A parametric optimisation study of urban geometry design to assess outdoor thermal comfort

•Height-to-Width ratio and orientation can reduce the UTCI by almost 6°C.•Orientation alone can reduce the UTCI of the same urban canyon by almost 2°C.•In hot-arid areas, H/W's of 3 to 6 are favoured in N-S, NE-SW and NW-SE canyons.•H/W's of 4 to 6 are best for southern flanks in narrow streets of E-W canyons.•H/W's of 0.3 to 3 are best for northern flanks in wide streets of E-W canyons. Over the last two decades, urban geometry has been shown to be a key determinant of the microclimatic conditions in urban areas. This study uses the Ladybug-tools, the plugins of Grasshopper3D to optimise building heights, street widths and orientation to maximise outdoor thermal comfort, represented by the diurnal average Universal Thermal Climate Index (UTCI). In the hot-arid climate of Cairo, Egypt, the optimised parameters of symmetrical and asymmetrical urban canyons are compared with the Egyptian Construction Act's design regulations. The results show a strong negative correlation between the height-to-width (H/W) ratios and the output UTCI, with R² = 0.71, and much stronger (R² = 0.91) if east-west orientations are excluded from the results, exceeding correlations previously reported for Cairo. Maximum UTCI reductions due to changing H/W and orientation approach ∼6°C. Considerable variation is shown in the strength of the correlation between UTCI and the asymmetrical H/W ratio of each flank, with R² = 0.81 for Southeast side compared to R² = 0.4 for Northwest side. Design recommendations are given urban planners based on using the optimised parameters that at least achieve a UTCI reduction benchmark that exceeds those resulting from using the regulations’ thresholds.