Urban heat island characteristics under different land use land covers in Dhaka, Bangladesh

Urban heat island (UHI) is a phenomenon of higher atmospheric and surface temperatures occurring in urban areas than in the surrounding rural areas due to urbanization. Rapid urbanization leads to increase in impermeable surfaces in the form of pavements and roads that may reduce natural vegetation cover and increase the surface temperature. Characterization of UHI is an important issue due to its harmful effects on human health and environment. The study was carried out to observe urban heat island (UHI) phenomenon under different land use land covers (built-up area, vegetated area, water body and bare soil) and its effect on human comfort in capital city Dhaka and its suburban area (Savar), Bangladesh. The last 30 years (1987-2016) temperature and relative humidity were collected from Bangladesh Meteorological Department (BMD) to investigate the presence of heat island phenomenon in the study areas. Air temperature and relative humidity under different land use land covers were measured using Thermo-hygro data logger from July to September, 2017 (10 July, 8 August and 8 September) in order to find out the thermal behavior of different land use land covers (built-up area, bare soil, vegetated area and water body). During the period of 1987-2016, annual average temperature increased by 0.024 °C / year in Dhaka, 0.0199 °C/year in Savar. In comparison with the rate of temperature increase with its suburban area, the rate of increase in Dhaka was higher, indicating the existence of urban heat island in this city throughout the study period (1987-2016). In Dhaka, the highest (3.5 °C) urban heat island intensity (UHII) was in 1999 and the lowest (0.6 °C) was in 2011. The built-up area exhibited the highest UHI effect in comparison with bare soil, vegetated area and water body. Highest UHII of Dhaka was 3.3 °C in built-up area and lowest was 0.2 °C in vegetated area. Thermo hygrometric index (THI) ranged from 26.2 °C to 32.6 for Dhaka city and 25.7 °C to 31.8 ° C for Savar sub-urban area indicating the more comfortable condition in Savar. From this study, it is found that land use land cover changes (from vegetated to impervious layer) in urban areas increased urban heat island phenomena compared to its surrounding sub-urban area.