In the past thirty years, the temperature in Phoenix, Arizona has exceeded 100 degrees at an average of 110 days per year. Extreme temperature events are not only caused by rising daytime temperatures but are exacerbated by higher nighttime lows due to the Urban Heat Island (UHI) effect. In urban areas made up of high concentrations of concrete, buildings absorb and store heat throughout the day. While areas with more vegetation observe significant cooling after the sun sets, UHI areas do not. In the evening, UHI areas offset natural nocturnal cooling patterns as buildings transfer the heat they have stored throughout the day back into the surrounding air at night. Factors that cause UHI, such as solar exposure, poor air flow, and insufficient sky exposure for heat loss, can be minimized by careful consideration of building geometry and relationship between congruent buildings and spaces.
Street canyons describe the width to height ratio of streets to adjacent buildings. These ratios play significant roles in the magnitude of the UHI affect. Traditional compact urban design has been based on the notion that lower solar exposure always yields lower ambient temperatures. However, while narrower street canyons of a 1:3 ratio or higher do block direct sunlight upon pedestrians, overall such high ratios are counterproductive to reducing the UHI effect, as deeper canyons do not provide sufficient sky view for facilitating heat escape. Street canyons of 1:2 proportions are ideal for balancing minimal solar exposure and maximum heat exchange.
Additionally, insufficient air ventilation causes ambient heat to stay trapped within UHIs, preventing natural cooler low temperatures at night. Urban air flow can be increased using building design and layout. Studies have shown that porous urban landscapes, which leave significant and intermittent spaces throughout the street wall volume, promote air flow that pushes out heat. Placing towers diagonally on a block, with open space on the other two corners produces optimum ventilation. - 11 minutes ago