Impacts of Atmospheric and Surface UHI

It should come as no surprise that anthropogenic modifications could have the potential for some negative side-effects. Up to this point, this series has focused on creating familiarity with what the Urban Heat Island actually is and how it should be viewed as a phenomenon. Nevertheless, the UHI does have its own set of repercussions and those need to be considered as scientific interest alone is not a considerable driving force for researching into the phenomenon. To fully comprehend the implications of the UHI, a new UHI formation would need to be introduced first.

Surface Heat Islands (SHI):

Up to this point, the UHI has been discussed from the perspective of the atmosphere with respect to formation and implications. When working with the UHI, unless explicitly stated otherwise, it will often be with regards to Atmospheric Heat Islands. In contrast, a form of UHI known as a Surface Heat Island (SHI) could also form in regions with high exposure to solar irradiance and high thermal absorption properties. 

The SHI refers to the increase in the temperature of the land surface as a result of direct exposure to to solar influence. By replacing the natural environmental surfaces with anthropogenic surfaces, the thermodynamic properties of the surface can shift. Greenery are much more proficient at dissipating the influence of the added solar input through the use of transpiration and moisture retention. Anthropogenic surfaces such as Asphalt, steel and glass have a high tendency to absorb heat and as a result would become much warmer than their green counterparts. Additionally, lower albedo surfaces also add to this effect as anthropogenic surfaces have a tendency to be dark (Roads). 

Impacts of UHI:

Urban heat islands, both atmospheric and surface have been characterised for being hazardous in more extreme circumstances. Atmospheric Heat Islands could represent an increase in temperatures of up to 12°C. On warm summer afternoons, a rise in temperature of such a degree, particularly near the equator could have dangerous implications towards one's health. The potential for heat stroke and dehydration alone should highlight the importance of mitigating the phenomenon. 

The increase in atmospheric temperature also increases the use of cooling mechanisms to cool the environment, which in turn releases more waste heat as a result, further fueling the UHI. Additionally, the increase in electricity consumption would induce a larger requirement for electricity generation, which in most countries is primarily achieved through fossil fuels. This results in an increase in waste pollution into the atmosphere, which could result in the development of photochemical smog, acid rain and further fuel climate change. Additionally heat sensitive equipment could also suffer as a result of the increased temperatures.

The surface heat island could also pose a threat as the temperatures experienced could reach up to (27–50°C) warmer than the atmospheric temperatures. This does not apply to shaded areas as direct sunlight is required for the SHI to form. Under shade, the temperatures experienced will reflect that of the atmospheric temperatures. Many urban surfaces have a high retention for heat, which could also lead to burn incidents for outdoor workers. 

Storm water runoff has also been found to be heated by potent surface heat islands on pavements and rooftops. High SHI could raise the stormwater temperature from 21°C to over 35°C, which when washed into freshwater communities within lakes, rivers, ponds or other sources populated by freshwater ecosystems, could result in serious repercussions to temperature sensitive communities. Water temperature has an effect on metabolic functions of some aquatic organisms and rapid changes in temperature as a result of heated water inflow could be very stressful to the communities, leading to sickness and even death.

The UHI is not naturally known to form extreme conditions regularly, but nevertheless, the potential for such exists and the repercussions do need to be taken into consideration. I decided to dedicate this post to help promote understanding for why the UHI, despite being often ignored, deserves recognition and attention. The effects are no doubt of lower significance in comparison to climate change or atmospheric pollution but the UHI still deserves to receive the attention it needs to be properly understood and dealt with. 

UHI Basics

Following up from last week's post, the fundamentals of heat islands in general should be somewhat familiar before progressing into that of urban heat islands. As a recap, the UHI refers primarily to the increase in the atmospheric temperature within an urbanised region as a result of anthropogenic modifications to the surrounding landscape. The temperatures experienced could vary depending on the degree of urbanisation in conjunction with some meteorological conditions such as cloud cover and wind activity.

The phenomenon is not particularly difficult to come across and has been studied across multiple research expeditions. The following lists a few notable examples of highly regarded research papers into UHI dynamics.

• A review on the generation, determination and mitigation of the urban heat island.
• Rizwan Ahmed Memon, Dennis Y.C. Leung, Liu Chunho (2008)
• Urban Heat Island Dynamics in Montreal and Vancouver
• T. R. Oke. (1974)
• City Size and the Urban Heat Island
• T. R. Oke (1973)

Nevertheless, as described by Oke (1974), The exact manner in which the thermal and radiative properties of the urban environment could operate in conjunction to form an urban climate is somewhat poorly understood. This fact remains true despite the plethora of studies that have delved into the subject. A prominent issue with urban heat islands is they are not consistent and tend to fluctuate regularly. Obtaining measurements requires ample effort and time and hence, identifying the conditions necessary for the formation of the UHI is of considerable importance. To date, the most reliable classification system is that imposed by Arnfield (2003):

• UHI is strongest during the evening
• UHI is stronger during the warmer half of the year
• UHI decreases with increasing wind speed
• UHI decreases with increasing cloud coverage

Despite Arnfield's suggestions for the optimal conditions to measure the UHI, they are not absolute and have been proven to be incorrect in certain circumstances. As mentioned earlier, considering the UHI is very dependant on the urban architecture and meteorological conditions, it's fairly optimistic to assume his classification could be used for most areas outside of his geographic scope. Some other conditions have also been poorly explored, such as humidity and the potential effect of traveling fronts (migrating air masses).

On average, the measured UHI rests at 1-2 ºC warmer than the surrounding rural regions but could reach up to 12 ºC in extreme conditions. To get a better understanding of what would need to happen for the conditions to become extreme, we would benefit from exploring the conditions associated with the UHI's formation.

Formation:

The UHI forms as a result of heat ineffectively escaping from an area over a span of time. To put this in context, we could compare the urban heat island to a normal heat island that could be found in densely populated rural regions (forests). 

Heat absorption:

• Urban regions are usually riddled with dark surfaces and efficient heat absorbing materials. Tarmac is a good example of this. 
• High albedo surfaces such as windows and reflective surfaces can also intensify heating in microscale regions (Figure 1).
• Pollutants can evoke cloud formation aiding in heat trapping.

Moisture: (Water has a high heat capacity and evaporation cools the surroundings)

• Urban regions tend to be more dry than forested rural regions. As most of the moisture is not retained in the system, evaporative cooling is lower.

Heat sources:

• Urban regions are warmed during the day by solar activity but also continue to be warmed uniformly due to technological machinery. Waste heat can accumulate and coupled with escaping heat being trapped, the regions could be warmed more efficiently. Buildings also extend much higher, increasing the surface area for heat absorption and release.
• Rural regions are only warmed by radiation from other warmed surfaces. Primary heating is limited to solar activity. Also less efficient at trapping heat than urban regions.

Wind activity:

Wind forcing has a tendency to blow warmed air from one region to another allowing for the air to be recycled and remove trapped air from a system.

• Urban regions are much more unnaturally structured. Wind is easily obstructed and slowed by having to veer around buildings and streets (figure 1). The weaker wind forcing lowers the degree at which the UHI is dissipated.
• Rural regions feature many gaps between structures allowing the wind to flow more smoothly.

Figure 1: A schematic view of the UHI at multiple scales in addition to the obstructions imposed by the urban architecture on wind forcing. (Source)

Summary

• The UHI is still poorly understood in all forms outside of a conceptual understanding
• The UHI tends to fluctuate and can vary from city to city depending on the architecture.
• Best conditions to measure the UHI are during calm, clear, summer evenings. 
• The thermal properties of the urban surfaces emphasize heat absorption and less efficient heat loss
• Urban moisture levels are lower on average, which increases UHI
• Urban areas obstruct wind forcing and hence the air is not easily regulated.
• Urban pollutants release condensation nuclei which aid cloud formation and hence, heat trapping

~Further reading~

I hope this post has helped in solidifying any concerns with regards to what the UHI is and what the key drivers for its formation are. Many researchers have attempted to mitigate the UHI by removing the conditions for formation as opposed to developing techniques for removal. These approaches will be discussed in future posts.

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.