Urban Heat Island (UHI) is a term used to describe urban environments in which background temperatures have significantly risen, due to the presence of heat-absorbing surfaces, unsustainable human activities, and few carbon sinks. This article discusses urban heat island definition, formation, impacts, examples and advantages, as outlined below;
Urban Heat Island Definition: 5 Ways to Define Urban Heat Island
Urban heat island is any urban or semi-urban area that experiences generally higher temperatures, higher heat wave-risk, and overall higher environmental impact of climate change, due to a combination of natural ecosystem conversion and human activities.
The above is a very basic format by which to describe the concept of urban heat island. It mentions the most essential factors which are involved in urban heat island occurrence; both as causes, effects, and characteristics.
Further clarity may however be provided by describing the mechanism behind urban heat islands, as is done briefly in the urban heat island definition below;
Urban heat island is an urban region affected by rapid heat absorption and retention, by concrete and other surfaces, as well as the convective circulation and continuous radiation of such heat (solar energy) from these surfaces .
While some of them have already been hinted in the two definitions above, a more detailed outline of the causes of urban heat island will be helpful toward the understanding of the concept.
Below is an alternative urban heat island definition that lists some of these causes;
Urban heat island is any area that is subjected to high thermal intensity that results from processes and factors like deforestation, urban development, overpopulation, electricity generation, energy wastage, and already-existent climatic conditions; and which occurs in the form of unnatural heat levels in urban areas .
A holistic yet simple description of the causes of urban heat island is; ‘human activities’.
While it can be described as an effect of these causative factors, there are some identifiable effects of urban heat island itself, by which it may be characterized.
In the urban heat island definition below, some of these effects are listed;
Urban heat island is an urban or semi-urban environment affected by the absorption, retention and radiation of large amounts of heat energy from manmade surfaces, with multiple effects that include; water and air quality decrease, higher energy demand for cooling purposes, alterations in seasonal patterns, aquatic ecosystem degradation, and human health problems like heat stroke .
While environmental degradation is the most predictable effect of urban heat island, it has been shown above that these effects extend beyond water and air pollution, to include socioeconomic impacts like higher energy consumption rates.
The above definition makes reference to ‘manmade surfaces’, indicating that a major reason behind urban heat island occurrence is the replacement of natural ground surfaces with concrete (as well as the replacement of plant biomass like forest trees with buildings and other manmade structures).
When these natural components are replaced with manmade ones, processes like carbon sequestration and photosynthesis that help mitigate greenhouse emission and global warming, become much-less prominent.
When viewed within the context of its effects, urban heat island may be rephrased as urban heat island effect.
Lastly, the possible solutions to urban heat island constitute another important factor that should be considered when analyzing the concept in any given scenario. Some of these possible solutions are mentioned in the alternative urban heat island definition below;
Urban heat island is an area experiencing significantly-high temperatures compared to adjacent zones, as a result of human encroachment into natural ecosystems, and which can be mitigated using any of various potential solutions that include; afforestation and reforestation, energy conservation, renewable energy development, energy efficiency-enhancement, and sustainable building applications like green roof and reflective surface installation.
How Heat Islands Form: Mechanism of Urban Heat Island
Heat islands form through a repetitive process of heat capture, absorption, radiation or re-emission, and convective-circulation.
In order for heat capture to occur, certain absorbent materials must be present.
The entire concept of urban heat island is arguably based on the presence of these materials; which are available because of the replacement and conversion of natural soil-composed ground surfaces, vegetation, and even some water bodies, to manmade alternatives.
Materials in urban areas that trap solar energy and contribute to the heat island effect include; concrete, asphalt, and dark-colored surfaces in general. Usually, the introduction of heat-absorbent materials into the urban ecosystem occurs simultaneously with the establishment of infrastructures like buildings, power plants, roads, and water dams.
These materials act as blackbodies in their absorption of heat energy.
They also subsequently re-emit or radiate the absorbed heat in a manner that is slightly similar to charging and discharging cycles in energy storage systems.
However, the discharge of stored heat in this case is not absolute.
Rather, the heat that is emitted is circulated within the environment, through cycles of convective air current-movement that lead to a condition whereby heat is trapped within a given area for extended time periods.
Impacts of Urban Heat Island
Impacts of urban heat island include;
1). Higher risk and rate of air pollution
2). Increased regional temperature
3). Health problems including respiratory ailments and heat stroke
4). Elevated greenhouse gas atmospheric concentration
5). Poor air quality and circulation
6). Aquatic ecosystem degradation
7). Alteration of seasonal patterns
8). Facilitation of both manmade environmental hazards like heat wave
9). Increased energy demand and consumption
Examples of Urban Heat Island
Examples of urban heat island are;
1). Extreme regional heating due to natural ecosystem encroachment in Bragança, Portugal 
2). Urban heat island conditions in Tehran, Iran 
3). High environmental temperatures accompanied by environmental pollution in Berlin, Germany 
Advantages of Urban Heat Island
The only advantage of urban heat island is the occurrence of comparatively-warm temperatures in affected regions during periods of winter or extreme cold.
Urban heat island is any area in which urbanization and industrialization have both led to increased regional temperature compared to adjacent areas.
Heat islands form by the passive trapping and gradual release of solar energy as heat, from manmade surfaces.
Impacts of urban heat island include; higher air pollution risk, increased regional temperature, human heath problems, elevated greenhouse gas concentration, poor air quality, aquatic ecosystem degradation, seasonal pattern-alteration, hazard facilitation, and increased energy demand.
Examples of urban heat island are; extreme regional heating in Bragança, Portugal; heat island conditions in Tehran, Iran; and high environmental temperatures in Berlin, Germany.
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