PRELIMINARY STUDY ON URBAN HEAT ISLAND
EFFECT IN HONG KONG
(2001 - 2002)
By Green Power
September 2002
GENERAL OVERVIEW
1. What is Urban Heat Island (UHI) Effect (城市熱島效應)?
(1.1)
In order to accommodate growing population in cities, vegetation is replaced by pavement,
buildings and other structures. These surfaces absorb, rather than reflect, the sun’s heat, causing
surface temperatures and overall ambient temperatures to rise. 1 Together with the enormous
amounts of heat generated by cars, factories, furnaces, lights, air conditioners and peoples in cities,
an urban heat island (UHI) is created surrounding by cooler suburban and rural areas.2
(1.2)
Such unique microclimate has been recorded in many big cities (Table 1) and is known as
urban heat island (UHI) effect.
1
The Heat Island Reduction Initiative, USEPA, http://www.epa.gov/globalwarming/actions/local/heatisland/
Miller, G.T. 1998, Living in the Environment – Principles, Connections, and Solutions, 10th Edition: US: Wadsworth
Publishing Company.
2
1
Table 1. Temperature differences between urban and suburban areas of some big cities.
Cities
Temperature
Difference
Urban
Population
Between
Temperature
Area (km²)
Record Time
(million)
Urban &
(C)
suburban (C)
-
Atlanta 阿特蘭大
(USA)3
340
0.425
Day
Night
Huntsville 漢斯維爾
(USA)4
383
1700
-
-
27 (air)
49 (ground)
13 (air)
24 (ground)
-
-
Day
15.5
45.1
29.6
Night*
6.9
24.0
17.1
Summer
months**
Winter
months**
4.0
-
-
3.2
-
-
Midnight***
10.0
-
-
12 Aug 96#
8.9
30.6
21.7
Winter months
1.9
-
-
0.18
Melbourne 墨爾本
(Australia)5
7
Suburban
Temperature
(C)
3.4
Wilkes-Barre
威克斯巴勒市
324
0.625
(USA)6
Tucson 圖桑
0.75
(USA)7
of 1985/86
Plant City
0.035
January 1981
8.4
-7.2
-15.6
1620
12
Winter
10.0
-
-
116
0.46
-
8.0
-
-
(USA)8
London 倫敦
(UK)9
Manchester 曼徹斯特
(UK)10
* 5 hours after sunset. **1985-1994
***One occasion with wind speed below 1.5m/s and little or no cloud
#Maximum temperature
3
http://news.bbc.co.uk/hi/english/sci/tech/specials/washington_2000/newsid_652000/652679.stm
http://wwwghcc.msfc.nasa.gov/land/heatisl/heatisl.htm
5
http://www.earthsci.unimelb.edu.au/~jon//WWW/uhi-melb.html
6
http://www.ems.psu.edu/~nese/ch14web.htm
7
http://uanews.opi.arizona.edu/cgi-bin/WebObjects/UANews.woa/wa/SRStoryDetails?ArticleID=2210
8
http://www.cybervox.org/archive/stormreports/199911-199912/0058.html
9
http://www.timesonline.co.uk/printFriendly/0,,1-2-412838,00.html
10
http://www.timesonline.co.uk/printFriendly/0,,1-2-412837,00.html
4
2
Causes of UHI Effect11
2.
(2.1)
During the day in rural areas, the solar energy absorbed near the ground evaporates water
from the vegetation and soil. Thus, while there is a net solar energy gain, this is
compensated to some degree by evaporative cooling. In cities, where there is less vegetation,
the buildings, streets and sidewalks absorb the majority of solar energy input. The removal
of trees and shrubs eliminates the natural cooling effects of shading and evapotranspiration
(蒸騰作用 a natural cooling process in which water transpires from a leaf’s surface and
(2.2)
evaporates into the surrounding air, reducing ambient temperature).
The cities have less water available for evaporation. Runoff is greater in the cities because
the pavements are largely impermeable. Thus, evaporative cooling is less which contributes
(2.3)
to the high air temperatures.
Waste heat from city buildings, cars and trains is another factor contributing to the warm
cities. Heat generated by these objects eventually makes its way into the atmosphere. This
(2.4)
heat contribution can be as much as one-third of that received from solar energy.
The thermal properties of buildings add heat to the air by conduction. Tar, asphalt, brick and
concrete are better conductors of heat than the vegetation of the rural area (Table 2). Dark
materials absorb more heat from the sun.12 Black surfaces in the sun can become up to 40C
hotter than the most reflective white surfaces.
Table 2. Albedo values of various urban environment surfaces.
Surface
Albedos*
Grass
0.25-0.30
Trees
0.15-0.18
Asphalt
0.05-0.20
Concrete
0.10-0.35
Brick/Stone
0.20-0.40
Corrugated roof
0.10-0.15
Highly reflective roof
0.60-0.70
Colored paint
0.15-0.35
White paint
0.50-0.90
Tar & gravel
0.03-0.18
Red/brown tile
0.10-0.35
*A high albedo indicates that much of the incident energy is reflected rather than absorbed.
11
12
http://cimss.ssec.wisc.edu/wxwise/heatisl.html
http://eande.lbl.gov/EAP/BEA/HIP/CoolRoofs/
3
(2.5)
The canyon structure that tall buildings create enhances the warming. During the day, solar
energy is trapped by multiple reflections off the buildings while the infrared heat losses are
reduced by absorption.
3.
Impacts of UHI Effects
(3.1)
Urbanization alters the local (and sometimes the regional) climate. Cities are generally
warmer, rainier, foggier, and cloudier than suburbs and nearby rural areas. 2 UHI effect raises urban
temperatures in the summertime, adversely affecting human health and the environment.
Additionally, UHI effect reduces livability by decreasing urban ventilation, increasing air pollution
levels, and increasing the risk of heat-related illness and mortality.1(USEPA)
High Temperatures13
(3.2) On warm summer days, the air in a city can be 3 -5C hotter than its surrounding areas. Since
there are fewer trees, shrubs, and other plants to shade buildings, intercept solar radiation, and cool
the air by “evapotranspiration”.
(3.3) Moreover, buildings and pavement made of dark materials absorb the sun’s rays instead of
reflecting them away, causing the temperature of the surfaces and the air around them to rise. Some
of the heat collected by the roof is transferred inside. Staying comfortable in under a dark shingle
roof often means more air conditioning and higher utility bills. These roofs also heat the air around
them, contributing to the UHI effect.
Air Quality14
(3.4) UHIs are not only uncomfortably hot, they are also smoggier. Smog is created by
photochemical reactions of pollutants in the air. These reactions are more likely to occur and
intensify at higher temperatures.
(3.5) In Los Angeles, for example, at temperatures below 21C, the concentration of smog
(measured as ozone) is below the national standard (120 ppb or 240 µg/m3) and above 35C all days
are smoggy. For every degree Fahrenheit (0.56C) the temperature rises above 70F (21C), the
incidence of smog increases by 3%. 15According to scientists’ calculation, when afternoon
temperatures are lowered 2 to 3 ºF, ozone levels fall by 7-10 ppb, a 6-8% drop.
(3.6) Because heat and sunlight increase the formation of ground-level ozone pollution and its
13
14
15
http://eande.lbl.gov/EAP/BEA/HIP/HighTemps/
http://eande.lbl.gov/EAP/BEA/HIP/Air Quality/
http://www.fraqmd.org/Heat%20Island.htm
4
precursor compounds, the UHI effect can increase ozone pollution to levels that threaten human
health; especially that of children, the elderly, asthmatics, and other vulnerable groups.1(USEPA)
(3.7) The dome of heat also traps pollutants, especially tiny solid particles (suspended particulate
matter), creating a dust dome above urban areas. If wind speeds increase, the dust dome elongates
downwind to form a dust plume, which can spread the city’s pollutants for hundreds of kilometers.
Energy Use16
(3.7) Higher temperatures in UHI bring with them increased energy use, mostly due to a greater
demand for air conditioning. As power plants burn more fossil fuels, they increase both the
pollution level and energy costs.
(3.8) 17Three to eight percent of urban electricity demand is use to compensate for increased
temperatures from UHI effect.
(3.9) On warm afternoons in Los Angeles, for example, the demand for electric power rises nearly
2% for every degree Fahrenheit (3.6% for every ºC) rise in the daily maximum temperature. Also,
peak power rises 3% for every 0.5F (10.8% for every 1C) rise in daily maximum temperature. In
total, about 1 -1.5 gigawatts of power are estimated to be used to compensate the impact of the UHI.
This increased power costs the Los Angeles ratepayers about US$100,000 per hour, or about
US$100 million per year.
(3.10) The Salt River Project estimates that for every degree F increase in temperature, the utility’s
610,000 residential customers pay US$3.2 to 3.8 million extra per month in cooling costs, or about
US$5 to 7 per customer per month.18
4.
Combating UHI Effect
(4.1)
Planting trees and bushes can help reduce urban temperatures as well as make cities greener.
(4.2)
Rooftop gardens can help to absorb heat and keeps temperatures inside the building low.
Chicago, IL's city hall has installed a 32,000 ft² rooftop garden expected to reduce annual air
conditioning costs by US$4,000. 1
Install reflective (or high albedo) and emissive roof on buildings to bounce incoming solar
radiation back into the atmosphere, reducing heat transfer through the building envelope.
This measure effectively cools building interiors, reducing air conditioning demand and
(4.3)
16
17
18
http://eande.lbl.gov/EAP/BEA/HIP/Energy Use/
http://64.224.189.27/cases/uhi.htm
The Arizona Republic on the Urban Heat Island Effect, http://www.sepp.org/reality/arizrepub.html
5
associated energy costs.
(4.4)
(4.5)
Increase the reflectivity of roads, driveways and other paved surfaces by switching to light
colored construction materials. Light colored materials reflect solar radiation and reduce
surface and ambient air temperatures.1 (USEPA)
One computer study by the University of California-Berkeley showed that summertime
(4.6)
(4.7)
(4.8)
temperatures in Los Angeles could drop up to 5C, simply by covering buildings and roads
with light-colored surfaces. The amount of smog also could be reduced by up to 10%-the
equivalent of removing more than 3 million cars from the road.18
Decreasing the use of electrical appliances (e.g. air conditioner).
Reducing air pollution, so to reduce urban temperature.
Amend state building code to provide energy conservation credits all builders need to
(4.9)
incorporate “cool’ designs in their projects.15
Sacramento, CA, Davis, CA, and Salt Lake City, UT have established parking lot shading
regulations and guidelines.1
HONG KONG’S STATUS
5. Past Temperature Records
(5.1) Although UHI effect has not been formally monitored, like other metropolis Hong Kong
possesses favourable factors for formation of UHIs.
(a) Hong Kong has a population of 6.7 million19 on a land area of about 1090 square kilometre.
Over 2 million people are living at the Victoria harbour Area, i.e. Kowloon and northern shore
of Hong Kong Island.
(b) Associated with the high-density population, urban areas in Hong Kong are extensively paved
with concrete and closely packed with high-rise buildings.
(c) Vegetation cover in the urban areas of Hong Kong is sparse.
(d) Hong Kong consumes large amount of energy through combustion of fossil fuels by the large
vehicle fleet and use of electricity in residential and commercial buildings. The waste heat
generated heats up Hong Kong’s urban areas.
(5.2) In order to illustrate the existence of UHI in urban Kowloon Peninsula (KLN), the past
temperature records of King’s Park Station (KP) were compared with those of suburban areas, i.e.
Tuen Mun (TM) and Sai Kung (SK) Station. TM and SK Station were selected because
(a) Both have a similar topography and close to seashore,
19
http://www.info.gov.hk/censtatd/eng/whatsnew/01c/01c_feature/01c_article.htm
6
(b) Both are less populated than KLN, hence produce less severe UHI effect, if any,
(c) Two stations can represent the meteorological situation of western and eastern territory.
(5.3) The differences in monthly mean maximum and minimum temperatures (Tmax and Tmin
respectively) between the above mentioned station (KP, TM and SK) and Hong Kong Observatory
were calculated and are shown in Figure 1 and 2. Such differences (Tmax and Tmin) could be
interpreted as indicators of existence of UHI. However, one must be reminded that the existence of
UHI should ideally be proved with a temperature profile (Two-dimensional ideally).
Figure 1. Differences in Mean Maximum Temperatures between
Selected Stations and Hong Kong Observatory
2
1
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 max (deg. C)
1.5
-1.5
Time (Month/Year)
King's Park-Hong Kong Observatory
Tuen Mun-Hong Kong Observatory
Sai Kung-Hong Kong Observatory
(5.4) From 1999 to 2002, generally Tmax‘s of KP were the highest, followed by TM’s and SK’s
were the lowest. This indicates that in daytime the maximum temperatures of KP are the highest,
followed by TM and then SK on average. Such discrepancy in Tmax is more prominent in winter
and early spring. Similar but less obvious pattern is also revealed in Figure 2 for Tmin. Therefore,
an UHI might exist around KP, i.e. KLN, especially in winter and early spring.
6. Temperature Profiles of Urban Areas
(6.1) In order to prove the formation of UHI, temperature profiles of KLN were measured and
recorded on 14 August 2001 and 15 July 2002 and Causeway Bay on 20 August 2001 and 13
August 2002. The methodology of simulating the temperature profiles is explained in Appendix 1.
The results are illustrated in Figure 3 to 6.
7
Figure 2. Differences in Mean Minimum Temperatures between
Selected Stations with Hong Kong Observatory
2
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Tmin (deg. C)
1
-1.5
-2
-2.5
Time (Month/Year)
King's Park-Hong Kong Observatory
Tuen Mun-Hong Kong Observatory
Sai Kung-Hong Kong Observatory
(6.2) The profile on KLN sketches from Hung Hom Ferry Pier in the East to Kowloon Station of
Airport Express in the West. That in Causeway Bay is from Paterson Street in the Northwest to Lai
Tak Tsuen in the Southeast.
(6.3) All these profiles exhibit a peak in the middle and declines towards both ends, though not to a
same extent. This clearly demonstrates that an UHI existed on KLN and Causeway Bay on the days
of measurement. The maximum differences in temperature of the profile are tabulated in Table 3.
(6.4) From Table 3, UHI effect is more prominent on KLN than in Causeway Bay. That may be
attributable to the larger built-up area on the former. In both Figure 3 and 4 for KLN, the eastern
end was cooler than the western one. That may be attributable to the easterly prevailing wind during
measurement that dragged the UHI dome toward west. The reason why northwestern end was
cooler than southeastern end in Figure 5 and 6 for Causeway Bay was assumed the similar.
Table 3. Maximum Temperature Difference of the Profile shown in Figure 3 to 6.
Location
Kowloon
Peninsula
Date
Measurement
Time
Maximum Temperature
Difference of the Profile (C)
Figure
No.
14Aug01
1120-1400
3.6
3
15Jul02
1130-1420
5.5
4
Causeway Bay 20Aug01
1350-1530
2.1
5
13Aug02
1420-1600
2.2
6
8
Figure 3. Temperature Profile of Kowloon Peninsula (1120-1400)
九龍半島東西溫差(14Aug01)
2.0
1.0
Distance (m)
0.0
-1300
-800
-300
200
700
1200
1700
-2.0
-3.0
Temp Diff (deg C)
溫差 (攝氏)
-1.0
-4.0
Figure 4. Temperature Profile of Kowloon Peninsula (1130-1420)
九龍半島東西溫差(15Jul02)
2.0
1.0
Distance (m)
0.0
-800
-300
200
700
-1.0
-2.0
-3.0
-4.0
Temp Diff (deg C)
溫差 (攝氏)
-1300
-5.0
9
1200
1700
Figure 5. Temperature Profile of Causeway Bay (1350-1530)
銅鑼灣海邊內陸溫差(20Aug01)
1.0
0.5
0.0
-400
-200
0
200
400
600
800
Temp Diff (deg C)
溫差/攝氏
-600
Distance (m)
-0.5
-1.0
Figure 6. Temperature Profile of Causeway Bay (1420-1600)
銅鑼灣海邊內陸溫差(13Aug02)
0.5
Distance (m)
0.0
-400
-200
0
200
-0.5
-1.0
-1.5
-2.0
Temp Diff (Deg C)
溫差 (攝氏)
-600
-2.5
10
400
600
800
7. Implications to Hong Kong
(7.1) In Figure 1, the magnitude of Tmax of KP increased slightly from 1999 to 2002. As the
monthly mean maximum temperature of HKO (Appendix 2) remained more or less the same in this
period, this indicated UHI effect of KLN is exacerbating.
(7.2) The formation of smog is highly sensitive to temperature. Ozone, a secondary air pollutants
formed in photochemical smog, will be higher in concentration under intense sunlight and high air
temperature. Maximum daily ozone concentrations of Central-western District and rural areas were
plotted against corresponding maximum daily temperatures and are show in Figure 7 and 8.
Figure 7. Graph of Daily Maximum Ozone Concentration against Daily Maximum
Temperature for Central-western in 2000
中西區
每日臭氧最高值與每日最高氣溫分佈
2000年
Max O3 Conc 臭氧最高值(ug/m3)
(Central-western中西區)
300
250
200
150
100
每升高攝氏1度
臭氧最高值升高 11 ug/m3
50
Max Temp 最高氣溫(Deg C 攝氏)
(HK Observatory 天文台)
0
0
5
10
15
20
25
30
(7.3) Two graphs display a similar pattern to other UHI12 in which all data points fall into lower
portion of “diagonal”. This pattern graphically proves the dependence of ozone concentration on
temperature. However, in most occasions high temperature did not lead to high ozone concentration
(i.e. most data points are far below the “diagonal”) because of the existence of unfavourable
meteorological conditions such as cloudiness, high wind speed, rains, etc.
11
35
Figure 7. Graph of Daily Maximum Ozone Concentration against Daily Maximum
Temperature for Rural Area in 2000
郊區
每日臭氧最高值與每日最高氣溫分佈
2000年
Max O3 Conc 臭氧最高值(ug/m3)
(Tap Mun 塔門)
300
250
200
每升高攝氏1度
臭氧最高值升高 5.5 ug/m3
150
100
50
0
0
5
10
15
20
25
30
35
40
Max Temp 最高氣溫(deg C 攝氏)
(Ta Kwu Ling打鼓嶺)
(7.4) If considering data of the year 2000, at temperature below 30C the concentration of ozone
always complied with Air Quality Objective of 240 g/m3 20. That means if ambient air
temperature can be kept low, adverse impacts caused by smog or ozone can be relieved to a certain
extent. In the recent high API days on 6 and 9 September 2002, the maximum temperatures when
high ozone concentrations were recorded were around 33C.
(7.5) Higher temperatures in UHI will bring about greater demand for air conditioning. With
reference to the data of Southern California, peak load of electricity demand increases by 2.9 % for
increase of every degree Celsius. When considering the UHI Effect of Kowloon Peninsula, peak
power may rise 10% for 3 to 4C increase in daily maximum temperature.
20
http://www.epd.gov.hk/epd/english/environmentinhk/air/air_quality/aq_obj.html
12
8. The Way Forward
Regarding the implication of UHI Effect on the public health, energy consumption, air pollution and
image of Hong Kong as an international metropolis, Green Power would like to urge the HKSAR
Government to take the following actions.
(a) Systematic and multi-purpose greening of urban areas so as to increase considerably the
vegetation cover (especially trees) of built-up areas and preserve the existing urban green areas
such as parks, trees on roadside and slopes. Plants can shade the sunlight with their leaves, cool
down the ambient air through evapotranspiration and absorb air pollution during gas exchange.
(b) Increase the reflectivity of pavement and building materials to reduce the absorption of solar
radiation in case that plantation is not possible.
(c) Take into account the UHI effect in design of the urban landscape to avoid canyon effect and
trapping of high temperature and air pollutants. Create favourable urban microclimate for
dispersal of heat and air pollutants.
(d) Lower the energy/electricity demand (especially for air conditioning) of buildings through
legislation or building codes so as to reduce the generation of waste heat from buildings.
(e) Lower the dependence of vehicles for commutation in urban areas to reduce emission of air
pollutants and waste heat.
(f) Long-term monitoring of the UHI effect and its adverse impacts over the territory.
(g) Incorporate the results of (f) in future town planning, transport planning for new towns and
redevelopment of old districts.
END
13
Appendix 1.
Methodology of simulating the temperature profiles
(A1) Locations along the profiles were chosen by the following criteria where air temperatures were measured.
(a) shaded
(b) concrete paved
(c) no major temperature interfering factors such as doorway of shopping malls, idling vehicles, exhaust ducts of
restaurants, etc.
(A2) Measurements were taken around noontime when air temperatures approaching maximum and steady readings.
(A3) Middle points of the profiles were reference points (Ref) where air temperatures were recorded continuously with
a digital thermometer during the measurement periods.
(A4) Air temperatures were recorded with the thermometer of the same model point by point along the profiles starting
from the reference point. After arriving one end of the profiles, measurements were taken point by point in opposite
direction, passing the reference point to the other end. Then measurements continued in reverse direction again until
reaching the reference points. In this way, two temperatures were recorded for each location along the profiles.
(A5) The “real time” differences in temperatures between each location and reference points were calculated and
averaged (because each location had two temperatures recorded). These temperatures were plotted against distances of
the locations to the reference points.
Point to Point distance (X) of KLN Profile
Location
Ref
Point to Point distance (X) of Causeway Bay Profile
X/m
0.00 佑寧堂
Location
X/m
P1
120.00 佐敦道/彌敦道
H7
680.00 維園道/警察俱樂部
P2
250.00 佐敦道/白加士街
H6
520.00 告士打道/百德新街
P3
480.00 英皇佐治五世公園
H5
430.00 百德新街/ 京士頓道
P4
640.00 佐敦道/渡船街
H4
370.00 百德新街/ 記利佐治街
P5
930.00 佐敦道/連翔道
H3
340.00 百德新街/ 怡和街
P6
1200.00 佐敦道/雅翔道
H2
240.00 怡和街/功德林
100.00 高士威道/德信街
P7
220.00 佐敦道/加士居道
H1
P8
460.00 衛理道/加士居道
Ref
K1
640.00 漆咸道/理工大學
H9
140.00 銅鑼灣徑巴士總站
K2
1070.00 暢行道/溫思勞街
H10
270.00 大坑道/嘉寧徑
K3
1180.00 紅磡南路/機利士南路
H11
510.00 大坑道/芝蘭閣
K4
1400.00 紅磡南路/寶來街
H12
670.00 福群道/真光中學
K5
1610.00 紅磡道/船景街
K6
1770.00 百合苑第五座
K7
1980.00 紅磡碼頭
14
0.00 中央圖書館
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2
Ju
l02
Ja
n-
Temper at ur e (deg. C )
Appendix 2
Monthly Mean Max and Min Temp. of HK Observatory
35.0
30.0
25.0
20.0
15.0
10.0
Time (Month/Year)
Mean Maxi Temp
15
Mean Mini Temp