Hot in the City

Our recent heatwave will likely have been more intense for people living in the city due to a phenomenon called the ‘Urban Heat Island’ (UHI) which leads to higher temperatures in our city centres when compared with suburban and rural surroundings, especially overnight. Early this year, I published a paper that characterised the Leeds UHI and modelled its potential impact on human comfort and energy consumption in buildings. In simple terms, the mass of the city’s built environment stores the daytime heat leading to warmer temperatures overnight, which can disrupt sleep and unfortunately exacerbate health problems for more vulnerable residents.  The UHI intensity is the difference between the temperature within the city centre and rural reference sites. Data from the 2013 heatwave were used to understand the extent of the UHI in Leeds, with analysis identifying a peak UHI intensity of 5.9 ˚C at 20:00 on 2^nd August 2013 and an average UHI intensity of 2.3 ˚C at the same time during the evening over the entire summer.

Whilst this paper was our first published piece of work in this field, we have lots of ongoing research that aims to provide us with a better understanding of our city environment. By working with Leeds City Council, we also have an extended network of temperature sensors installed across the city region collecting data every hour; if you look up you may see these attached to road signs and trees. The sensors are housed in what look like small white beehives, these are actually shields that protect the sensors from direct solar short-wave radiation (you can see these in the picture of Felix Thomas and I as we set off to install the first of these sensors over two years ago!). They are deliberately divided between green and grey (man-made) spaces within the urban environment. Adding more green space helps to mitigate against the UHI as plants can heat up and cool down much more efficiently than heavy man-made materials. We have been collecting these data since the summer of 2019 when we had around 20 sensors in the city centre, we have now expanded this network to include 71 sensors across the wider city region.

I will be (virtually) presenting initial results from the urban sensors at this year’s Ecocity World Summit 2021 that is being held in Rotterdam this September. Although the green space in the city centre is limited to very small areas, data from the city centre suggests that the green space is on average, across the summer months, approximately 0.5 ˚C cooler than built-up areas, and up to 3 ˚C cooler on very hot days. We hope that as this data set increases it will not only help to support the retention of existing green spaces, but also the introduction of more urban green infrastructure. The data are also potentially useful for other researchers in the fields of geography, planning, health and social sciences.

As well as the work described above, we are also monitoring internal temperatures in 80 apartments across the city, again in partnership with LCC. This will help us to understand the extent of any overheating in these buildings but also to explore the relationship between the external temperature, grey and green spaces. We would like to expand this monitoring programme so if you live within the Leeds city area and would like a set of air temperature sensors installed in your home, please get in touch: j.m.parker@leedsbeckett.ac.uk. In addition to monitoring the urban temperatures, we are also beginning to monitor the air quality within the city. Longer term, these sensors will provide us with a high-resolution combined data set which will increase our understanding of the urban environment in Leeds, hopefully to the benefit of people who live and work in the city.