A Final Word

As you might expect, every series eventually reaches its end. I’m very proud to have had the opportunity to discuss such an overlooked property of our atmosphere. It’s always nice to experience the subtleties in our surroundings, regardless of how dynamic or prevalent they may be. 

I’d like to thank anyone who took the time to read through any of the blogs and hope you’ve at least had a chance to broaden your perspective on what you may have never otherwise ever heard about. To any new readers, I encourage you to have a browse of what the series has to offer as it was designed from scratch to cater to anyone, assuming you read them in order. 

It has truly been a pleasure and I sincerely hope that someone may have benefited, even if only very slightly from my interpretations of one of the most overlooked atmospheric phenomena. To be so close and yet know so little is a poor state to be in. 

Until further notice, I bid you all a farewell!

-Muwaffaq

A Retake on UHI and UCV Formation Conditions (Plymouth)

Understanding the UHI's ideal formation mechanisms have always been vague with a general idea of a good time to take some measurements. In my dissertation as an Ocean Science student in May 2016, Time, Season, Wind speed and Cloud Coverage were studied within Plymouth using eulerian field measurements through secondary fixed weather stations at 3 hour intervals for the summer and winter months of 2015/2016 to determine their ideal formation conditions. This post briefly will focus on the results of that study to help paint a clearer picture of the conditions.

To keep things simple, i'll be mainly producing the results of my paper without much discussion on the mechanisms of formation. For a very detailed explanation, please comment on this post requesting the paper, which includes a very detailed statistical analysis of the trends, formations and mechanisms.

 Season and Time of day:

This section could be summarised very easily with a single image (Figure 1). The UHI and UCV do appear to be inverse of one another with highest intensity UHI forming at roughly 21:00 - 00:00 on average both within the Summer and Winter, whilst the UCV forms within the 12:00 - 15:00 hours of the day in both seasons. The error bars are to 95% significance.

Another factor is that the intensities were not only weaker in the winter but also very unreliable. The error bars were massive suggesting readings were not uniform and difficult to predict. both UHI and UCV are also less intense as suggested by Arnfield (2003).

Figure 1: A trend indicating the progression of the state of the UHI at 3 hour intervals throughout the course of a 24 hour period using hourly results over 3 summer months and 3 winter months with errors bars at 95% confidence.

Wind and Cloud Coverage:

The results of this section (Figure 2) might come across as shocking considering the readings appear to contradict Arnfield and many studies but in fact, they do not. It appears that at high wind speed, a UHI will always be present whilst at low, it may not. In truth, what actually happening is the wind speed is theoretically the same in both urban and rural regions but different in practice. The tall structures of the city provide shielding, weakening the wind speed and hence allow some heat to remain. In the rural region, the wind is unobstructed as Devon is mainly a grassy field hence the any residual heat will be dispersed and the difference will always be present. The UHI will always be present in high wind conditions, albeit a weak one regardless of season for a dense city. nevertheless, it's far from ideal and unlikely to be the maximum intensity as some heat will still be removed by the wind. The UCV will also never form in high wind conditions for the same reasons. 

Cloud coverage readings agree with Arnfield's suggestion as the UHI appears strongest in clear conditions but appears to have a very minor role on UHI formation in comparison to the other conditions. It also appears that during the winter, clear conditions have a strong potential for UCV formation in comparison to UHI suggesting that cloud coverage could play a role in UHI mitigation, albeit a weak one. 

Figure 2: Graph indicating the range of UHI/UCV that could be experienced by a system within the summer and winter under varying wind conditions and cloud coverage

Concluding remarks:

The readings were taken under controlled conditions and given very heavy statistical significance tests before being incorporated into the study. Essentially the study did not falsify anything suggested by Arnfield but did manage to expand somewhat on the significance of conditions. To summarise, here are the ideal UHI and UCV conditions as suggested by the study:

UHI:

• UHI readings are strongest between 21:00 - 00:00 during the summer and 21:00 in the winter.

• UHI readings are more reliable and higher (by up to 0.5 ºC in Plymouth within the Summer than in the Winter

• UHI is strongest during calm conditions. Increasing wind weakens a strong UHI but always leaves a resulting weak UHI at high wind speed (>35 km/h)

• UHI is strongest during clear conditions. Cloud coverage has a weak effect on UHI but can weaken a strong UHI, particularly in the summer.

UCV:

• UCV is strongest during the mid-day (12:00 - 15:00) during the summer and between (09:00 - 15:00) in the winter.

• UCV is not identifiably stronger during the summer months but appears more reliably.

• UCV is strongest under clear conditions and cannot form under winds >35 km/h

• UCV is strongest under clear conditions but the effect of cloud coverage on UCV is weak.