Urban microclimates: causes

By Matt Burdett, 4 November 2019

On this page, we look at urban microclimates and the urban heat island effect, and how they arise.

  • Causeway Bay, Hong Kong. The gaps between tall buildings create ‘canyons’ which funnel air and can trap heat. Souce: By the author.

Defining microclimates

A microclimate is the climate of any small area that is different to its surrounding area. An urban heat island is one type of microclimate. Other types include:

  • Snow-capped mountains: mountains are colder than the surrounding lowlands
  • Woodlands: trees give shelter which reduces wind speed, and create shade making temperatures at ground level cooler.
  • Bodies of water: lakes (and any large bodies of water) make summers cooler, and warmer in winter due to water heating up and cooling slowly.
  • Vegetation: Seasonal vegetation changes during the year effect rural microclimates (trees losing leaves etc.)
  • Agriculture: Changes of farming at different types of year (e.g. ploughing) can change the vegetation coverage and also the albedo of the land

Urban heat islands

An urban heat island is a type of microclimate that is created when an urban area becomes warmer than the surrounding area. It is common in larger urban areas such as Tokyo, New York City and Paris. This can be seen easily on a map such as this one of London:

  • Isothermic map of London, showing the temperature rising towards the centre of the city. Source: Bhatt, 2016

As the graph below shows, the temperature to the centre of the urban area is higher than the surrounding area.

  • The temperature variation over an urban area. Source: EPA, 2017.

Key terms about urban heat islands

There are several important key terms in this area of study:

  • Urban Heat Island – the increased temperature over urban areas compared to surrounding rural areas. It is called an island because in map view, it appears as though the urban area is an ‘island’ of heat in a colder ‘sea’ of the rural area surrounding it.
  • Urban Heat Island Intensity – the amount of the temperature difference. A large intensity means the temperature is a lot warmer in the urban area compared to the surrounding rural area.
  • Structures – Any artificial construction including buildings, scaffolding, landscaped vegetation, roads, paths and covered farmland (e.g. a greenhouse or plasticulture)
  • Human activities – any undertaking being done by humans, including the effects of non-climate related activities such as transport and agriculture.
  • Microclimate – the average weather conditions of a small area.
  • Aspect – the direction in which something faces. In the northern hemisphere, south facing buildings receive more sunlight and therefore warmth; they have a ‘southerly aspect’.
  • Urban canyons – the tall buildings in urban areas create streets that act like canyons or deep valleys. Canyons create shade and thus cooler temperatures, and can funnel winds increasing their speeds. A good example is Queen’s Road Central in Hong Kong, or 7th Avenue around Times Square in New York.
  • Albedo – the reflectivity of a surface. Dark surfaces have a lower albedo and absorb more heat.

Features of microclimates

The features of a microclimate can include variations in:

  • Temperature
  • Wind speed
  • Wind direction
  • Humidity
  • …and any other weather-related measurement such as the cloud coverage of the visible sky. Note: total cloud cover is not a microclimate feature because it measures the whole sky, not the sky in a small location.

A single weather event is not necessarily linked to the microclimate. Climate refers to the average conditions over a long period of time (usually 30 years); sudden changes in the weather in an urban area are therefore not part of the urban microclimate unless they generally happen more often or more severely because of the urban features.

Causes of urban microclimates

Urban microclimates are caused by several factors.

Albedo

Albedo refers to the amount of energy that is reflected from a surface. Urban areas often have darker surfaces than their surrounding rural areas. The darker surfaces have a low albedo (they don’t reflect much energy) which leads to energy being absorbed and warming up the urban environment. Remember: high albedo means high reflectivity, so less energy absorbed at the surface. A low albedo means low reflectivity, so more energy absorbed at the surface.

The graph below shows the albedo of various surfaces. Urban activities can alter natural and rural albedo, such as clearing snow or removing bodies of water.

  • Percentage of diffusely reflected sunlight in relation to various surface conditions. Source: Grobe, 2000.

Building material

Construction materials can affect albedo, not just through their colour but also their specific heat capacity and thermal conductivity. Specific heat capacity is a scientific term that relates to the amount of heat energy (usually measured in joules) that is required to raise the temperature of a material by 1 Kelvin.

Materials such as concrete have a low specific heat capacity: they warm up very quickly with a small amount of heat, which is then reradiated almost immediately. Concrete surfaces cause temperatures to rapidly rise when they are heated by the sun, then rapidly cool when the sun goes down. Further details on the specific heat capacity of various materials can be found at the Designing Buildings Wiki.

Building density and height

Buildings are an important blockage of wind. The obstacle created by dense buildings causes the wind to be pushed over and around the city, which can lead to lower urban wind speeds. As the wind is diverted around corners, it can also change direction.

Wind is affected from ground level, and the point in the atmosphere at which wind is no longer affected by buildings is known as the Urban Boundary Layer. The diagrams below show how wind patterns will change due to the interference of buildings.

  • Wind patterns (speed and direction) are affected by buildings in urban areas. Source: Bhatt, 2016

However, tall buildings can also funnel wind through the gaps between them, causing gusts and wind speeds to increase. This creates part of the ‘urban canyon’ effect that people often experience when turning a street corner in a city centre, when the wind is suddenly much stronger.

Another part of the urban canyon effect is that the buildings create shade. The heat from the sun never gets to ground level, which prevents temperatures from getting higher at the street level.

Anthropogenic heat

Perhaps the most obvious and certainly one of the most important causes of urban heat islands is anthropogenic heat. Anthropogenic heat refers to any heat produced by humans and their activities.

The heat created by the bodies of humans is a very small impact on the urban microclimate, and is really only noticed at crowded events like football matches – and even then it is very small.

Most anthropogenic heat comes from two sources:

  • Mechanical heat, produced by vehicles, air conditioning units and so on
  • Ambient heat, produced by the heating systems of buildings in cooler climates

Regardless of whether people are trying to heat up or cool down their inside environment, the result is: it gets warmer on the outside!

Air conditioning

Although air conditioning makes individual indoor spaces cooler, the overall impact is to increase the urban temperature. All air conditioning works on the basis of using a mechanism to push air around gas coolants. A byproduct of this mechanism is to produce heat, especially from the fan that pushes the air through the system. Air conditioning units commonly have an indoor air circulator and an outdoor one that prevents the heat generated remaining inside.)

  • This graph shows a model of the temperature variation when air conditioning and non-air conditioning is taken into account. Source: NASA, 2010.

Drainage systems

Water acts like a giant heat ‘sponge’, absorbing energy during warm periods and gradually releasing it during cool periods. Cities with effective drainage systems have less surface water as the rainfall (or floodwater) is removed quickly, so the urban area will experience faster warming and cooling as a result. Cities without effective drainage will see a more gradual rise and fall in temperature.

In some cities, large lakes are artificially created for leisure purposes. The Serpentine in Hyde Park in London was created in 1730; the Jacqueline Kennedy Onassis Reservoir in Central Park in New York was built in the 1850s and ‘60s. These bodies of water can absorb heat from the local environment during the day, and reradiate it at night. They can also add water vapour through evaporation, which increases the humidity in the immediate vicinity of the lake.


Sources

Bhatt, K., 2016. The Impact Of Buildings In Their Surrounding Microclimates: The Case Of Mandeville And Horsenden Schools. Architectural Association School of Architecture https://www.researchgate.net/publication/324783077_THE_IMPACT_OF_BUILDINGS_IN_THEIR_SURROUNDING_MICROCLIMATES_THE_CASE_OF_MANDEVILLE_AND_HORSENDEN_SCHOOLS Accessed 4 November 2019.

EPA [United States Environmental Protection Agency], 2017. Learn About Heat Islands. https://www.epa.gov/heat-islands/learn-about-heat-islands Accessed 29 June 2019.

Grobe, 2000. Percentage of reflected sun light in relation to the various surface conditions of the earth. https://commons.wikimedia.org/wiki/File:Albedo-e_hg.png Accessed 31 January 2018

NASA, 2010. Satellites Pinpoint Drivers of Urban Heat Islands in the Northeast. https://www.nasa.gov/topics/earth/features/heat-island-sprawl.html Accessed 29 June 2019.


Urban microclimates: causes: Learning activities

Questions

  1. Define the term ‘microclimate’. [1]
  2. Identify three examples of natural microclimates. [3]
  3. Define the term ‘urban heat island’. [1]
  4. Define the term ‘urban heat island intensity (UHII)’. [1]
  5. Suggest four features of a typical microclimate. [2]
  6. Distinguish between the meaning of ‘weather’ and ‘climate’. [2]
  7. Explain the impact of the colour of a surface on the amount of energy it absorbs. [3]
  8. Describe the effect of low specific heat capacity on the ability of a surface to absorb heat energy. [2]
  9. Explain how building height and building density affect wind patterns and wind speeds. [6]
  10. Define the term ‘anthropogenic heat’. [1]
  11. Identify the two main sources of anthropogenic heat. [2]
  12. Explain how reducing the indoor temperature using air conditioning can contribute to the urban heat island effect. [3]
  13. Do human changes to the amount of surface water usually cause the urban heat island intensity to go up or down? Explain your answer. [4]

Other tasks

A microclimate study is a popular focus for a school project. Look at a map of your school or local area. Create a series of hypotheses about the impact of various features of the chosen urban area on specific features of the microclimate, such as temperature, wind speed and wind direction.

Going further

How can you study the features of a microclimate in your local area? Use YouTube and other online sources to establish methods for primary data collection. Take measurements, ensuring you use an appropriate sampling method.


© Matthew Burdett, 2019. All rights reserved.

All secondary material on this site is clearly referenced and may be subject to copyright restrictions by the original authors. All original material on this page is subject to copyright.