Influence of urban morphology and sea breeze on hot humid microclimate: the case of Colombo, Sri Lanka

Urbanisation leads to increased thermal stress in hot-humid climates due to increased surface and air temperatures and reduced wind speed. We examined the influence of urban morphology and sea breeze on the microclimate of Colombo, Sri Lanka. Air and surface temperatures, humidity and wind speed were measured at 1 rural and 5 urban sites during the warmest season. The urban sites differed in their height to width (H/W) ratio, ground cover and distance to the sea. Intra-urban air temperature differences were greatest during the daytime. A maximum intra-urban difference of 7 K was recorded on clear days. Maximum temperatures tended to decrease with increasing H/W ratio and proximity to the sea. All urban sites experienced a nocturnal urban heat island (UHI) when the sky was clear or partly cloudy. The temperature differences between sunlit and shaded urban surfaces reached 20 K, which shows the importance of shade in urban canyons (reducing long-wave radiation from surfaces). Within the urban areas, the vapour pressure was high (> 30 hPa) and showed little diurnal variation. Wind speeds were low (< 2 m s(-1)) and tended to decrease with increasing H/W ratio. Shading is proposed as the main strategy for lowering air and radiant temperatures; this can be achieved by deeper canyons, covered walkways and shade trees. It is also suggested to open up wind corridors perpendicular to the sea to facilitate deeper sea breeze penetration.