The Urban Heat Island Effect

A visualization of the urban heat island effect (via science.nasa.gov).

Most of us look towards the night sky as a reprieve from a long day of the Sun’s warmth, but it is becoming increasingly harder for city-dwellers to experience this relief. Instead, they face a radiating heat that creeps into their homes, forcing them into a discomfort scarcely known by those who live in the countryside. This is not solely a byproduct of global warming, but a localized phenomenon known as the Urban Heat Island (UHI) effect. While it may seem like a simple consequence that comes with city life, this is a structural failure with detrimental effects on society.

To understand this occurrence, one must first look at the surface. Our environment naturally reduces the impact of solar energy through reflection and biological cooling. Grasslands have a relatively high albedo—the measure of a surface’s ability to reflect solar radiation—while forests have a low albedo, primarily absorbing heat through their dark canopies. Furthermore, most terrestrial plants perform evapotranspiration (ET), a process likened to human sweating. Through this process, plants absorb water through roots and release the vapor through their leaf pores (stomata), consuming heat and cooling the surrounding atmosphere.

In contrast, landscapes found in urban areas have largely impervious surfaces, meaning they are man-made and prevent water from infiltrating the soil. Materials like aged-asphalt and dark roofing membranes have incredibly low albedo, absorbing upwards of 90% of the solar radiation that hits them. According to the NASA Earth Observatory, once impervious surface area exceeds about 35% of an urban area’s land, the daytime temperature difference compared to fully vegetated land begins to increase noticeably, rising from roughly 2.3°F (about 1.3°C) to around 2.9°F (about 1.6°C) as impervious cover reaches 65%.

Although the temperature may be hotter during the daytime, the mechanics of the UHI effect are most pronounced once the sun goes down. This is known as the Nocturnal Heat Island. While rural soils and plants release heat just as quickly as they absorb it, the concrete and steel found in urban areas possess high thermal mass. As a result, they spend the daytime accumulating solar energy and spend the nighttime slowly discharging it. This causes nightly temperatures in cities to be 1-7°F (0.6-3.9°C) higher than the surrounding areas.

While the sun is the primary energy source, cities generate their own internal heat called anthropogenic heat. This term, coined in the 1980s, refers to waste heat from buildings, machinery, combustion, humans, and paradoxically, our attempts to stay cool.

Outdoor temperatures rise due to the UHI effect and citizens use air conditioning to combat the heat. Air conditioning (AC) units do not eradicate this heat; they simply pull heat from inside buildings and release it outside. Apartments and closely built houses are common in cities, so when everyone runs their AC at the same time, hot exhaust is pumped into narrow urban canyons, effectively creating a feedback loop. The exhaust raises the temperature during the night, forcing the units to work harder the following day. Air conditioning also contributes to the broader issue of global warming. According to the International Energy Agency, AC used 7% of the world’s electricity in 2022, causing 2.7% of energy-related carbon dioxide emissions. So, while AC usage makes extreme heat bearable, the drawbacks are just as significant as the benefits

An aerial view of a cityscape covered in asphalt and concrete.

Public health is also affected by urban heat islands. UHIs intensify extreme heat, which is the leading cause of weather-related death in the United States. High temperatures can lead to heat stroke, difficulty breathing, and cramps, especially in vulnerable populations such as the elderly. The cramped architecture cities have, which prevent wind distribution, also play a role in UHIs. Higher temperatures hasten the chemical reaction that produces ground-level smog as well, which contains ozone. Ozone is a pollutant that has a plethora of negative effects on humans, including shortness of breath, coughing, and the development of asthma, COPD, and various metabolic disorders. 

As a response to these effects, some cities have begun taking a proactive approach. Within this approach, engineers are looking at material science to improve citizens’ comfort and health.

• Cool Pavements: Los Angeles and Phoenix are paving the way in the usage of cool pavements to combat the urban heat island effect. The cities are using light colored asphalt and reflective coating to reduce the amount of solar radiation the streets absorb. Other cities around the country have adopted permeable roads to allow water to filter and evaporate, cooling the atmosphere and pavement.

• Blue-Green Infrastructure: This concept integrates water sources (blue) with vegetation (green) to sustain urban development while protecting the environment and its inhabitants. Green roofs (made of plants) are alternatives to traditional heat-absorbing materials, and they reduce air pollution. Living walls serve the same purpose, though they are often criticized for being expensive and difficult to maintain.

• The Solar Reflectance Index (SRI): This is a scale measuring a material’s ability to reflect solar heat. It is calculated using the roof’s solar reflectance (SR) and thermal emittance (TE) to provide an indication of how cool the material will stay in the sun. Many building codes, including California’s Building Efficiency Standards and the International Green Construction Code, reference SRI and even have SRI requirements for roofing products.

By understanding the thermodynamics behind urban heat islands, we can begin to redesign our cities on a larger scale. As we face an increasingly warmer global climate, the ability to cool our cities should be at the forefront of architectural innovation. We cannot eradicate urban infrastructure, but moving forward, we can advocate for cleaner approaches that prioritize the environment.