The Urban Heat Island Effect: Causes, Impacts, & Solutions

Which has hotter temperatures in general, the city or the countryside?

The city, right?

If I ask you why, you’d say, “Because there are more buildings and fewer trees in the city.”

You are right, but this phenomenon has more depth to it, and it’s called The Urban Heat Island Effect.

In this post, I will walk you through what the Urban Heat Island Effect is, what causes it, its impact, and how it can be mitigated.

What is the Urban Heat Island(UHI) Effect?

Simply put, the term refers to city areas being hotter than the surrounding rural areas. 

This temperature difference between urban and rural environments is bigger during nighttime. 

It’s also more prominent when the winds are weak, and when there are “block” conditions preventing weather patterns from moving on, especially during summer and winter. 

Not all cities display this phenomenon in an equal manner. 

The climate change we are experiencing globally today is not the cause of UHI, but it does intensify it. 

What Causes The UHI Effect?

So, how come city areas get hotter than suburban and country regions?

The city areas absorb and contain more heat from the sunlight, that’s why. 

What makes cities such heat-absorbing areas?

Let’s find out.

Thermal Bulk Properties of The Construction Materials & Urban Canyon Effect 

Most of the materials used in constructing buildings and transportation infrastructure are dark colored, like black, brown, and grey. Take asphalt, brick, steel etc for example. Such dark materials absorb heat from sunlight fast.

Also, most of these construction materials have impervious surfaces, meaning the surfaces are kind of sealed off, so water can’t get through. So, they can’t be cooled by the natural flow of water through condensation and evaporation. 

These materials carry heat through them quite easily and can hold a lot of heat in them, too. In technical terms, they have higher thermal conductivity and heat capacity.

The tall buildings in the urban areas provide a higher surface area for solar radiation to be absorbed and reflected, too, contributing to the UHI effect. This phenomenon is called the Urban Canyon Effect.

All this contributes to city areas heating up quickly and cooling down slowly.

Reduced Vegetation Cover

Another reason why city areas tend to be hotter is that there are not as many trees there as in the countryside.

So, the urban areas lack the shade the trees can provide and get more exposed to solar radiation, heating up more and faster.

Trees also provide a cooling service as they absorb ground water through the roots and release it as vapor from the leaves, absorbing heat from the surrounding environment to transform that water into vapor. 

Most cities lack this cooling through evapotranspiration because there are not many trees. 

Air Pollution 

Fossil fuel-powered transportation and various industrial operations can pollute the air around urban areas, and this air pollution contributes to the UHI too. 

Various pollutants such as gasses like Nitrous oxide, Volatile Organic Compounds (VOCs) like Benzine, and particulate matter like dust and soot trap heat and prevent it from dissipating at night, contributing to making these city areas heat up more.

Impacts of Urban Heat Island Effect 

So, how does UHI affect the environment and humankind?

Let’s find out:

Changing Local Weather & Climate 

As the air in city areas heats up, expands, and moves upwards due to UHI, they drive in cooler air from the surrounding rural areas, causing the speeds and directions of local winds to change. 

UHI affects cloud formation and fog too. 

As the air around cities expands due to higher temperatures and moves upwards, it carries moisture from the surface with it. The heat makes this moisture stay up in the air in its gaseous state longer.

This vertical mixing and retention of moisture results in more cloud formation, as the moist and warm air moving up eventually cools down at higher altitudes, condensing the water vapor into clouds. Some particulate contaminants in the air around cities aid in this too. 

This increased cloud formation can result in increased rainfall downwind of cities too.

Reduced nighttime cooling due to UHI can delay the formation of fog, as it makes it harder for the air to reach the dew point temperatures.

Effect on Human Health 

The Urban Heat Island Effect can cause various physical and mental health issues among city residents.

UHI can prolong and intensify the heat waves within urban environments, exposing people to extremely high temperatures. The nighttime effect of UHI can be especially dangerous during a heatwave.

This exposure to high temperatures can cause heat illnesses like heat stroke, heat exhaustion, heat syncope, and heat cramps.

Studies have shown that heat waves kill more people in the US than any other weather event such as tornados, hurricanes, etc.

Exposure to extreme heat can affect people’s mental health too, leading to aggression, increased domestic violence, and substance abuse. It also has the potential to interfere with school performance and education.

Effect on Water Sources & Aquatic Life Forms

UHI also can heat the rainwater runoff that ends up in local water bodies, increasing the temperatures of those water bodies in return and negatively affecting the organisms that inhabit them.

Hot rooftops, rain gutters, and pavements are the main culprits of this heated rainwater runoff.

There have been incidents where significant numbers of aquatic animals died after this hot stormwater ended up in nearby lakes, rivers, and streams.

Effect on Animals 

Interestingly, UHI works in some animals’ favor.

Some colonizing animals find the warmer winter conditions in the cities due to the Urban Heat Island Effect preferable to their usual wilderness habitats that are colder in winter.

Insects are another kind of animal that can be benefited by UHI. Being unable to keep their body temperatures optimum in cold conditions like warm-blooded animals, some insects find the hotter urban areas more favorable.

Economical Impacts 

The Urban Heat Island Effect puts a significant burden on the economy due to the increased demand it creates for refrigeration and air conditioning.

It’s also been found that residual heat from the air conditioners can lead to higher nighttime temperatures.  

Mitigating the Urban Heat Island Effect 

So, how can the UHI be mitigated and its impacts reduced?

Changing Urban Development & Construction Strategies 

Changing how the city area infrastructure is planned and constructed is one option for reducing the Urban Heat Island Effect.

One such change is using white or reflective materials to build buildings, roofs, pavements, and roads, so they will not absorb as much heat. 

Painting rooftops white or applying reflective coatings on rooftops and using reflective materials to build roofs(cool roofs) are also viable options.

Green infrastructure is also effective in reducing the UHI effect. 

This means using layers of small plants on tops of structures like roofs, payments, and parking lot grounds, insulating those surfaces from solar radiation.

Also, constructing city buildings and other infrastructures in a way that they spread over larger areas rather than being crammed together in small spaces is another part of this changed approach to urban development.

Vegetating the Cityscapes

Planting trees in city areas is another effective way of battling the UHI, as trees can significantly reduce surface and air temperatures by providing shade and cooling through evapotranspiration.

In addition, the planted trees can help battle global warming too by absorbing carbon dioxide from the atmosphere.

Quick Recap 

The term Urban Heat Island (UHI) Effect refers to the phenomenon of urban areas having warmer temperatures compared to surrounding rural areas. 

This is mainly due to urban design that encourages heat absorption from solar radiation and retaining it.

The UHI effect has significant impacts on local weather, human health, local water bodies, and the organisms that occupy them, as well as the economy.

To mitigate the UHI effect, using less heat-absorbing and more reflective materials for urban development, establishing green infrastructure, and planting trees in cityscapes can be used.