UHI's (Urban Heat Islands) Temperature

Specification point: Climate on a local scale: urban climates

This spec point is split into 4 sections, temperature, precipitation, air quality and winds. In this post I will talk about temperature.

An urban heat island is generally described as a climatic dome which is warmer in comparison to surrounding rural areas. This is a result of several reasons relating to the human environment of urban areas.

• Building materials such as brick, tarmac and concrete act to absorb large quantities of heat during the day due to their high thermal conductivity and heat capacity. This heat is slowly radiated out at night whereby the contracts in rural-urban diurnal temperature ranges will be less within urban centres as radiated heat will warm the cities during the night. This causes a change in the energy balance where urban areas will exhibit high temperatures than rural areas.
• The geometric features of tall buildings can provide surfaces in which radiating heat is trapped within the urban canopy layer creating what is known as the 'urban canyon effect' where temperatures are elevated above their mean latitude average.
• Heat from both industry and buildings which burn fuel too generate heat with contribute to increased temperatures within urban areas. Similarly, as people generate heat and there is a large density of populations within cities, this too attributes toward increasing temperatures.
• Air pollution from industry and cars increases cloud cover creating a 'pollution dome' allowing the entrance of short wave radiation but preventing its reflected back into the atmosphere. In the events of anticyclonic weather conditions in which the localised climate remains static, the creation of photochemical smog resulting from pollutants can trigger severe health problems such as asthma and bronchitis.
• The impermeable surfaces of urban areas mean water is disposed via complex drainage systems. Whilst useful, this change means the natural moisture budget of urban areas is disrupted as reduced evapotranspiration from vegetation  means more energy is available to heat the atmosphere of urban areas rather than evaporating surface water.

What is also important to note is that heat islands vary seasonally and diurnally. As mentioned, the effect of a UHI is greatest under high-pressure anticyclonic weather conditions especially where temperature inversions maintain a stable, static climate.

UHI's also vary over space. The UHI effect has been likened to a cliff in which upon entrance to an urban areas, temperatures generally increase 2-4 degrees for every km toward the city centre. However, there are variations within this pattern that reflect the distribution of green space and industry (Green space will see lower temperatures, industry higher). This could be a result of differing albedo effects of the physical environment compared with the human environment as man-made structure, consisting of dark materials, tends to absorb insolation and re-radiate this energy increasing the heat output within the urban area. In contrast, green areas can be highly reflective and reflect radiation back into the atmosphere.