Climatic behavior of various urban parks during hot and humid summer in the mediterranean city of...

INTERNATIONAL JOURNAL OF CLIMATOLOGY

Int. J. Climatol. 26: 1695–1711 (2006)

Published online 16 May 2006 in Wiley InterScience

(www.interscience.wiley.com) DOI: 10.1002/joc.1330

CLIMATIC BEHAVIOR OF VARIOUS URBAN PARKS DURING HOT ANDHUMID SUMMER IN THE MEDITERRANEAN CITY OF TEL AVIV, ISRAEL

ODED POTCHTER,* PNINIT COHEN and ARIEH BITAN

Unit for Climatology, Applied Climatology and Environmental Aspects, Department of Geography and the Human Environment, TelAviv University, Tel Aviv, Israel

Received 20 September 2005Revised 10 February 2006

Accepted 15 February 2006

ABSTRACT

This study examines the climatic behavior of different designs of urban parks during hot and humid summer conditions,and their influence on human comfort in Tel Aviv, Israel. The research was conducted in three different types of urbanparks: a park with grass and a few low trees, a park with medium sized trees and a park with high and wide-canopiedtrees. The results showed that an urban park that contains high trees with a wide canopy has the maximum coolingeffect during daytime, reduces temperatures by up to 3.5 °C and lowers heat stress values despite increasing relativehumidity values. An urban park that contains dense, medium sized trees can also reduce temperatures during daytimeby up to 2.5 °C as well as slightly lower heat stress. However, during nighttime it can create uncomfortable climaticconditions owing to the reduction of wind velocity and increase in relative humidity. An urban park covered with grasscan be warmer and sometimes even more humid than the built-up area during the day, which increases heat stress values.Copyright 2006 Royal Meteorological Society.

KEY WORDS: urban parks; grass; well-treed park; heat stress; Mediterranean climate

1. INTRODUCTION

The urban heat island (UHI) is defined as the difference between temperatures measured in the urban spaceand those measured in the nonurban space surrounding it (Oke, 1987). The UHI, which is characterizedby amplification of temperature values, lower relative humidity values and moderation of wind velocity, cancreate unpleasant microclimatic conditions for humans. On the other hand, green areas within the urban space,can create a cooling effect, lowering temperatures and increasing relative humidity (Oke, 1987). The coolingeffect of urban parks creates a ‘park cool island’ (PCI), and is most pronounced in an urban area that hasdeveloped a UHI (Spronken-Smith and Oke, 1998). In cities with uncomfortable, hot summers, green areasoffer the potential of cooling the built-up areas. The potential cooling effect of urban parks is particularlyimportant in urban areas located in hot and humid climates, owing to the fact that through the process ofcooling by the vegetation, the relative humidity values increase, a parameter that can have a negative effecton human comfort.

1.1. Background

Previous studies examined the cooling effect of green areas within the city on the urban climate (Givoni,1991). Studies in hot, humid cities show that the PCI effect can be substantial. In Kuala Lumpur, well-treedparks were 4–5 °C cooler than the built-up areas during the afternoon, while at nighttime they were only 1 °C

* Correspondence to: Oded Potchter, Unit for Climatology, Applied Climatology and Environmental Aspects, Department of Geographyand the Human Environment, Tel Aviv University, Tel Aviv, Israel; e-mail: [email protected]

Copyright 2006 Royal Meteorological Society

1696 O. POTCHTER, P. COHEN AND A. BITAN

cooler (Sham, 1991). The study of Padmanabhamurty (1991) examined microclimate in the tropical city ofNew Delhi and found that urban forests lower temperatures by 2.5 °C in comparison to built-up environments.In Kumamoto, Japan, an urban park covered with broad-leaf trees had a maximum PCI of 3 °C during theday. No data on temperature differences at night were given (Saito et al., 1991). Other studies in Japanshowed that there is a direct correlation between the size, density and quality of the vegetation cover andair temperature in and around the urban parks. Park temperatures during daytime were lower by up to 3 °Cin comparison to the built-up areas, and the temperature reductions could be observed up to 50 m from theparks’ edges (Katayama et al., 1993). On the other hand, in Sacramento, all park types developed a PCI of2–3 °C during daytime. However, after sunset the PCI grew rapidly, especially at dry sites, up to 5 °C, andin extreme cases the PCI reached 6.5 °C in ideal conditions above open area and grass (Spronken-Smith andOke, 1998). Recent studies emphasize the necessity of investigating the climatic behavior of different typesof vegetation and grass as well as the effect of irrigated green areas on the climatic parameters in the parks(Spronken-Smith et al., 2000; Chudnovsky et al., 2004). A study in the subtropical climate in the city ofGaborone, Botswana, showed that at midday, densely vegetated areas were up to 2 °C cooler than rural sites(Jonsson, 2004), while parts of the city with sparse vegetation were warmer than the countryside. Studiesconducted in Gan Meir Park in Tel Aviv, Israel, showed that the park’s cooling effect was between 0.8 and4 °C, and more pronounced during daytime than at nighttime (Potchter et al., 1999).

Most studies emphasize the positive effect of the green areas on the urban climate. However, few studieshave suggested that vegetated areas can be warmer than the surrounding built-up environment creatingunpleasant microclimatic conditions (Grimmond et al., 1996; Jauregui, 1991; Potchter et al., 2002). Thepotential cooling effect, as well as the potential warming effect of urban parks is particularly important inurban areas located in hot climates, where the UHI can develop undesirable climatic conditions.

1.2. Objectives

The aim of this research was to study the climatic behavior of different urban parks with various typesof trees and vegetation coverage by measuring the climatic parameters within both urban parks and thesurrounding built-up area. On the basis of collected data, the influence of each park design on human comfortlevels was examined. The study focused on hot and humid climate conditions that are typical of coastalMediterranean cities.

2. METHODOLOGY

2.1. Study area

The study was conducted in the city of Tel Aviv, Israel, located at 32°06′N 34°47′E. Tel Aviv is situatedon a plain along the east coast of the Mediterranean Sea and has a subtropical climate, Csa according toKoppen classification (Potchter and Saaroni, 1998). The city is the core of the largest metropolitan area inIsrael with a population of 1.6 million (Statistical Abstract of Israel, 2003). The climatic conditions of TelAviv are a result of the combined effect of the geographical location, the influences of the Mediterranean Seaand urbanization. Meteorological data that were taken at Tel Aviv airport (Sde Dov station), located at thenorthern part of the city, 0.5 km from the coast line, showed that the city suffers from hot and humid climaticconditions during the summer: Daily average maximum temperatures are around 29.0 °C, while daily averageminimum relative humidity is around 61%. Daily minimum temperatures are around 22.2 °C, while dailyaverage maximum relative humidity is around 83%. The wind regime is influenced by the Mediterranean Seabreeze, which blows during the day from the west and northwest, whereas during the night, it is affected bya light land breeze, which blows from the east and southeast (Bitan and Rubin, 1994). The city of Tel Avivdevelops an UHI during the summer at both midday and at night (Bitan et al., 1992).

Copyright 2006 Royal Meteorological Society Int. J. Climatol. 26: 1695–1711 (2006)DOI: 10.1002/joc

CLIMATIC BEHAVIOR OF TEL AVIV URBAN PARKS DURING SUMMER 1697

2.2. Research units’ classification

In order to examine the aim of the research, three urban parks that represented the common types of treesand vegetation coverage in Tel Aviv were selected. All parks are surrounded by densely built areas and locatedabout 0.8–1.4 km from the coastline (Figure 1):

Park A (25 000 m2): covered by grass and a few low trees (young Olive trees and young Bombax cebiatrees), mostly without shade, located in a residential neighborhood at the northern suburb of the city.

Park B (35 000 m2): covered by dense medium sized Tipuana tipu trees, partly shaded (65% of the groundarea), and located in a residential neighborhood at the northwest of the city.

Park C (28 000 m2): covered at the park center by high and wide-canopied Ficus microcarpa trees, wellshaded (95% of the ground area), and located in the city center.

2.3. Tools and methodology

Two methods of investigation were used in this research: (1) setting of fixed meteorological stations and(2) mobile measurements. On the basis of the data collected from stations, comfort conditions were calculated.

2.3.1. Setting of fixed meteorological stations. Six Campbell automatic meteorological stations were set upin the study area. Two fixed meteorological stations were situated in each research unit, one in the center of thepark and the second in the built-up area next to the park. The meteorological stations measured temperature,black ball temperature and relative humidity at a height of 2 m and wind direction and wind velocity at aheight of 2.5 m.

The instruments used in these meteorological stations included Campbell HMP45C-type temperature andrelative humidity sensors. The sensors were installed inside a Gill Multi-Plate 41 002 unaspirated radiationshield. The temperature and relative humidity accuracy of the sensors is ±0.2 °C and ±2%, respectively.Wind velocity and direction were measured using Young 05 103 anemometer. Sensor accuracy was ±0.3 m/sfor the velocity and 3° for the direction. Readings were taken every second with the resulting data averagedand stored every 5 min using a Campbell 21X data logger. For wind direction, the modal direction wasstored. All the instruments were calibrated, checked and compared before and after the experiment underthe same conditions. According to specifications of the screen manufacturer (Young, 1998), the screen mayoverestimate the air temperatures during the daytime hours (up to 1.5 °C under calm conditions and less than0.7 °C and 0.4 °C with winds of 2 and 3 m/s, respectively). In order to avoid erroneous conclusions stemmingfrom the screen effect, temperatures of stations exposed to different wind velocities were compared to those ofa station sheltered from the wind. Temperature differences were less than 0.2 °C, indicating that the effect ofthe reduced ventilation in the exposed screens in the park was negligible. It should also be noted that duringmost hours during daytime, wind velocity was above 2 m/s, so the screens were naturally ventilated. BlackBall temperature was measured by a T type thermocouple (copper-constantan), whose accuracy is ±0.5 °C inthe range of 0–40 °C and ±1.0 °C in the rest of the span, and which is inserted into a 6-inch-diameter hollowcopper sphere, painted with matte black paint.

2.3.2. Mobile measurements. Mobile measurements were taken by foot along one route in each park. Theroute started from the built-up area through the park center to the facing built-up area passing by the fixedmeteorological stations situated along the route (Figure 2). The measurements were conducted simultaneouslyin all three parks. Measurements of temperature and relative humidity were taken at a height of 2 m using anNTC-DT029 temperature sensor (accuracy of ±0.1 °C) and DT041 relative humidity sensor (accuracy of ±2%)and a response time of 15 s (at 20 °C, 90% response). The sensors were installed inside a Gill Multi-Plate41 002 unaspirated radiation shield. The length of each route was approximately 750 m. Readings were takenat 10 points along each route, after standing 60 s at the measuring point, until the sensor stabilized, allowinga reliable reading. Measurements along the entire route took less than 30 min. The two fixed meteorologicalstations situated along the route (in the park center and in the built-up area) served as reference points.In cases where the temperature differences recorded at the fixed meteorological station and mobile routeexceeded ±0.3 °C during the time it took to take the mobile measurements, corrections were made to the data



Tel Aviv

Haifa

Jerusalem

M e

d i

t e r

r a n

e a

n S

e a

Park C

Park B

Park A

0 1 2

N

Eilat

Figure 1. The city map of Tel Aviv and location of the research units. The pictures demonstrate the typical landscape of investigatedparks

to compensate for these possible inconsistencies. The data were gathered by the Fourier MultiLog electronicdata logger. These measurements were carried out three times a day (at 11:00, 15:00 and 21:00 h on 6 June



N

N

N

Park A

10

10

10

9

9

9

8

8

8

7

7

7

6

6

6

5

5

5

4

4

4

3

3

3

2

2

2

1

1

1

A

A

A

B

B

B

0 50 100 m

0 50 100 m

0 50 100 m

Park B

Park C

Figure 2. Location of the fixed meteorological stations (marked in letters) and the routes of the traverses (measurement points markedby numbers)

2002 and at 04:30, 11:00 and 15:00 h on 7 June 2002, according to local solar time, LST). Figure 2 displaysthe location of the fixed meteorological stations and the routes of the traverses taken by foot in each researchunit. Before each experiment, all three sets of instruments (sensors and data loggers) were calibrated andcompared in order to avoid reading mistakes and inaccuracy of measurements.



2.4. Study time

The research was conducted during the summer season that is characterized in Israel by relatively stableweather conditions with minor daily changes. A pretest was conducted on 22–23 June 2000. The mainmeasurements were taken during the summer at two different meteorological events; one in two consistentdays between 31 July and 1 August 2001, during typical summer stable weather conditions, clear skies, hightemperature and high relative humidity values, accompanied by low wind velocities. The second measurementswere taken on two consistent days between 6 and 7 June 2002 during early summer conditions; moderateweather conditions on 6 June, and ‘sharav’ conditions (hot and dry spell) on 7 June. In both cases the resultswere similar.

During 2001, park C was undergoing renovations and there were no observations from the stationarymetrological sites.

However, the results of 2002 give us the opportunity to evaluate two different weather conditions in onemeasurement period and therefore, this paper will discuss mainly the results of the 2002 measurements.

3. RESULTS

3.1. Results of the fixed meteorological stations

Temperature comparison between the three parks from 6 to 7 June 2002 showed that during daytime thelowest temperatures were measured in park type C (well treed), while the highest temperatures were measuredin park type A (grass). These results were more pronounced in the second day during the heat wave whenthe maximum temperature in park C was 27.2 °C in comparison to 30.2 °C in park A, a difference of 3.0 °C(Figure 3(a)), despite the fact that the built-up area temperature values around park C on the same day weresimilar to those of the built-up area near park A (Figure 3(b)). Temperatures in park B (medium sized trees)were warmer by 0.8 °C than park C on the first day and warmer by 2.0 °C on the second day. However, thetemperatures measured in the built-up area around park B were similar in the first day to those measuredaround park C (at 15:00 h, 26 °C at built area near park B and 25.8 °C near park C) and warmer up to 1.5 °Con the second day (at 15:00 h, 28.5 °C near park C and 30 °C near park B). During the night, the minimumtemperature in park C was 20.0 °C, the highest among the three parks. The temperature values of the built-uparea at the city center (near park C) were higher than temperatures at the suburb (near park A and B), probablydue to the UHI effect. The minimum temperature measured in park A was 19.4 °C while that in park B was17.3 °C, the lowest among the three parks. It should be noted that the temperature in the built-up area aroundpark B was also the lowest in comparison to the built-up area around parks A and B.

In order to examine the climatic behavior of each park type, an hourly temperature difference between thecenter of the park and its surrounding built-up environment was calculated. Figure 4 presents the results ofthe measurements taken in 2001 and 2002 for parks A and B and measurements taken in 2002 for park C.

Both parks B and C, which contain trees, were cooler then their surrounding built-up area during all hoursof the day and night. The temperature differences during daytime were higher than those at nighttime. ParkC (well treed) was cooler by up to 2.5 °C than its surrounding built-up area in June 2002.

However, the results of the 2002 measurements in park C do not represent the cooling potential of thispark. Former measurements that had been taken in this park in 1997, 1998 and 1999, during typical summerclimate conditions, showed that the cooling effect of this park reached 3–4 °C (Potchter et al., 1999). Asmentioned before, during the rehabilitation of this park in 2001, the park suffered from massive pruning,which probably affected its cooling ability, and comparison between the results of 2002 measurements tothose of 2000 is very interesting (Potchter et al., 2002). As can be noted in Figure 4(c), in June 2000 thepark was cooler up to 3.5 °C than its surrounding area.

The temperatures differences between Park B (medium sized trees) and its surrounding built-up area weresimilar in both 2001 and 2002. Park B was cooler than its surrounding built-up area by up to 2.3 °C. Duringnighttime, the temperature differences between the park center and the surrounding built-up area, in both parkB and C were smaller in comparison to the differences during the day. Park B was up to 1.3 °C cooler thanits surrounding built-up area while park C was cooler by only up to 0.6 °C than its surrounding built-up area.



8:00

10:0

0

12:0

0

14:0

0

16:0

0

14:0

0

16:0

0

18:0

0

20:0

0

22:0

0

0:00

2:00

4:00

6:00

8:00

10:0

0

12:0

0

Hour

32

30

28

26

24

22

20

18

16

Tem

pera

ture

(°C

)

32

30

28

26

24

22

20

18

16

Tem

pera

ture

(°C

)

Park A - center Park B - center Park C - center

8:00

10:0

0

12:0

0

14:0

0

16:0

0

14:0

0

16:0

0

18:0

0

20:0

0

22:0

0

0:00

2:00

4:00

6:00

8:00

10:0

0

12:0

0

Hour

Park C - built upPark A - built up Park B - built up

(a)

(b)

Figure 3. Hourly temperatures in: (a) the different park types’ centers and (b) the surrounding built-up areas, during 6–7 June 2002

The results in park A (grass) were more complicated, and some differences occurred from one day toanother and from year to year. It should be noted that in both measurement periods the park was irrigated.In 2001, the irrigation was for about 1 h at midnight, while in 2002 the irrigation lasted from midnightto early morning until the ground was soggy. At both measurement periods, the park was cooler than thebuilt-up environment during nighttime. In 2001, the park was cooler than its surrounding built-up area byup to 1.7 °C before midnight (22:30), while in 2002 the park was cooler only by up to 0.6 °C (at midnight).The results during daytime showed that in some hours of the day the park was slightly cooler, while at other



2

1.5

1

0.5

0

-0.5

-1

-1.5

-2

0

-1

-2

0.5

-0.5

-1.5

-2.5

-3

-3.5

12:0

0

14:0

0

16:0

0

18:0

0

20:0

0

22:0

0

0:00

2:00

4:00

6:00

8:00

10:0

0

12:0

0

14:0

0

16:0

0

July 31 - August 1, 2001 June 6-7, 2002

Hour

Tem

p di

ffere

nce

(°C

)T

emp

diffe

renc

e (°

C)

(a)

12:0

0

14:0

0

16:0

0

18:0

0

20:0

0

22:0

0

0:00

2:00

4:00

6:00

8:00

10:0

0

12:0

0

14:0

0

16:0

0

Hour

July 31 - August 1, 2001 June 6-7, 2002

(b)Park B

Park A

Park C0

-1

-2

0.5

-0.5

-1.5

-2.5

-3

-3.5

Tem

p di

ffere

nce

(°C

)

(c)

8:00

10:0

0

12:0

0

14:0

0

16:0

0

18:0

0

20:0

0

22:0

0

0:00

2:00

4:00

6:00

8:00

10:0

0

12:0

0

14:0

0

16:0

0

Hour6-7 June 2002 22-23 June 2000

Figure 4. Hourly temperature differences between the center of the parks and their surrounding built-up environments during selectedperiods, in: 4(a). park type A (grass), 4(b). park type B (medium trees) and 4(c). park type C (well treed)



hours of the day the park was warmer than its surrounding built-up area. On the first day of the measurementperiod in 2001, the park was cooler than its surrounding area during most hours of the day, and reached amaximum value of 0.8 °C on the first day (15:00 h). On the second day of the 2001 measurements, a differentphenomenon was found; from the morning until midday the park was warmer than the built-up area by upto 0.6 °C and then the park became cooler than the surrounding built-up area by up to 1.0 °C. At the secondmeasurement period in 2002, the park was warmer than its surrounding area during most daytime hours onboth days by up to 1.3 °C and only during the afternoon of the last day of measurements, the park center wasslightly cooler than the built-up area by 0.5–1.0 °C (Figure 4(a)).

Measurements of black ball temperatures were taken during 6–7 June 2002 in each park center and inits built-up surrounding area in order to measure the mean radiant temperature and to indicate the shadingefficiency of each park (Figure 5). In general, there are two patterns of diurnal behavior, one includes parkB and C and the other includes the built-up environment and park A.

The main temperature differences between the stations occurred during daytime. A cycle of 24-h mea-surements from 6 to 7 June 2002 showed that at 08:30 h the black ball temperature differences betweenthe stations were 5 °C. The differences then increased rapidly and between 12:00 to 14:00 h the temperaturedifferences were the highest, 27 °C at the treed urban parks and 47 °C at the built-up area around park A andB. As mentioned in the preceding text, the temperatures at park A (grass) were similar to the built-up areaand the maximum temperature, 46 °C, was recorded at 15:00 h. During the afternoon, temperatures reducedrapidly and at 20:00 h, the gap between the various stations was 4 °C. After sunset, the trend of the tempera-ture differences changed and the temperatures in the treed parks (B and C) were higher than the temperaturesof the built-up area. The temperatures in park A were the lowest and a minimum temperature of 14.5 °Cwas recorded at 05:00 h. After sunrise, temperatures in all the stations started rising, while the rapid changeoccurred in the grass park.

8:00

10:0

0

12:0

0

14:0

0

16:0

0

18:0

0

20:0

0

22:0

0

0:00

2.00

4:00

6:00

8:00

Park A - built-up areaPark B - built-up areaPark C - built-up area

Park A - centerPark B - centerPark C - center

10

15

20

25

30

35

40

45

50

Rad

iatio

n te

mpe

ratu

re (

°C)

Hour

Figure 5. Black ball hourly temperature at the center of each park type and its surrounding built-up environment during the measurementperiod of 6–7 June 2002



Measurements of relative humidity showed that all park types were found to be more humid than thebuilt-up surrounding areas (Figure 6). Relative humidity comparison between the three parks in 6–7 June2002 showed that during the day the values in park C were the highest, while during the night the values inpark B were the highest. The minimum relative humidity measured in park C was 56% and the maximumvalue was 81%, while the minimum relative humidity measured in park B was 48% and the maximum valuewas 89%. The minimum relative humidity measured in park A (grass) was 50% and the maximum value was87% (the maximum values were measured during the nighttime while the minimum values were measuredthe second day during a hot and dry spell).

The relative humidity values in the well-treed park (type C) were 8–10% higher than those in the built-uparea during the day, and 5% during the night. Inside park type B, the values were higher during most of theday by 5–7% than those in the built-up area, and 2–5% higher during the night. The relative humidity valuesin the grass park were higher in the first day of measurements by 2–3% than the surrounding built-up area.After sunset the park became more humid, by up to 5% more than the built-up area. At 04:30 h, immediatelyafter irrigation started, the relative humidity values increased and the park was 8% more humid than thebuilt-up area (irrigation done by sprinklers). During a hot and dry spell during the second day, the park wasup to 13% more humid than the built-up area, despite the fact that the park was warmer than the built-up area.

Owing to the fact that the relative humidity values represent the relationship between the humidity andthe temperature, it is necessary to calculate the values of the absolute humidity in order to estimate thecontribution of the different types of vegetation to the moisture conditions and their influence on humancomfort.

Comparison of absolute humidity differences between the park center and the built-up environment in thedifferent park types showed that during the first day of the measurement period in 2002, the values in all

90

85

80

75

70

65

60

55

50

45

40

8:00

10:0

0

12:0

0

14:0

0

16:0

0

18:0

0

20:0

0

22:0

0

0:00

2:00

4:00

6:00

8:00

10:0

0

12:0

0

14:0

0

16:0

0

Hour

Rel

ativ

e hu

mid

ity (

%)

Park A - built-up areaPark B - built-up areaPark C - built-up area

Park A - centerPark B - centerPark C - center

Figure 6. Hourly relative humidity values in the different park types and in the surrounding built-up areas during 6–7 June 2002



the stations were quite similar and consistent. The absolute humidity values in park C were most of the timehigher by 0.2–0.7 g/kg (dry air) than the surrounding built-up area during most of the measurement period.The values in park B were slightly lower, by an average of 0.13 g/kg (dry air) than the built-up area until08:30 h on the second day of the measurement, and then surprisingly the values in the built-up area werehigher by up to 1.1 g/kg (dry air) than those of the park probably owing to the fact that private gardens withinthe residential area were irrigated during the heat wave. The absolute humidity values in park A (grass) werehigher by 0.2–0.5 g/kg (dry air) then the surrounding built-up area from the beginning of the measurementuntil 04:00 h. At 04:30 h after irrigation started, the absolute humidity at the park increased, and values atthe park were higher up to 3.7 g/kg (dry air) then the surrounding built-up area (Figure 7).

Wind measurements were taken in the center of each park, approximately 100 m from the built-up area.Therefore, the influence of the buildings was reduced and allowed the examining of the impact of each parkdesign on the wind regime. During the day, the wind velocity in all park types was higher than that duringthe night as a result of the diurnal synoptic wind regime and the sea breeze, which increased wind velocityduring daytime and reduced it during nighttime (Figure 8). During the day, the wind velocity differencesbetween the parks were relatively small; during the night, the differences were more pronounced despite theweak wind velocity. Wind measurements of 6–7 June 2002 showed that, in general, wind velocity in park Bwas the lowest during the day and night in comparison to the other parks. From 22:00 h until the morning,wind velocity in park type C was between 1 and 2 m/s; in park type A, between 0 and 1 m/s and in park typeB (medium sized trees) the wind velocity was the lowest and calm conditions were measured during the 9 h,from 22:00 until 07:00 h. From the morning, wind velocity in all stations increased again and reached highervalues than those of the first day. In the built-up area near all the parks, wind velocity values were lower then1 m/s. It should be noted that wind velocity in park C was higher than that measured in the built-up area,owing to the high canopy design which offered little resistance to near-surface airflow. In park A, despitelack of trees, wind velocity was lower than in park C, possibly because of the height and density of buildings.

Since urban parks are recreation areas, it is important to estimate their potential for producing comfortableclimatic conditions and help in reducing the climatic heat stress caused by the urban environment. The thermalcomfort index used in order to examine the climatic conditions in each park and in its built-up area was thepredicted mean vote (PMV). This index, calculated from Fanger’s (1970) comfort equation, is one of the

4

-2

3

2

1

0

-1Abs

olut

e hu

mid

ity d

iffer

ence

sg/

kg (

dry

air)

9:00

15:0

0

11:0

0

13:0

0

11:0

0

13:0

0

15:0

0

17:0

0

19:0

0

21:0

0

23:0

0

1:00

3:00

5:00

7:00

9:00

Hour


Figure 7. Hourly absolute humidity differences between the park center and the built-up environment in the different park types during6–7 June 2002



6

0

5

4

3

2

1

Win

d ve

loci

ty (

m/s

)

8:00

14:0

0

16:0

0

10:0

0

12:0

0

10:0

0

12:0

0

14:0

0

16:0

0

18:0

0

20:0

0

22:0

0

0:00

2:00

4:00

6:00

8:00

Hour


Figure 8. Hourly wind velocity in the center of the different park types during 6–7 June 2002

most frequently used indices in human biometeorology. Through parameterization of the complex radiationfluxes by Jendritzky et al. (1990), Fanger’s model has become applicable to outdoor situations under thename ‘Klima-Michel model’ (KMM). Six input parameters are required for comfort assessment: four climaticparameters; air temperature, vapor pressure, mean radiant temperature and wind velocity, and two parametersrelated to the occupants of the environment; metabolic heat production and clothing insulation. The PMVindex predicts the expected comfort vote on a seven-step psychophysical scale (Table I):

The mean radiant temperature (Tmrt) and PMV were calculated, using the PC modeling program RayMan(Matzarakis et al., 2000; Matzarakis, 2002). Tmrt, which is required in the human energy balance model andfor comfort assessment, is defined as the uniform temperature of a surrounding surface emitting black-bodyradiation (emission coefficient ε = 1) (Matzarakis et al., 2000). Mean radiant temperature is the temperatureof surfaces surrounding a person. It is also a measure of the radiant heat exchange between a person andhis surroundings. Tmrt is the most important meteorological input parameter for human energy balance during

Table I. Relation of PMV, thermal perception and percentage ofpersons in thermal discomfort according to Fanger (from Jendritzky

et al., 1990)

PMV Thermal perception Persons indiscomfort (%)

Less than −2.5 Severe cold stress >80–1.5–2.5 Medium cold stress >50–0.5–1.5 Light cold stress >10–0.5–0.5 Comfortable <100.5–1.5 Light heat stress >101.5–2.5 Medium heat stress >50More than 2.5 Severe heat stress >80



summer weather conditions and thus has a strong influence on thermophysiological indices such as the PMV(Mayer, 1993).

The calculation of the thermal sensation according to the model requires adjustment of the followingconstants: The body surface has been standardized to 1.9 m2, which represents a human with a height of1.75 m and a bodyweight of 75 kg (Hoppe, 1984). The metabolic rate (Met) has been fixed at an averagevalue of 65 W/m2 for a sitting person. The insulation factor of the clothing (Icl) has been standardized to 0.5for light summer clothing (Jendritzky et al., 1990). An Icl of 0.8 has been chosen in case of a PMV<−0.5(cold, uncomfortable situations at night). The factor (fcl) that describes the covered body surface to uncoveredbody surface has been fixed at 1.1.

The results present the different human climatic sensations in each park type. During the day, both parksthat contain trees (park B and C) were more comfortable than the surrounding built-up area (Figure 9). Duringthe first day, the PMV values in both parks indicate a comfort situation (∼0.5), while in the built-up area thePMV values were one level higher (between 1 and 1.5). During the hot spell on the second day, the PMVvalues in parks B and C were defined as light heat stress (between 0.8 and 1.4), while at the built-up area thePMV values were defined as medium heat stress (between 1.7 and 2.5). In park A (grass), the phenomenawere different. During the first day, the comfort level in the park center and in its surrounding built-up areawas similar – light heat stress. During the second day, the PMV values in park A were higher than the built-uparea by 0.2–0.4 units, which in this case was one level higher, (medium in the built-up area and severe inthe park). During the night, the PMV values were comfortable in all stations with the exception of park A(grass), which was found to be slightly cooler.

3.2. Results of mobile measurements

Figure 10 presents results of the mobile measurements during the early afternoon on 6 June 2002 and atsundown on 7 June 2002. The results present the different temperature patterns of each park. In the early

3

-1

2.5

2

1.5

0.5

-0.5

PM

V

9:00

15:0

0

17:0

0

11:0

0

13:0

0

11:0

0

13:0

0

15:0

0

17:0

0

19:0

0

21:0

0

23:0

0

1:00

3:00

5:00

7:00

9:00

Hour

Park A - center

Park B - center

Park C - center

Park A - built-up area

Park B - built-up area

Park C - built-up area

1

0

SEVERE HEAT STRESS

MEDIUM HEAT STRESS

LIGHT HEAT STRESS

LIGHT COLD STRESS

COMFORTABLE

Figure 9. Hourly heat stress values according to the PMV index in the different park types and in their surrounding built-up areas during6–7 June 2002



28

27

26

25

24

23

22

21

20

19

18

17

Tem

pera

ture

(°C

)

1 2 3 4 5 6 7 8 9 10

Measurement points

Park A Park B Park C

hr 04:30

hr 15:00

Figure 10. Temperature values taken by mobile measurements at the three different park types on 6–7 June 2002, at 15:00 and at 04:30

afternoon between 15:00 and 15:30 h, the greatest temperature reduction of 2.0 °C was found in park C (welltreed). The temperature reduction in park B (medium sized trees) was up to 1.2 °C. In the center of park A(grass), the temperature values were slightly higher than those measured in the built-up area east of the parkand higher by 0.7 °C than the temperature in the built-up area west of the park. At dawn between 04:30 and05:00 h, the greatest temperature reduction of 1.2 °C was found in park B (medium sized trees), in park A(grass) temperature reduction was 0.9 °C, while temperature reduction in park C (well treed) was negligible,at only 0.3 °C. It should be noted that comparison of temperatures between the three parks is complicated dueto the fact that park C is located in the city center and therefore the temperature values in the park and itssurrounding built-up area were higher by 2 °C than the temperatures in the suburb where park B and A arelocated.

4. DISCUSSION

The hypothesis of this study was that parks within urban environments will be cooler than their surroundingbuilt-up areas and will create a PCI. Oke (1987) described this phenomenon as the ‘oasis effect’ and suggestedthat an isolated moisture source will always find itself cooler than its surrounding arid region. Studies showedthat even in hot, humid cities, parks can develop PCI effect (Sham, 1991; Saito et al., 1991; Sundersingh,1991) which can be explained by the oasis effect caused by the vegetation (Spronken-Smith et al., 2000).The assumption that urban parks will always be more humid than its surrounding arid region was found.However, the finding of this study showed that moisture sources, like urban parks, will not always be coolerthan their surrounding arid regions. It was found that urban parks containing trees can reduce temperatures. Itwas also found that the maximum cooling effect of urban parks containing trees occurred during the daytime.These results can be explained by the rule of two climatic factors; humidity and radiation. During the day,the relative humidity values are lower than those during the night and therefore the gap between dry bulbtemperature and wet bulb temperature is higher than during the night; therefore, the cooling effect is strongerduring daytime. The second factor is the shading effect of trees that prevent heating by the solar radiation(Figure 5) but it is meaningless during the night.



The maximum cooling effect of both treed parks was up to 2.5 °C in park C (well-treed park) and up to 2 °Cin park B during the afternoon. During the night, the cooling effect in both parks was weaker than that duringthe day, with temperatures between 0.5 and 1.2 °C, though the cooling effect of park type B was slightlystronger during nighttime than park type C. These results can be explained by the canopy character of thedifferent park types. The dominant tree in park C is the Ficus tree, whose canopy is denser and thicker thanthe canopy of the Tipuana tipu tree that is dominant in park B (Figure 1). Therefore, park C has a lower skyview factor, which creates better shading efficiency during the day but prevents radiation cooling during thenight.

It should be noted that measurements in 2002 (mentioned above) were taken after park C underwent massivepruning. Comparison of the 2002 results with those of June 2000 measurements, before pruning, showed thatthe cooling effect of this park reached higher values (2.5 °C in 2002 and 3.5 °C in 2000), and lasted for morehours than in 2002 (Figure 4(c)). This comparison emphasizes that the cooling effect of trees is influencedby foliage density, trees height and size and canopy volume.

Park type A (grass) demonstrates completely different climatic behavior. During daytime, with typicalsummer climate conditions, park type A (grass) was found to be slightly warmer (up to 1 °C) than its built-upsurrounding area. During the ‘sharav’ (hot and dry weather), the park was found to be warmer by up to 1.4 °Cthan the built-up area. This can be explained by two reasons, first – the grass park is completely exposedto sun radiation while the built-up area is partly shaded by the buildings, second – due to the decomposingprocess, the grass park produced heat that contributed to air temperature (Ben-Dor et al., 1997). During thenight, park type A was found to be slightly cooler than the built-up area (Figure 3). On 6 June with typicalsummer climate conditions, the temperature in the built-up area around park A was lower by 1.5 °C thanthat of the built-up area around park C. During ‘sharav’ conditions, the temperature in the center of park A(grass) was higher by up to 3 °C than that of park C (well treed), while the temperature in the built-up areaaround park A was higher by only 0.5 °C than the temperature in the built-up area near park C. With regardto humidity, all three parks were more humid than the built-up area and created a moisture island. As can beseen from the absolute humidity differences (Figure 7), park C was slightly more humid than the other parkswith the same climatic conditions. However, after park A was irrigated, a prompt increase in humidity valueswas observed. The fact that parks with grass often require irrigation has a direct influence on their humidityvalues.

As urban parks serve as recreation areas, there is special importance with regard to the climatic comfortconditions for humans inside them. It was found that the poorest comfort conditions prevailed at the grasspark during the day, as temperature values in this park were higher by up to 3 °C than the coolest park (C),despite the fact that the wind velocity in park A was the highest among the other areas. The differences inthe predicted thermal sensation between the three parks emphasizes the role of sun radiation as one of themain negative factors that causes uncomfortable climatic conditions for human sensation. It can be seen thatthe black ball temperature values during periods of sunshine at the treed parks were much lower than thoseof the built-up area around them, while the values at the grass park were similar to those in the surroundingbuilt-up environment (Figure 5). It also should be noted that black ball temperatures in park A (grass) wereeven higher than those at built-up areas near park C. This can be explained by the lack of shading inside parktype A in comparison to that in the urban canyon street in the city center.

The results of the study emphasize the contradiction between the cooling effect of park C and B and thewarming effect of park A. Parks that contain trees can serve as a friendly urban environmental element,especially during the day, due to reduction of air temperature and black ball temperature. On the other hand,grass parks can be considered as an unfriendly element in the urban environment, owing to increasing airtemperature and high black ball temperature accompanied with an increase in humidity values.

The results of the study showed another contradiction that requires explanation. It is clear that urban parkswith trees will develop a cool island and therefore will be more comfortable for humans than the built-upenvironments. This study points out that despite the cooling effect of the urban park during some hours ofthe night, the park with medium sized trees can have a negative effect on human climatic comfort due toreduction of wind velocity and an increase in relative humidly.



5. CONCLUSIONS

Urban parks with high and wide-canopied trees (type C) have the maximum cooling effect during the hottesthours of the day and have a positive effect on human climatic comfort. Parks with grass and only a few lowtrees were found to be warmer than the surrounding built-up areas and other park types, especially during theday, and have a negative effect on human climatic comfort during the day. A park with medium sized treeshas a cooling effect during the day and therefore has a positive effect on human comfort. However, duringthe night this park type is very humid and suffers from lack of ventilation, and therefore has a negative effecton human comfort.

One of the research findings is that during all hours urban parks in hot and humid coastal Mediterraneanclimate are ‘humidity islands’ but not necessarily cool islands as Oke suggested (Oke, 1987; Spronken-Smithand Oke, 1998).

The results of this study suggest that the most useful type of urban park in a coastal Mediterranean area,from the climatic point of view, is a park with high and wide-canopied trees. The results also suggest avoidingparks with grass coverage and without trees in a coastal Mediterranean area.

ACKNOWLEDGEMENTS

The authors wish to thank Professor Baruch Givoni for his fruitful advice and remarks. The authors extendtheir gratitude to the students of Bet Berl College of Education for their participation in the field survey.The authors are grateful to Mr Yaron Yaakov for his technical assistance in the measurement process andpreparation of the figures.

REFERENCES

Ben-Dor E, Inar Y, Chen Y. 1997. The reflectance spectra of organic matter in the visible near-infrared and short wave infrared region(400–2500 nm) during a controlled decomposition process. Remote Sensing of Environment 61(1): 1–15.

Bitan A, Rubin S. 1994. Climatic Atlas of Israel for Physical and Environmental Planning and Design. Ramot Publishing Company, TelAviv University: Tel Aviv, Israel.

Bitan A, Noy L, Turk R. 1992. The impact of the seashore on the climate of Tel-Aviv. Scientific reports of the Institute for Meteorologyand Climate Research of the Karlsruhe University 16: 147–160.

Chudnovsky A, Ben-Dor E, Saaroni H. 2004. Diurnal thermal behavior of selected urban objects using remote sensing measurements.Energy and Buildings 36(11): 1063–1074.

Fanger PO. 1970. Thermal comfort. Danish Technical Press: Copenhagen, Denmark.Givoni B. 1991. Impact of planted areas on urban environment quality: A review. Atmospheric Environment 25B(3): 289–299.Grimmond CBS, Souch C, Hubble MD. 1996. Influence of tree cover on summertime surface energy balance fluxes, San Gabriel, Los

Angeles. Climate Research 6: 45–57.Hoppe P. 1984. Die Energiebilanz des Menschen. Wissenschaftliche Mitteilungen des Meteorologischen Institutes der Universitat

Munchen: Munchen; 49.Jauregui E. 1991. Influence of large urban parks on temperature and convective precipitation in tropical cities. Energy and buildings

15–16: 457–463.Jendritzky G, Menz G, Schirmer H, Schmidt-Kessen W. 1990. Methodik zur Raumlichen Bewertung der Thermischen Komponente im

Bioklima des Menschen, Bd. 114. Beitrage der Akademie fur Raumforschung und Landesplanung: Hannover, Germany.Jonsson P. 2004. Vegetation as an urban climate control in the subtropical city of Gaborone, Botswana. International Journal of

Climatology 24: 1307–1322, DOI:10.1002/joc.1064.Katayama T, Ishii A, Hayashi T, Tsutsumi J. 1993. Field surveys on cooling effects of vegetation in an urban area. Journal of Thermal

Biology 18(5/6): 571–576.Matzarakis A. 2002. Validation of modelled mean radiant temperature within urban structures. Proceeding of the Fourth Symposium on

the Urban Environment, Norfolk, Virginia, 20–24 May. AMS: Boston, MA; 72–73, abstract no. 7.3.Matzarakis A, Rutz F, Mayer H. 2000. Estimation and calculation of the mean radiant temperature within urban structures. In

Biometeorology and Urban Climatology at the Turn of the Millennium, de Dear RJ, Kalma JD, Oke TR, Aulicems A (eds). SelectedPapers from Conference ICB-ICUC’99, Sydney, Australia. World Climate Application and Service Program. (WCASP-50), vol. 1026.World Meteorological Organisation (WMO)/TD: Sydney, Australia; 273–278.

Mayer H. 1993. Urban bioclimatology. Experientia 49: 957–963.Oke TR. 1987. Boundary Layer Climates, 2nd edition. Methuen: New York; 435.Padmanabhamurty B. 1991. Microclimate in tropical urban complexes. Energy and Buildings 15–16: 83–92.Potchter O, Saaroni H. 1998. An examination of the map of climatic regions of Israel, according to the Koppen classification. Studies

in the Geography of Israel 15: 179–194. (Hebrew).Potchter O, Yaacov Y, Bitan A. 1999. Daily and seasonal climatic behavior of small urban parks in a Mediterranean climate: A case

study of Gan-Meir Park, Tel-Aviv, Israel. In Proceedings of the 15th International Congress of Biometeorology and InternationalConference on Urban Climatology , Sydney, Australia, 8–12 November; ICUC 6.3.



Potchter O, Saaroni H, Yaakov Y, Lunka A, Bitan A. 2002. The use of vegetation in open space as a tool for improving climatic qualityin metropolitan areas: the case study of Tel Aviv. Studies in the Geography of Israel 16: 362–377. (Hebrew).

Saito I, Ishihara O, Katayama T. 1991. Study of the effect of green areas on thermal environment. Energy and Buildings 15–16:493–498.

Sham S. 1991. Urban climatology in Malaysia: a review. Energy and Buildings 15–16: 105–117.Spronken-Smith RA, Oke TR. 1998. The thermal regime of urban parks in two cities with different summer climates. International

Journal of Remote Sensing 19(11): 2085–2104.Spronken-Smith RA, Oke TR, Lowry WP. 2000. Advection and the surface energy balance across an irrigated urban park. International

Journal of Climatology 20: 1033–1047.Statistical Abstract of Israel. 2003. Central Bureau of Statistics, Jerusalem. Vol. 54.Sundersingh SD. 1991. Effect of heat islands over urban Madras and measures for its mitigation. Energy and Buildings 15–16: 245–252.Young Co. RM 1998. Technical Data on Model 41002 Gill Multi-Plate Radiation Shield. Travers City, Michigan, USA.


Climatic behavior of various urban parks during hot and humid summer in the mediterranean city of...

Documents

Transcript of Climatic behavior of various urban parks during hot and humid summer in the mediterranean city of...