Characterising the Urban Heat Island in a low-rise indigenous city using remote sensing

The study evaluates urban temperatures in the low-rise, semi-arid city of Kano, Nigeria. Massive urban growth over four decades raises concern about impacts of urbanisation on households, given more frequent heat waves and poor access to electricity supply. MODIS Land Surface Temperatures ( LST )s are examined to characterise potential Urban Heat Island (UHI) effects, and changes in urban temperatures over two decades. As the study demonstrates strong relationships between satellite LST and air temperatures, LST is used to evaluate the effects of urbanisation on local climate. Over the 2-decade study, daily maximum air temperatures have risen by at least 1.5 °C in urban areas, compared to 0.8 °C in rural areas. Annual nighttime LST over much of the urban area has increased by up to 3.2 °C at 10:30 pm and summer shows an increase of up to 3.6 °C. Greater temperature increases for urban than rural areas, and for nighttime than daytime, are attributed to UHI effects. The observed summer daytime Surface Urban Cool Island (SUCI) is surprising, as daytime SUCIs elsewhere are attributed to either being high rise or well vegetated, which Kano is not. Furthermore, air temperatures from local climate stations indicate a summer daytime UHI approximating 9 °C. The disparity may be due to use of galvanised metal roofing, which has lower emissivity than values in satellite LST calibrations. The observed summer nighttime Surface Urban Heat Island Intensity (SUHII) averaging 6.6 °C may be an underestimate, as real surface temperatures would be higher than image calibrated values. • There are few studies of Urban Heat Islands in indigenous cities. • MODIS Land Surface Temperatures (LST) is well correlated with air temperatures. • The observed summer daytime Surface Urban Cool Island (SUCI) is surprising. • The SUCI is attributed to low emissivity of metal roofs giving anomalous LST values. • The summer nighttime SUHII of 6.6 °C could be an underestimate and is concerning.