. Sample Electric Load Data or New Orleans, LA (NOPSI, 1995). North American Urban

Heat Island Summit. Toronto, Canada. 1-4 May 2002. Data courtesy Entergy Corporation.


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15 Taha, H. and L.S. Kalkstein, S.C. Sheridan, and E. Wong. 2004. The Potential o Urban Environ-

mental Controls in Alleviating Heat-wave Health Eects in Five US Regions. Presented at the

American Meteorological Society Fith Conerence on Urban Environment. 25 August. See also

NOAA. 1995. Natural Disaster Survey Report: July 1995 Heat Wave. Retrieved 20 June 2008

rom .

16 Kalkstein, L.S. and S.C. Sheridan. 2003. The Impact o Heat Island Reduction Strategies on

Health-Debilitating Oppressive Air Masses in Urban Areas. Prepared or U.S. EPA Heat Island

Reduction Initiative.

17 Kalkstein, L.S. 1991. A New Approach to Evaluate the Impact o Climate upon Human Mortality.

Environmental Health Perspectives 96: 145-50.

18 CDC. 2004. Extreme Heat: A Prevention Guide to Promote Your Personal Health and Saety.

Retrieved 27 July 2007 rom .

19 Roa-Espinosa, A., T.B. Wilson, J.M. Norman, and Kenneth Johnson. 2003. Predicting the Im-

pact o Urban Development on Stream Temperature Using a Thermal Urban Runo Model

(TURM). National Conerence on Urban Stormwater: Enhancing Programs at the Local

Level. February 17-20. Chicago, IL. Retrieved 17 Jul. 2008 rom .20 EPA. 2003. Beating the Heat: Mitigating Thermal Impacts. Nonpoint Source News-Notes. 72:23-26.

21 EPA. 2003. Beating the Heat: Mitigating Thermal Impacts. Nonpoint Source News-Notes. 72:23-26.

Documents

Reducing Urban Heat Islands: Urban Heat Island Basics