Have you ever wondered why it’s so much warmer when you visit your family in the city vs when you visit family out in the countryside? Cities have been getting warmer and warmer as the years have gone by. This year, cities such as London have been consistently hitting temperatures of above 300C and there’s a couple of reasons why. The obvious one is climate change: temperatures are rising, and the earth is getting warmer in general. The less obvious reason is due to a phenomenon known as the urban heat island effect.

What is the Urban Heat Island effect?

The anthropogenic impact that humans have had on the environment, specifically the atmospheric environment, is widespread and well documented with one of the major climatological impacts being the creation/generation of Urban Heat Islands.

The urban heat island effect is a type of heat accumulation phenomenon that refers to the relatively warm air temperature near the ground and results in towns and cities being much warmer than their surrounding rural and countryside areas. It is a phenomenon that occurs in cities and is generated through many factors associated with the physical make-up of the city such as increased anthropogenic heat, increased number of dark and impervious surfaces, and urbanisation.

The urban heat island effect is an almost ubiquitous phenomenon as it is observed in cities within every climatic region and it occurs as a result of human modifications to the surface and atmospheric properties due to human development.

The urban energy balance of cities (balance of incoming and outgoing energy flux from an urban system) influences the climates of cities and the ways they interact with the surrounding areas. The energy that is generated through anthropogenic heat, and is absorbed by the urban surface system, is physically balanced by the evaporation of moisture in the atmosphere, the storage of heat and the warming of the air above the surface, which influences the ways in which cities utilise energy.

The increased heating results in a decrease in the need for heating in the winter but increases the need for cooling in the summer (the urban heat island effect sees its greatest intensity in the summer). This leads to increased release of anthropogenic heat and an intensification of the urban heat island effect.  These are major determining factors on the net positive thermal balance within cities.

How does the Urban Heat Island effect form?

There are many factors that cause the formation of the urban heat island effect. The magnitude and intensity of these factors may also be intensified due to global warming. These factors include: 

Increased anthropogenic heat from buildings (e.g. air conditioner units) and cars (increased traffic releasing heat):

In addition to the absorption and trapping of incoming solar radiation, the release of anthropogenic heat is a significant contributor to the urban heat island effect. Cities require a lot of energy in order to function properly. This energy is dissipated as heat and is intensified by solar radiation. This heat accumulates and becomes trapped by buildings and other urban structures and is released slowly at night.

Increased roughness in the city

The buildings and structures in cities act as obstacles for wind and stores of heat. As a result, they are characterised as influencers of the roughness parameter. The roughness parameter refers to the roughness of a surface and is characterised as the area up until the height of urban roughness.

Cities are characterised as being very “rough” surfaces. This is due to the tall buildings and structures within them that act as obstacles for wind, affecting the air flow and reducing wind velocity. Suburbs are also seen as “rough” but not as “rough” as urban areas. This is due to their densely packed, low rise buildings. Smooth surfaces are seen as having the lowest “roughness” and this is one of the characteristics that accounts for the differences in temperature and wind speed values between urban and rural landscapes.

Increased urbanity/urbanisation

The large number of dark and impervious surfaces within cities such as buildings and pavements absorb a great amount of solar radiation which is then stored and released during the night. This causes the greatest temperature differences between the city and the countryside during this time.

Decreased evaporation and evapotranspiration

This is due to a reduction in the number of trees and green spaces present within cities. Urban areas are characterised by large buildings and constructions that reduce the local vegetation coverage. As a result, they are impacted by a large amount of solar radiation absorption (long wave radiation). 

Impervious surfaces within urban areas cause increased runoff and the shedding of water more easily instead of it being absorbed. This is the opposite effect of soils and vegetation. Water is evaporated from soils and evapotranspiration occurs in leaves. The removal of trees and vegetation to be replaced with less permeable/impervious surfaces results in the minimisation and loss of the natural cooling effects produced by evaporation and shading that is provided by trees and vegetation

Rural environments and green areas on the other hand, experience lower air temperatures due to increased evapotranspiration fluxes. As a result, these areas experience a reduced/lower urban heat island effect.

Reduced albedo

Albedo refers to the reflectivity of a surface. Surfaces with a high albedo reflect more solar radiation from the sun back into the atmosphere, while surfaces with a lower albedo absorb the incoming solar radiation. Darker surfaces have lower albedos than white (or lighter) surfaces.

Due to the large number of dark surfaces within cities, such as buildings, there is a reduction in albedo of the city. This reduction in the albedo of cities results in more absorption of heat rather than reflection of heat occurring. This leads to higher temperatures within cities.

Impacts of the Urban Heat Island effect

The urban heat island effect impacts people and cities in a number of ways. High temperatures in urban areas affects the health and wellbeing of urban dwellers, especially vulnerable people such as the old and the poor, and can also affect leisure activities within the urban environment.

It also impacts the local meteorology of cities, increasing humidity, changing wind patterns, forming clouds and fog, and altering precipitation rates. It may also enhance urban air pollution which would further exacerbate the pressure that the urban heat island places on the health of the inhabitants of the city.

Final thoughts

Urban development is a major cause of the large differences between urban and rural temperatures. This development, in addition to potentially more frequent extreme climatic events occurring such as heat waves, will have a major impact on cities moving forward.

The Urban Heat Island effect is a phenomenon that occurs all over the world and is having a significant impact on the temperatures in cities. Cities are becoming warmer due to climate change and this heating effect is being exacerbated by the urban heat island effect. There are, however, ways to mitigate the effects of this phenomenon and many cities have come up with interesting and innovative ways to combat the heating of their cities.

By having a better understanding of what causes this phenomenon to occur and the underlying physical principles that affect it, better decisions can be made to combat its effects by both the city and its inhabitants.