It is normal for local weather conditions to vary from regional weather patterns. However, an urban heat island, commonly encountered in densely populated areas, creates an entirely different environment.

The term “urban heat island” may not be that familiar, but anyone living in a city or densely populated area has already experienced this phenomenon. And unlike other atmospheric conditions, this localized weather occurrence is entirely the result of human activity.

(We already discussed the difference between local and regional weather in detail in a separate article. You can read more about these differences in this article.)

The term is not used to describe an area of a specific size in which weather conditions are reported, but rather an environment created by human activity that develops its own microclimate.

In this article, we look at what exactly an urban heat island is, how it forms, and its impact on local atmospheric conditions.

Before looking at how this phenomenon occurs and what its impact is, one first needs to define what an urban heat island is.

Urban Heat Island Definition​​​​​​​​​

An urban heat island can be defined as a densely populated area that is substantially warmer than the surrounding region. Higher temperatures are mainly a result of urban infrastructure and human activity. Temperatures are generally higher throughout the day but most noticeable during the evening.

Infrastructure (concrete buildings, glass, and asphalt roads) is the primary contributor due to its ability to maintain and trap heat within this environment. This is also why the difference in temperature is wider during the night than during the day.

Another contributor to urban heat islands is human activity, such as the heat produced by transportation, the heating and cooling of buildings, and, to a much smaller extent, human bodies.

Now that you have a clear idea of what an urban heat island is, we can drill down into the specifics to find out how this microclimate is created.

We already identified the main factors responsible for the formation of a UHI (Urban Heat Island). But to see how exactly these factors create this effect, they need to be examined in more detail. 

Infrastructure can be considered the primary cause of the UHI effect. The majority of a city is covered by concrete, asphalt, glass, and metal.

The attributes of concrete and asphalt allow them to absorb and maintain heat throughout the day and well into the night. This makes the temperatures in the city significantly higher than in the surrounding rural area.

Reflective materials like glass and metal allow heat to be reflected and scattered onto the already warm concrete, asphalt, and other heat-absorbent materials in the city.

As a city has a high concentration of people living and working in a relatively small area, a higher volume of transportation is required.

This means more buses, cars, and trains, which all make use of heat-generating engines. The sheer volume of transportation in a city makes it clear how much extra heat is generated and contributes to the UHI effect.

To a much lesser extent, our own bodies also release heat into the surrounding environment. The air we inhale is warmed by our bodies, which means the air we exhale is warmer and contains more carbon dioxide and moisture.

A similar process occurs with the internal combustion engines used by buses and motor vehicles. The hot exhaust gases released by these vehicles contain pollutants and also add heat to the surrounding environment.

The one thing rural regions have in abundance that metropolitan areas don’t is vegetation. And it’s vegetation that helps the environment to cool off throughout the day. Plants and trees absorb carbon dioxide and release fresh oxygen-rich air into the atmosphere.

But of more importance is the evapotranspiration (evaporation and transpiration) process. The process involves soil, vegetation, treetops, and plant leaves themselves, releasing moisture into the atmosphere, which has a significant cooling effect.

Traditional metropolitan and other densely populated urban areas typically contain much less vegetation than surrounding rural regions. In contrast, urban surfaces contribute to heating rather than cooling the surrounding air, as is the case when plenty of plants and trees are present.

The streets and alleyways spread throughout the city and, flanked by tall buildings, create an airflow similar to that found in a canyon, hence the name “urban or street canyon.”

This occurrence greatly contributes to the UHI effect. The tall buildings inhibit the free flow of air through the city, causing the heat generated by the city’s infrastructure to be trapped in the air above the streets and between the buildings.

There are many other factors also contributing to the creation of the Urban Heat Island Effect, but the ones just mentioned play the biggest part.

During the description at the beginning of the article, the temperature difference between a UHI and the surrounding rural areas was mentioned. By looking at the characteristics of an urban heat island, you will be able to better understand exactly how it functions.

The air in a metropolitan area is not only warmer than that of the surrounding rural areas, but the temperature difference also changes throughout the day. The annual mean air temperature of a large city can be around 1–3° Celsius (1.8–5.4° Fahrenheit) higher than that of its surroundings.

During the evenings, though, the temperature difference can increase to as much as 12° Celsius (22° Fahrenheit), particularly on clear, calm nights. The larger nighttime difference is mainly due to the different rates at which urban and rural surfaces cool down.

During the evenings, though, the average yearly difference in temperature can increase to  12° Celsius (22° Fahrenheit). The big difference in nighttime temperature is mainly due to the way in which the surface areas cool down.

Here, two familiar factors have the greatest impact:

A) Urban Infrastructure

The concrete, asphalt, and reflective materials that cause metropolitan areas to heat up more than rural areas also maintain the heat and radiate the warm air well into the night, causing the city to remain warm.

The tall buildings also restrict vital air movement, trapping heat between them. The normal upward movement of warm air from a heated surface, called convection, helps to remove heat from the surface. Although convection still occurs in cities, tall buildings can restrict ventilation and prevent the heat from dispersing effectively.

B) Lack Of Vegetation

The benefits of vegetation and its ability to cool down rural areas have already been explained in detail. The lack of vegetation in densely populated urban environments and metropolitan areas further diminishes these areas’ capability to cool down effectively.

(City planners are realizing this shortcoming, and modern cities are designed to incorporate “green zones,” which contain dense sections of green vegetation. Any area that can be further utilized to accommodate plants and trees is increasingly being used. 

Rooftop gardens are one such case where the roofs of buildings are converted into living gardens rich in leafy plants, grass, and trees. The cumulative effect of incorporating vegetation throughout the city can make a huge impact on effectively cooling it down.)

Unfortunately, the higher temperatures associated with an urban heat island can worsen air pollution and encourage the formation of photochemical smog. When stable atmospheric conditions prevent the air from mixing effectively, pollutants can also accumulate near the surface, negatively affecting both human health and the environment.

The brown haze you see hanging over a city may be photochemical smog. Photochemical smog forms when sunlight triggers reactions between nitrogen oxides and volatile organic compounds, which are commonly released by vehicles and other sources that burn fossil fuels.

Photochemical smog is not the only air-quality concern in urban environments. Carbon dioxide is a greenhouse gas that contributes to global warming and climate change, although it is not normally a direct health hazard at typical outdoor concentrations.

Carbon dioxide is produced by the burning of fossil fuels (primarily through transport emissions, but also through the burning of wood and coal). To find out more about carbon dioxide and its wide-ranging effects, you can read the full article here.

The higher temperatures associated with an urban heat island can also encourage the photochemical reactions that produce ground-level ozone. Although ozone in the upper atmosphere protects the Earth from the Sun’s harmful UV radiation, ground-level ozone can cause a range of respiratory problems when inhaled.

A range of other harmful pollutants may also accumulate in urban air, especially when calm or stable atmospheric conditions limit their dispersal. These include sulfur dioxide, particulate matter, lead, hydrocarbons, and carbon monoxide.

Indirectly, the higher surface temperatures within an urban heat island can also affect the water quality of rivers, ponds, and dams in or near the city. Water that ends up in these bodies of water may begin its journey some distance away.

As rain falls in a metropolitan area that is experiencing the UHI effect, the raindrops fall on warm rooftops, pavements, and streets. They run down warm drainpipes before being collected by drains and carried away by stormwater runoff.

The water can travel for miles before reaching a river, dam, or pond. It has been determined that the UHI effect can warm the rainwater from 21° Celsius (70° Fahrenheit) all the way up to 35° Celsius (95° Fahrenheit).

Aquatic life is very sensitive to changes in water temperature, and an influx of warm water can be devastating, endangering the survival of many fish and other aquatic species.

A further danger comes from standing water in warm temperatures. These conditions can allow harmful bacteria and other disease-causing organisms to multiply and spread, especially when the water is contaminated by animal carcasses or excrement.

Especially in rural communities that rely on water from rivers and reservoirs close to the city, people are particularly vulnerable to waterborne diseases that can spread under these conditions.

You can learn more about waterborne diseases, which are a major cause of fatalities in India during the monsoon season and are often linked to contaminated standing water, in this article. Look towards the end of the article under the “Monsoon Section.”

The first and most important characteristic of an Urban Heat Island is the elevated temperature. It is the much warmer conditions that have the most profound effect on human health and well-being.

This is particularly obvious during the summer months, when the UHI effect causes the highest temperatures to be recorded. During this period, people with developing or weakened immune systems are most prone to heat-related illnesses.

Children and the elderly fall into this group. Children (and adults) can suffer from heatstroke, headaches and dizziness, cramps, and respiratory problems. Although not all of these conditions are fatal, severe heat-related illness can cause long-term organ damage.

People, especially the elderly, with existing health conditions like asthma, diabetes, and cardiovascular disease, are especially hard hit by the increased temperature.

Every year, fatalities increase during periods of extreme heat, especially among the elderly. This is a worldwide phenomenon, particularly in densely populated urban areas where the UHI effect can intensify already high temperatures.

There should be no doubt left in your mind that the urban heat island effect is very real and can have a significant impact on both the environment and human health.

Although not on such a large scale, the UHI still has a significant impact on its immediate surroundings. It should not, however, be confused with global warming and climate change, which are primarily caused by the buildup of greenhouse gases in the atmosphere.

The difference between the microclimate of Urban Heat Islands and nearby rural areas can be so significant that many people are turning to home weather stations to measure the atmospheric conditions in their surroundings.

You can read more about why you may need the convenience and the added assurance of a home weather station in this article.

I trust you now have a clear and thorough understanding of what an urban heat island is, how it is created, and the effects it has on the weather and its surroundings.

Until next time, keep your eye on the weather!