What is an urban heat island and why does this occur?
Temperatures tend to be warmer in urban than surrounding rural areas, particularly when it is calm, clear and at night. These warmer temperatures in cities are referred to as the urban heat island. On average in cities temperatures are one to three degrees centigrade warmer, but on occasions may be as much as 10 C warmer.
Temperatures tend to be warmer in urban than surrounding rural areas, particularly when it is calm, clear and at night. These warmer temperatures in cities are referred to as the urban heat island. On average in cities temperatures are one to three degrees centigrade warmer, but on occasions may be as much as 10 C warmer.
There are many reasons why urban heat islands occur - the five main reasons are related to:
- The building materials used to construct cities - bricks, concrete, asphalt etc. – have high heat capacities and ability to conduct heat. These materials absorb and store large amounts of heat in the day which is slowly released at night
- The vertical walls and roofs of buildings increase the area exposed at the surface, further enhancing the absorption and storage of heat. They also reduce the ability for surface cooling - energy emitted from the surface is trapped in the urban canyons
- The removal of vegetation and high impervious surface covers (parking lots, buildings, roads that don’t allow water to infiltrate) reduces water availability, limiting evapotranspiration. This means that available energy heats the building fabric and air; it is not used up evaporating water
- Human activities release heat - from traffic, from factories, from homes (heating and air conditioning). This is an additional source of heat not available in rural areas
- Air pollution. Human activities lead to ejection of pollutants and dust into the atmosphere; this may lead to retention of heat in the lower atmosphere
Luke Howard was the first person to identify the Urban Heat Island phenomenon
Luke Howard made daily recordings of the pressure, temperature, humidity, precipitation and evaporation at locations (including Tottenham) outside London from 1806 onwards. In addition he maintained a diary of his observations and collated newspaper articles on any event of meteorological interest.
He published his observations in a volume entitled the Climate of London and such is the importance of his observations that the book remains an invaluable resource for urban climate scientists today.
The foreword to IAUC edition of Luke Howard's Climate of London notes:
In summary, he identifies four causes for the observed differences in air temperature:
1. Anthropogenic sources of heat resulting in atmospheric warming, particularly in winter.
2. The geometry of urban surfaces which ‘traps’ radiation and obstructs ‘free radiation to the sky’.
3. The effect of urban ‘roughness’ in impeding the passage of ‘the light winds of summer’.
4. The availability of moisture for evaporation in the country.
While the first cause is invoked to explain the excess warmth of London in the Winter, the latter three are used to explain the fact that ‘London does not wholly lose its superiority of temperature, by the extinction of the fires in Spring’.
He published his observations in a volume entitled the Climate of London and such is the importance of his observations that the book remains an invaluable resource for urban climate scientists today.
The foreword to IAUC edition of Luke Howard's Climate of London notes:
In summary, he identifies four causes for the observed differences in air temperature:
1. Anthropogenic sources of heat resulting in atmospheric warming, particularly in winter.
2. The geometry of urban surfaces which ‘traps’ radiation and obstructs ‘free radiation to the sky’.
3. The effect of urban ‘roughness’ in impeding the passage of ‘the light winds of summer’.
4. The availability of moisture for evaporation in the country.
While the first cause is invoked to explain the excess warmth of London in the Winter, the latter three are used to explain the fact that ‘London does not wholly lose its superiority of temperature, by the extinction of the fires in Spring’.
Finally the foreword observes:
It is a pity that Howard had no means of recording wind velocity except by direct observation. With detailed wind information he would certainly have examined the correspondence between ∆Tu-r and wind-speed, to which he alludes. In addition, he had no comparative data to examine rates of evaporation or differences in humidity. His examination of the urban effect was therefore largely limited to temperature (he had little trust in the available urban rainfall data). Howard did not attempt to formalize his analysis by examining the relative magnitudes of the causes he hypothesized (such as the anthropogenic contribution). Moreover, he did not consider the impact of urban construction materials on the thermal properties of the city’s surfaces. Despite this, Howard identified virtually all of the factors that are responsible for the UHI – that he did so in 1820, at the very beginning of the scientific study of weather and climate is remarkable. By any measure, ‘Luke Howard’s account is monumental’.
It is a pity that Howard had no means of recording wind velocity except by direct observation. With detailed wind information he would certainly have examined the correspondence between ∆Tu-r and wind-speed, to which he alludes. In addition, he had no comparative data to examine rates of evaporation or differences in humidity. His examination of the urban effect was therefore largely limited to temperature (he had little trust in the available urban rainfall data). Howard did not attempt to formalize his analysis by examining the relative magnitudes of the causes he hypothesized (such as the anthropogenic contribution). Moreover, he did not consider the impact of urban construction materials on the thermal properties of the city’s surfaces. Despite this, Howard identified virtually all of the factors that are responsible for the UHI – that he did so in 1820, at the very beginning of the scientific study of weather and climate is remarkable. By any measure, ‘Luke Howard’s account is monumental’.
Download the International Association for Urban Climate (IAUC) edition of
Luke Howard's Climate of London based on the 1833 edition here
Luke Howard's Climate of London based on the 1833 edition here
For further reading on Urban Heat Islands click here