Abstract
High air temperature and high humidity, combined with low wind speeds, are common trends in the tropical urban climates, which collectively govern heat-induced health risks and outdoor thermal comfort under the given hygrothermal conditions. The impact of different urban land-uses on air temperatures is well-documented by many studies focusing on the urban heat island phenomenon; however, an integrated study of air temperature and humidity, i.e., the human-perceived temperatures, in different land-use areas is essential to understand the impact of hot and humid tropical urban climates on the thermal comfort of urban dwellers for an appraisal of potential health risks and the associated building energy use potential. In this study, we show through near-surface monitoring how these factors vary in distinct land-use areas of Kuala Lumpur city, characterized by different morphological features (high-rise vs. low-rise; compact vs. open), level of anthropogenic heating and evapotranspiration (built-up vs. green areas), and building materials (concrete buildings vs. traditional Malay homes in timber) based on the calculated heat index (HI), apparent temperature (TApp) and equivalent temperature (TE) values in wet and dry seasons. The results show that the felt-like temperatures are almost always higher than the air temperatures in all land-use areas, and this difference is highest in daytime temperatures in green areas during the dry season, by up to about 8 °C (HI)/5 °C (TApp). The TE values are also up to 9% higher in these areas than in built-up areas. We conclude that tackling urban heat island without compromising thermal comfort levels, hence encouraging energy use reduction in buildings to cope with outdoor conditions requires a careful management of humidity levels, as well as a careful selection of building morphology and materials.
Highlights
An urban heat island (UHI) is often defined as the significant temperature differential between urban and surrounding rural areas [1]
While there are no general risk categories for apparent temperature, similar to the ones we report above for heat index, previous studies suggest that the apparent temperature thresholds for defining levels, beyond which mortality risk increases significantly can be as low as 30.7 ◦ C for Taipei [60], 27 ◦ C for Korea, and 29.4 ◦ C for the Mediterranean basin [6], highlighting the potentially dangerous levels of apparent temperatures in all land-use areas we investigate in Kuala Lumpur
Our results show that while these areas offer the best thermal comfort in the evening and at night, regardless the season, the daytime thermal comfort in open, grass covered green areas (Botanic Gardens and Tugu Negara) is comparable to built-up areas in the wet season, while it is poorer than the built-up areas in the dry season, especially from early-morning to mid-afternoon, with higher felt-like temperatures of up to 8 ◦ C/5 ◦ C
Summary
An urban heat island (UHI) is often defined as the significant temperature differential between urban and surrounding rural areas [1]. Despite recent sporadic studies that demonstrated that humid heat is increasingly a global trend [11] and highlighted urban moisture as a prevalent issue and an aggravator of heat island impact [12,13,14,15] with important implications on health and energy use [16], the scholarly discourse on urban microclimate under current and future climatic trends is still heavily dominated by air temperatures alone In this respect, our presuppositions with regards to the thermal comfort in different land-use areas need to be revisited, especially in tropical and subtropical cities (compared to mid-latitude cities), because (1) high air temperatures and UHI in tropical and subtropical cities is an almost year-round critical phenomenon, (2) humidity levels are rather high due to frequent and intense precipitation, and (3) ventilation is low due to overall lower wind speed values, further exacerbating the adverse impact of humidity on perceived temperatures
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