ORIGINAL PAPER Effect of asymmetrical street aspect ratios on microclimates in hot, humid regions Adeb Qaid & Dilshan R. Ossen Received: 17 January 2014 /Revised: 15 July 2014 /Accepted: 15 July 2014 # ISB 2014 Abstract Asymmetrical street aspect ratios, i.e. different height-to-width (H 1 /W-H 2 /W) ratios, have not received much attention in the study of urban climates. Putrajaya Boulevard (northeast to southwest orientation) in Malaysia was selected to study the influence of six asymmetrical aspect ratio scenar- ios on the street microclimate using the Envi-met three-di- mensional microclimate model (V3.1 Beta). Putrajaya Boule- vard suffers from high surface and air temperature during the day due to the orientation, the low aspect ratio and the wide sky view factor. These issues are a common dilemma in many boulevards. Further, low and high symmetrical streets are incompatible with tropical regions as they offer conflicting properties during the day and at night. These scenarios are examined, therefore, to find asymmetrical streets which are able to reduce the impact of the day microclimate on boule- vards, and as an alternative strategy fulfilling tropical day and night climatic conditions. Asymmetrical streets are better than low symmetrical streets in enhancing wind flow and blocking solar radiation, when tall buildings confront winds direction or solar altitudes. Therefore, mitigating heat islands or improving microclimates in asymmetrical streets based on tall buildings position which captures wind or caste shades. In northeast to southwest direction, aspect ratios of 0.8–2 reduce the morning microclimate and night heat islands yet the negative effects during the day are greater than the positive effects in the night. An aspect ratio of 2–0.8 reduces the temperature of surfaces by 10 to 14 °C and the air by 4.7 °C, recommended for enhancing boulevard microclimates and mitigating tropical heat islands. Keywords Asymmetrical Street H 1 /W-H 2 /W . Tropical climate . Urban microclimate . Putrajaya Boulevard . Numerical modelling Introduction Urban heat island can undermine a comfortable lifestyle (Yang et al. 2010). Creating thermally comfortable environments and quality microclimates is a complex task confronting urban designers and climatologists. The causes of heat islands and poor microclimates on a citywide scale are complex and mostly related to the configuration of urban streets (Goh and Chang 1999; Oke 1981). The configuration of urban geome- tries is usually represented by height-to-width (H/W) aspect ratios. These H/W aspect ratios are the main physical factor affecting incoming and outgoing radiation, radiation flux and wind flow (Xi et al. 2012; Johansson 2006); (Marciotto et al. 2010). They also affect the behaviour of air and surface temperatures (Bourbia and Awbi 2004a; Andreou and Axarli 2012). They are a physical factor in urban canyons introduced to control the urban heat islands, ameliorate urban microcli- mates and enhance outdoor thermal comfort. The available knowledge about the relationship between microclimates and H/W ratios is largely confined to symmet- rical H/W aspect ratios; information about asymmetrical H/W aspect ratios is very limited. Low and high symmetrical street H/W aspect ratios have been used in several studies that present conflicting day and night time needs in the tropics and seasonal needs elsewhere. The uniqueness of these studies is in highlighting the influence of asymmetrical H/W aspect ratios on microclimates and heat islands in tropical regions. Tropical climates have not been discussed in studies on asym- metrical streets. This represents a serious difference between the present work and the previous research of Ali-Toudert and Mayer (2007) and Swaid (1992). Further, these studies A. Qaid (*) : D. R. Ossen Faculty of Built Environment, Universiti Teknologi Malaysia, Skudai, 81310, Johor Johor bahru, Malaysia e-mail: adeebqaid@gmail.com Int J Biometeorol DOI 10.1007/s00484-014-0878-5