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.