What is urban heat island effect and why we need to tackle it?

Photo by Ashkan Forouzani on Unsplash

Part #1 – Singapore

Introduction to my master thesis project

Contemporary Singapore is a unique metropolis that will disappear under bulldozers paving the way to „urbanization”. In consequence, the residents live here in the shadow of skyscrapers. And this is just a metaphorical shadow. The shadow of concrete is different from the shadow of a tree – hides from the sun but it cannot be from the omnipresent humidity, stuffiness and sweltering heat.

According to Channel News Asia (sixth-widest reach among television news channels in Asia), Singapore gets warmer twice faster than the rest of the world. It is not only due to its tropical location but also the rapid urbanisation over the past decades. Many scientists even predict that before 2100 it can be even too warm to run outdoors. 

The project COOL IT DOWN proposes solutions to this challenge, which is presented as a toolbox. The toolbox uses Singapore as a reference point, but its content is universal. The rapid warming of the cities is a global phenomenon that needs to be addressed. 

The toolbox is presented in three different design scales: LARGE, MEDIUM and SMALL. The LARGE scale comprises master planning and a set of guidelines on the aerial scale to influence air temperature or ventilation. The MEDIUM scale includes building typologies that can effectively provide wind and protect from heat and solar radiation. The SMALL scale focuses on the design on a street level that can positively influence the microclimate. 

The master plan is a proposal for an enormous 166 ha site in a central area in the bustling metropolis. It is a fully artificial land that has a key role in the future development of the city. Having in mind the possible negative consequences of concrete the design site emphasizes the importance of greenery in overcoming the given challenge.

What is an urban heat island?

There tends to be a different climate in cities than in most natural landscapes, and the densely built-up city is usually warmer than the surroundings. This is the result of retained long wave thermal radiation, anthropogenic heat and reduced evaporation. It is mostly due to a lack of open soil surfaces, water or greenery. This heat phenomenon is called an urban heat island.

However, even within a city, there are areas with different density, green structure or the presence of wind corridors and consequently with different air temperatures. This is why the term heat archipelago is also used in literature and refers to the fact that cities have various heat areas and one should not consider only the whole city scale.

During the summer season, radiation itself is mostly responsible for the urban heat island effect. It is mostly received and stored during the day and it is released as longwave radiation at night. During the daytime, urban heat islands are a little warmer than the countryside. The heat island effect is strongest on hot, cloudless days with little wind as the radiations hit directly the surfaces.

People often believe that warmer cities are much better and there are no troubles in a warmer climate. Actually, the problems connected to urban heat may be way bigger. Heat stress leads to inevitable health issues, food spoils quicker, there can be mosquitos plagues and people dehydrate faster.

Moreover, overheating leads to infrastructural challenges, asphalt wears out and deforms, train tracks can bend, the usage of power to provide air conditioning skyrockets and street furniture can be too warm to even use them. Concluding, these problems lead to enormous costs both in money and human capital.

The urban heat island can occur both during summer and winter. During the latter, it is mostly created by the anthropogenic heat coming from residential heating, transportation or industry. However, due to the location of Singapore and lack of winter, I will focus on the summer season

Singaporean context

If the current rate of carbon emissions continues to rise, Singapore’s daily temperature will reach 35 to 37 degrees Celsius by 2100. There is a reason to worry as because of the high humidity the temperature feels to be even bigger. Because of the high humidity, more seawater evaporates and is present in the air. When both humidity and air temperature rise, our bodies struggle to cope. With higher humidity, we get a higher stress level. What does it mean for the people of Singapore?

Even they would struggle to adjust. With such temperatures and humidity of 60% and more only the professional runners that train in special chambers with higher temperature would manage to function in such conditions, but not for a long time.

Why is Singapore getting warmer faster than in other countries? According to NASA satellite key data, since 1950 Singapore has been heating up twice as much as the rest of the world. It was not only due to the tropical climate but especially because of rapid urbanisation and development. In a nutshell, the tropical jungle was replaced by a concrete one. It resulted in a vast urban heat island effect.

Dark vision

There are some unprotected forest areas in Singapore that cannot be removed or reduced if we want to, not even reduce the UHI effect, but at least just to keep the existing conditions and temperature. Nowadays 80% of Singaporeans live in HDB flats in big concrete towns around the island. The rapid urbanisation and increase of population led to a shift in typology towards huge residential towers.

Moreover, in 20 years, over 700 hectares of the rainforest will be turned into 42 000 new homes. Every development like that will add up to the environmental cost and urban heat island, helping to reach the 2100 dark vision. There is a dilemma: should Singapore continue the rapid development but decrease the quality of the citizens’ life in the future?

Due to Singapore’s tropical location, it lacks the four or even two seasons cycle. As a consequence, it is warm but sometimes It is also a rainy summer throughout the year. It brings many challenges. The record high temperature might come to as high as 36 degrees Celsius, recorded in March. It’s also only 4 degrees more than the average temperature for this month. Due to the climate and humidity, the rainfall is also quite big.

The average yearly precipitation exceeds 2,100 mm, whereas in many European cities it stays between 600 and 800 mm. The average humidity is around 83% but daily it can get even to 100. The monthly sunshine hours number does not drop to less than 130, which is also quite remarkable in comparison to for ex. Öresund Region where the number falls under 40 in winter months. Given the extraordinary weather conditions and analyzing different charts, we need to emphasize the numerous challenges that Singapore is facing with a very specific and extreme climate.


Yet, the most challenging and most inevitable challenge is the environmental one. Due to rapid changes, the enormous amount of concrete used to build all the new developments, the scarcity of land and a tropical climate, Singapore is heading straight towards a huge climate crisis.

I would like to finish this short article with some worrying challenges that Singapore (and also other countries, especially in the similar climate zone) needs to solve:

• How to decarbonise?

• How to end Singapore’s role in the oil and gas industry?

• How to meet the Paris agreement goals?

• Why was Singapore reclaiming so much land and is still doing it?

These are just a few questions undermining the huge success of Singapore over the last couple of decades. Answering all those questions is not a task for this master thesis – the questions should be rather answered by the government. 

Or maybe, in your opinion, there is anyone else that should try answering those questions?


  1. My master thesis: https://issuu.com/marcinzebrowski/docs/master_thesis_booklet_marcin_zebrowski

Do you want to listen to the podcast episode, that the article bases on? Click below.

57: How is air conditioning heating our cities?

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