Designing "liveable" cities as climate change effects are felt all over the world has become a priority to city authorities as ways are sought to reduce rising temperatures in urban areas. Urban Heat Island (UHI) effect occurs when there is a difference in temperature between rural and urban areas. In urban areas, impervious surfaces absorb heat during the day and release it at night, making urban areas warmer compared to rural areas which cool faster at night. This Urban Heat Island effect is particularly noticeable at night. Noticeable negative effects of Urban Heat Islands include health problems, air pollution, water shortages and higher energy requirements. The main objective of this research paper was to analyze the spatial and temporal relationship between Land Surface Temperature (LST) and Normalized Density Vegetation Index (NDVI) and Built-Up Density Index (BDI) in Upper-Hill, Nairobi Kenya. The changes in land cover would be represented by analyzing the two indices NDVI and BDI. Results showed the greatest increase in temperature within Upper-Hill of up to 3.96˚C between the years 2015 and 2017. There was also an increase in NDVI and BDI to study changes in land cover due to urbanization, reduces the time taken to manually classify moderate resolution satellite imagery.
Abstract. Urban areas have been cited to be contributors to changing climatic conditions. Conversion of vegetated areas to impervious surfaces causes an alteration of surface temperatures, where impervious surfaces absorb and store incoming radiation during the day, releasing it slowly at night. This causes the urban heat island effect where temperatures in urban areas are higher than rural areas during night. Extreme weather conditions experienced in urban areas include heat waves, intense rainfall resulting to flash floods, destruction of infrastructure facilities. The research involved the use of time-series of stereo-aerial images acquired in 1978, 2000 and 2017. Building footprints were digitized and using digital elevation models, building heights were derived and volumes calculated. Road volumes were calculated using minimum standards of road construction and the total built-up volume was determined. The ground coverage ratio (GCR) and built-up volume densities (BVD) of the built-up areas was calculated from area and volumes from road and building values. Results indicate the mean GCR and BVD in Upper Hill is within the low density range. Mean building height from 1978 to 2017 are in the low-rise range with significant rise in building height and volumes between 1998 and 2017. The 3D built-up intensity expansion rate ranged between −4.28% to 56.80% per annum with a mean of 19.6% per annum. Roads and pavements also have a significant influence on the built-up density and should therefore be considered in planning policies.
Abstract. Urban heat island is the difference in thermal temperature between rural and urban areas. The urbanization process alters the material type with impervious surfaces being absorbers of incoming radiation during the day and emitting it at night. The research involved the use of time-series satellite imagery from Sentinel, Landsat, ASTER and MODIS for the period 1986, 1995, 2000, 2005, 2011, 2015 and 2017 over the Upper Hill, Nairobi. Morning, afternoon and night land surface temperatures (LST) were calculated for each of these years and analyzed together with the land cover. The mean albedo was calculated to determine the relationship between each land cover and mean LST. The contribution index was calculated to determine whether a land contributed positively or negatively to the mean LST in Upper Hill. Results indicated that built-up land cover had increased from 1986 to 2017 by 0.86% per annum while forest land cover had decreased by 0.99% per annum. Sparse grassland had higher albedo and LST values of 0.81 and 27.9 °C respectively, whereas water had lower albedo and LST values of 0.09 and 25.1 °C. Water had the lowest mean LST during the day but highest mean LST in the afternoon and night in each of the years due to its high thermal capacity. Bare ground tends to have a higher contribution index compared to other land covers, while forest land cover has a negative contribution index, indicating the impact land cover types have on LST and the urban heat island effect.
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