Abstract:
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.
