Tall buildings are the signs of economic prosperity and technological superiority. The tall building is the most dominating symbol of the cities and a human-made marvel that defies gravity by reaching to the clouds. It embodies unrelenting human aspirations to build even higher. In line with rising population density, and advancements in engineering, height limits around the world are being revisited and revised to maximise space for commercial and residential growth. However, like anything else, skyscrapers are a good thing in moderation: Done right, they contribute to the vital mix of great cities. Tall buildings have their own sets of negatives as pointed out by some modern urban planners and environmentalists. While tall buildings can be good for cities, they are by no means a cure-all.
Tall buildings are urban canyons
According to United States’ Environmental Protection Agency, “The dimensions and spacing of buildings within a city influence wind flow and urban materials’ ability to absorb and release solar energy. In heavily developed areas, surfaces and structures obstructed by neighboring buildings become large thermal masses that cannot release their heat readily. Cities with many narrow streets and tall buildings become urban canyons, which can block natural wind flow that would bring cooling effects.”
University Corporation for Atmospheric Research says, “Urban canyons—the tall canyons formed by city buildings trap radiant energy in their walls. Comparisons of this “canyon effect” in European and North American cities suggest that areas with denser and taller buildings will more rapidly develop heat islands.”
The IPCC report found that the single biggest contributor to amplifying heat and warming in cities is “urban geometry,” the relationship between city layouts, building construction, and density. The main problem driving the so-called “heat-island effect” is tall buildings. They create urban canyons, blocking winds from cooling things down and locking in heat. Urban centers can range as much as 22 degrees warmer than nearby rural areas. Stoked by climate change, extreme heat kills more people in the U.S. than any other weather event. The report points to cities all around the world — especially Tehran, Iran and Kolkata, India — that are warmer than their surroundings.
They interfere with ventilation
To put it in simple words, tall buildings is a collection of spaces stacked vertically, the heat emitted by those buildings is greater than its footprint. Further, skyscrapers made of concrete, pavement, and other materials absorb the sun’s heat. They hold onto that heat longer than water or vegetation does. Some heat is released at night, but these materials don’t completely cool down before the sun hits them again. This will change the temperature around tall buildings to be warmer.
Tall buildings strongly change pedestrian-level winds in the surrounding streets and the flow field above the roofs of the low-lying buildings. This affects pollutant pathways and the overall ventilation potential of cities.
In fact, large, uniform blocks of tall buildings create “urban canyons” that interfere with ventilation and trap heat and pollutants. Improved ventilation is important to allow heat and pollutants to dissipate and to increase thermal comfort (even when ventilation does not reduce the absolute air temperature, higher wind speed can increase thermal comfort).
What happens is that the little light that does reach deep inside these urban canyons gets reflected back up at the (often very reflective) walls of the buildings themselves. These buildings absorb the heat and then release it back into the surrounding area, causing the areas between tall buildings to retain more heat. This is easily confirmed by touching a tall building at night—it stays warm well after sunset.
Studies show that the increase of building’s height will bring a significant cooling effect in this height range. When the building’s height exceeds 66m, its effect on land surface temperature will be greatly weakened. This is due to the influence of building shadows, local wind disturbances, and the layout of buildings.
Problem occurs when there is no proper planning nor any holistic view about city development. Planners can turn urban canyons into “ventilation corridors” that carry wind throughout the city by orienting major streets parallel to prevailing winds and New Clark City in Philippines is a living example for this. In Philippines, where the government is converting a military base into a new planned community, New Clark City, planners are taking into account sun and wind patterns in different seasons to make outdoor spaces more comfortable and usable in the hot and humid climate. There is need for other city planners to emulate the Philippines’ example in their planning and avoid the formation of urban heat island.