Observing changes in the urban thermal environment from space

Land surface temperature climate data provides insight into the changing landscapes of the urban environment.

Prof John Remedios and Dr Mike Perry

University of Leicester

Prof. John Remedios is Head and Professor of Earth Observation Science (EOS), Department of Physics and Astronomy. Director of the National Centre for Earth Observation.

Dr Mike Perry is a researcher at the University of Leicester specialising in Land Surface Temperature, Urban Heat Island and Urban Energy Balance.

Understanding the distribution of heat within an urban area and how this influences the urban energy balance is of significant importance for human health and comfort. This also has implications for future urban development as a consequence of climate change.

The urban heat island (UHI) is a term that refers to an area of industrial of urban cover which, due to anthropological activity experiences generally higher temperatures than seen in nearby rural regions. To characterise the UHI, regular accurate data is required at the city scale. Land surface temperature (LST) data provides a regular and robust source of information over an entire city. LST is not as directly linked to thermal comfort, heat risk and pollution as the air temperature. However it is still an important factor and provides a robust way to determine thermal information about the urban environment, with spatial and temporal consistency.

Figure 1 below shows an LST image of London on the 1 July 2009. This coincides with a heat wave which affected much of the UK. The satellite image (taken from AATSR,Advanced Along-Track Scanning Radiometer) shows not only the heightened temperatures, but also gives an indication of the spatial distribution of the heat. The inner city shows marked warmer temperatures to those of the surrounding suburbs and vegetated areas. Also there is a cool region which follows the course of the river seen on this map at 51.50 degrees north.

Figure 1- LST from AATSR over London UK – July 1st 2009. (D. Ghent)

These snapshots are useful but do not represent the only use of satellites in looking at the thermal environment we live in. Figure 2, shown below, is a time series over London for the full lifetime of the AATSR instrument which has been specifically designed for climate observations. The fluctuations seen in these time series indicate how the mean temperature of the city is changing year on year.

Figure 2 – Daytime time series for LST over London – Full AATSR lifetime (D.Ghent)

Analysis of such data allows us to understand the chief times and magnitudes of thermal conditions whilst pinpointing times of special interest from which we can identify observations at higher spatial resolution. The higher spatial resolution observations, such as those seen in Figures 3 and 4, provide additional information about the spatial distribution of heat within the UHI, allowing greater insight into the structure of cities.

Figure 3. LANDSAT 7 Land Surface Temperature data for London April 2011, at 90 m resolution. Dark red = lower temperature, light red = higher temperature.

Land surface emissivity (LSE) data, is required to ensure accurate land surface temperature (LST) estimation. Figure 4 shows LST for Phoenix, Arizona, this data was produced usng LSEs also retrieved from satellite, improving the accuracy of the LST, as well as broadening our understanding of the thermal infra-red properties of the materials used within cities and how these (for instance building facades and the materials that form our roads and pavements) can impact on the urban heat island. It should be noted that whilst hotter materials emit more thermal infrared radiation, this emission is also influenced by the emissivity properties of the material itself.