An Introduction To Heat Islands - An Unnatural Phenomenon?

When it comes down to discussing the potential impacts of anthropogenic modifications to the natural landscape, it’s a common opinion to be opposed to said changes in hopes of protecting the native wildlife but not very many people consider the implications these modifications could have on the surrounding atmosphere itself.

The Urban Heat Island refers to the increase in the temperatures experienced within an urban region in comparison to its rural counterpart. The key mistake people tend to make here is falling into the misconception that simply developing in a region will force a UHI to develop but this is more of an issue with understanding the nature of heat islands in general. Development itself does not cause the UHI, but rather our approach to developments and urbanisation do. To properly understand what an Urban Heat Island is, it would help to look into a few key fundamentals of meteorology first.

Schematic diagram of the formation of the Heat Island. The UHI experiences a plume-like formation which extends from the epicentre of the most urbanised region. Temperatures experienced in each respective region is subject to the degree of development that has taken place in sad region.

All matter absorb and radiate heat at different rates depending upon their physical characteristics. Specific heat capacity, albedo (reflectivity) and exposure to other heat sources can affect the absorption and release of heat into the system; all of which are strongly modified towards the warmer end of the spectrum within urbanised regions. Within a temperate grassland for example, the energy absorbed at the surface can easily be radiated outwards during the evening allowing for the atmosphere to cool naturally. This follows the system of the Terrestrial Heat Budget very well but this is where most people tend to flub with their interpretations of the urban heat island.

The Terrestrial heat budget highlighting the balance of incoming and outgoing energy from the terrestrial system.

Non-Urban Heat Islands

To understand how Heat Islands work in our anthropogenic biome (Cities), we would benefit from exploring the effects on other "Natural" biomes.

Grasslands are very efficient at losing heat as one of the key features for heat island formation; Physical Heat trapping, is absent. As a result, they operate as an ideal example of the geographical climate in that region. Heat radiated is easily lost into the upper atmospheres (with slight absorption and re-radiation downwards by clouds and within the stratosphere and so on…). They also lack external heat sources due to being very flat. This in turn means they aren’t likely to be heated externally during the evening whilst heat is continually being lost through thermal radiation.

In contrast to a grassland, or desert (Which is somewhat similar to a grassland for the same reasons), biomes with heavy foliage such as jungles and forests do in fact experience heat islands of their own but are not studied in detail due to not posing much risk to humans or the environment. Absorbed heat can often have difficulty escaping due to being re-absorbed and reradiated by the surrounding foliage. As a result, a small plume like formation develops within the entangled regions of these areas where the temperature is slightly warmer than it naturally would if it was an open space.

Some may argue that these regions aren’t as effectively warmed as open regions due to all of the foliage but that’s not entirely true. The degree at which heat is absorbed is more closely linked to the specific heat capacity and albedo of the surface exposed and most jungle and forest trees experience similar if not more absorption due to these factors in particular (As seen below). 

List of albedo experienced by different exposed surfaces. Higher albedo signifies higher reflectivity and lower heat absorption. Source (Oke, 1992, Ahrens, 2006)

 The other factor is that warmed air cannot be removed as effectively due to wind obstructions but this will be discussed in more detail in some later posts to help focus on the topic in a bit more detail.

Urbanisation simply produces an exaggerated form of this phenomenon meaning that the Urban Heat Island (UHI), despite occupying a very unnatural region is a natural side-effect. 

I plan to delve much deeper into how the UHI forms and its varying intensities and so on in the following posts but I felt it would be appropriate to begin by putting this phenomenon into its natural context. I believe it would be easier to understand the UHI by viewing it as a natural atmospheric side effect rather than an unusual hazard plaguing urban societies.

Despite the fact that I’ve defended the UHI as being a natural response to an unnatural situation, this does not mean that it carries no repercussions and these will be discussed in detail further down the line. I hope to spread my current knowledge of the UHI through this series of posts and hope to also expand my own understanding of the UHI and the atmosphere in general as we progress further into this series.