COOLER Cities #04: Outdoor Thermal Comfort Study Before Tree Planting
- Danial Mohd Ali
- 24 hours ago
- 8 min read
Updated: 11 minutes ago
Introduction: Why George Town Needs to Cool Down
George Town is internationally recognized for its historic alleys, vibrant street art, and colonial-era architecture. These narrow streets and hardscape-dominated spaces create a distinctive urban character—but they also present challenges in terms of thermal comfort, especially under Malaysia’s hot and humid tropical climate. As this COOLER Cities series continues, the next stages will explore how radiation, shading and vegetation interact within these heritage-rich urban spaces and how strategic tree planting targets to reshape the outdoor thermal experience of George Town by lowering the overall urban temperature by approx. 1.5 degree Celsius and the lowering the shaded areas by 5-7 degree Celsius.
In response, the city council together with ThinkCity and other stakeholders initiated a 4-kilometre landscape tree-planting programme encircling the historic core of George Town. The intention is clear: Improve pedestrian comfort through increased shading and microclimatic moderation.
As part of this initiative, IEN Consultants was commissioned to carry out a baseline outdoor thermal comfort study prior to any physical intervention such as tree planting and landscaping. The baseline is critical, without understanding existing conditions, it is impossible to quantify how effective the future tree planting will be once the trees mature. The baseline study therefore establishes a reference point against which post-intervention measurements can be meaningfully compared.
This fourth entry in the COOLER Cities series documents how heat is experienced in George Town today, why solar radiation plays such a dominant role, and how shade emerges as the single most powerful tool for improving outdoor comfort.
Short-Wave and Long-Wave Radiation: The Real Drivers of Urban Heat
To understand why George Town feels so hot during the day—and why it often remains uncomfortable well into the evening—it is necessary to distinguish between two types of radiation.
Short-wave radiation originates from the sun and includes visible and near-infrared light. It is the primary source of daytime heating, rapidly raising the temperature of roads, pavements, and building façades when direct sunlight is present.
Once absorbed, this energy does not disappear. Urban surfaces re-emit it as long-wave radiation or thermal infrared heat. This re-radiated heat is felt by pedestrians even after sunset, particularly in areas dominated by dark, dense materials such as asphalt and concrete.
The illustration above shows how people in an outdoor environment is affected by direct solar radiation as well as heat radiation from the surroundings.
Measurements in George Town clearly show this effect. On sunny days, surfaces exposed to direct sunlight store significant amounts of heat in the paved surfaces, which are then slowly released in the evening. This explains why thermal discomfort persists even when air temperatures begin to fall—a defining characteristic of the urban heat island effect, which can see 3-4°C higher temperatures inside the city compared to the country-side.
Measuring the Urban Climate: From Weather Stations to Street Level
Urban heat is not experienced at weather-station height (roof level) —it is felt at pedestrian level. To capture both the broader climatic context and the highly localised conditions along George Town’s streets, a multi-layered measurement strategy was adopted.
A fixed weather station was installed on the unshaded rooftop in central George Town, recording air temperature, humidity, wind, rainfall and solar radiation at regular intervals collecting measurements over a span of 2 months. This provided continuous background data and allowed comparison with longer-term climate records obtained from the state governments nearest weather station at Padang Kota.
At street level in downtown George Town, two data loggers were deployed in Gat Lebuh China and Gat Lebuh Gereja streets to record pedestrian level air temperature and humidity levels at a ten-minute interval. The loggers were placed in the shade, so the measurements would not be directly affected by solar exposure.
Most critically, a small mobile measurement device—known as the Comfort Cube (see photo and refer to our earlier article COOLER Cities #03) — was used to do spot-measurements of thermal comfort at 25 strategically selected locations across central George Town. These points were agreed jointly with the project partners and were chosen based on future pedestrian routes, planned tree planting locations and expected shading patterns. Measurements were conducted on sunny days in late July 2025 and on overcast days early September 2025. Measurements were done in these four time slots across the day:
07:00 – 9:00 (Morning)
11:00 – 13:00 (Midday)
14:00 – 16:00 (Afternoon)
18:00 – 20:00 (Evening)
Together, these measurements provide a detailed picture of how heat builds up, persists and is experienced in different parts of the city and different times of the day.
Choosing How to Define Outdoor Comfort: Why UTCI Was Used
Outdoor thermal comfort cannot be described by air temperature alone. Wind, humidity, and—most importantly in the tropics—radiant heat from the sun and surrounding surfaces all play decisive roles.
For this study, the Universal Thermal Climate Index (UTCI) was selected as the primary comfort indicator. Unlike simpler indices, UTCI integrates air temperature, humidity, wind speed, and both short-wave (solar) and long-wave (thermal) radiation into a single equivalent perceived temperature that reflects the physiological response of the human body.
UTCI is widely used in urban climate research and is particularly useful for comparing different urban scenarios—such as before and after landscape interventions. While it is acknowledged that UTCI tends to be conservative in tropical climates, especially when compared with how acclimatised populations perceive heat, its strength lies in consistency. For this project, it is the change in UTCI after tree planting, rather than the absolute value, that matters most.
What the Measurements Reveal: Sunny vs. Overcast Days
The contrast between sunny and overcast conditions in George Town could not be more striking.
On sunny days, UTCI values across most locations reached levels corresponding to strong or very strong heat stress from late morning through the afternoon. Even early mornings showed signs of discomfort, while evenings remained warm due to stored heat being released from surrounding surfaces.
On the overcast day, however, conditions were significantly different. With cloud cover limiting short-wave radiation, perceived temperatures dropped by approximately 8–10°C during peak hours. Extreme heat stress was entirely avoided, and many locations shifted into ranges that are considered acceptable for outdoor activity.
This comparison confirms that solar radiation—not air temperature alone—is the dominant driver of outdoor thermal discomfort in George Town.
The Power of Shade: A Street-Level Reality Check
One of the clearest findings emerged from a simple comparison between two nearby measurement points along the same street, Jalan Masjid Kapitan Keling. Both locations shared similar urban geometry and orientation. The only meaningful difference was shade.
The unshaded location recorded a UTCI approximately 5°C higher than its shaded counterpart during midday. In thermal comfort terms, this single difference shifted conditions from very strong to strong heat stress.
This result powerfully demonstrates that shade—whether from trees or built structures—is not a marginal improvement. It is a decisive factor that can fundamentally change how streets are experienced.
How People Feel: Street Surveys and Human Adaptation
To complement the instrument measurements, street surveys were conducted with over 500 respondents across central George Town. Participants were asked how they perceived the thermal conditions at the time of the survey and what factors contributed most to their discomfort.
Interestingly, while measurements consistently indicated moderate to strong heat stress, a significant portion of the street survey respondents described the current outdoor thermal conditions as acceptable. In fact, 80% stated that the thermal environment was acceptable, yet, 74% of them also stated they were feeling a moderate, strong, very strong or extreme heat stress; refer to the graph below. This shows a high level of acclimatization among residents and visitors to the tropical climate.
At the same time, when asked what bothered them most, the answers were unequivocal: heat from the sun, lack of shade and lack of trees dominated responses. When invited to suggest improvements, nearly four out of five respondents proposed the same solution—plant more trees.
This apparent contradiction highlights an important reality of tropical cities: people may accept heat as a fact of life, but they are acutely aware of how much better streets could feel with adequate shading.
Conclusion: Setting the Baseline for a Cooler George Town
While the Universal Thermal Climate Index (UTCI) is one of the most advanced tools for assessing outdoor thermal comfort, its application in tropical climates such as Malaysia remains contested. In George Town, UTCI consistently classified outdoor conditions as experiencing strong to very strong heat stress, even when many residents perceived the thermal environment as acceptable. This mismatch reflects a known limitation of UTCI in the tropics, where long-term physiological and behavioural acclimatization plays a significant role. According to Wolfgang Kessling of Transsolar, this issue was raised with the UTCI committee more than a decade ago, yet the model was retained for global consistency. Kessling notes that conditions such as 32°C in the shade with sufficient air movement can be perfectly comfortable in tropical settings, despite being labelled as heat stress by UTCI. In dense urban streets, where measured wind speeds are often low, UTCI tends to overstate discomfort—highlighting that while the index may exaggerate absolute heat stress, it remains a robust tool for comparing relative improvements particularly before and after interventions such as urban tree planting.
This baseline study confirms that outdoor heat stress is a daily challenge in George Town, particularly under sunny conditions. Exposure to solar radiation and stored heat from urban surfaces are the primary contributors to discomfort, while shade consistently emerges as the most effective mitigation strategy.
The findings strongly validate George Towns tree-planting initiative. By intercepting solar radiation before it reaches the ground, trees provide welcomed shade and lowers the urban heat island effect, significantly improving pedestrian comfort in the day and at night.
Most importantly, this study establishes a robust reference point. When measurements are repeated in the coming years, the cooling effect of the new urban landscape can be quantified with confidence. In doing so, George Town takes a critical step toward becoming not just a historic city—but a cooler, more liveable one.
Acknowledgements & References:
We would like to thank Think City Sdn Bhd for an excellent, enjoyable and professional collaboration on this outdoor thermal comfort study. We are particularly thankful to Matt Benson, Yogeswary Chellappan, Ilana Aqilah Abd Ghafar, Melissa Sivaraj and Sofia Castelo.
The author would like to thank the following IEN colleagues for undertaking the project work and/or contributing to this article: Bryan Ellama, Cindel Soo Yung Yung and Gregers Reimann.
“Penang Wins Global Awards” press release with states the George Town temperature reduction targets in this quote: “The goal is that after 6-8 years of completion (planned for 2025), temperatures will be reduced by approximately 1.5 degrees Celsius in all urban areas and 5-7 degrees Celsius in shaded areas. Stormwater management is also expected to be improved, substantially reducing flood risks.” Link: https://thinkcity.com.my/about/media-centre/press-releases/penang-wins-global-climathon-awards#:~:text=The%20project%20will%20be%20assessed,improved%2C%20substantially%20reducing%20flood%20risks.
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