EFFECTS OF URBAN MORPHOLOGY ON MICROCLIMATE IN WARM-HUMID CLIMATES A CASE OF ZANZIBAR

The urban geometry alters the microclimate conditions in urban centres; this alteration of the urban climate can have severe impacts on the people who use outdoor spaces within a city. The improper function of the geometry factors can increase the harshness of climate change, which is more critical in coastal cities of tropical regions; this affects the urban liveability especially in the warm-humid climates where outdoor activities are possible throughout the whole year. Szokolay (2004) states that warm and humid urban environments require a mix of heavy weight and light weight construction together with ventilation strategies in order to adequately respond to the local climate. As streets cover more than a quarter of these urban areas, designing them plays an important role in creating the urban microclimate. The geometry of the street; height to width ratio, as well as length to width ratio, and orientation directly influence the outdoor and indoor environment, solar access, patterns of irradiation, permeability to airflow for urban ventilation as well as the potential for cooling of the whole urban system. The street geometry and orientation are the single most important factors in providing a pleasant microclimate at pedestrian level in an urban canyon. This research paper empirically examines the effects of the existing urban forms of Stone Town and Ng’ambo on the microclimate of Zanzibar city and highlights some of the adaptation strategies that could improve the comfort levels at the street. From the measured microclimatic data in Stone Town and Ng’ambo, the intra-urban air temperatures differences were greatest during daytime; a difference of 3.30C was recorded between the hottest and coolest street canyon. The daytime temperatures were lowest in the deep canyon and the North - South oriented one while the greatest wind speeds were found in the wide canyons. Wider streets provide better mixing of air and consequently better airflow in the urban canyon. The height to width ratio affects the quantity of solar energy obtained by the street surfaces hence the higher air temperature readings in the wider canyons; decrease in this ratio increases solar access. The orientation causes differences in the distribution of the total radiation over the street surfaces; the East- West oriented streets received higher percentages of diurnal radiation on the vertical surfaces of the streets. In light of this, designing of urban streets appropriately to utilise solar access and channel airflow is vital and essential for the creation of comfortable conditions outdoors.

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