What is the Urban Heat Island Effect?

Written By Teacher: Teresa Pettitt-Kenney

Hi there! My name is Teresa and I just finished my Bachelor's degree in Environmental Science and am excited to pursue environmental education in the future! I am extremely passionate about climate change, equitable climate action, and how education can work to address these issues. 

Why does the city always feel much hotter than the countryside? Answer your students' curious questions by exploring the concept of urban heat islands. Connect this concept to environmental justice, plant transpiration, or the climate impact of concrete. Focus on positive solutions with younger students in this design thinking lesson about nature in cities. Investigate relatable real-world issues with secondary students in this social studies lesson on tree inequality in cities. 

Written By: MIT Environmental Solutions Initiative

The MIT Climate Change Engagement Program, a part of MIT Climate HQ, provides the public with nonpartisan, easy-to-understand, and scientifically-grounded information on climate change and its solutions.

The urban heat island effect is a phenomenon whereby cities experience higher air temperatures than the surrounding countryside. This effect can be quite noticeable. On average, cities tend to be 1-7°F warmer during the daytime. This difference continues well into the night, during which cities can still be as much as 5°F warmer than the areas around them.1 Scientists refer to areas afflicted by these higher temperatures as urban heat islands.

People living in urban heat islands are especially vulnerable to the effects of climate change. As the planet warms, urban heat islands will only intensify those higher temperatures. And since nearly 70% of humanity will live in cities by 2050, finding ways to alleviate urban heat islands has become even more pressing.

What causes urban heat islands?

As cities grow, new development such as roads and buildings displace natural surfaces like trees, ponds, and soil. This change in the local environment results in a corresponding change in the local climate. That’s because these natural surfaces help moderate air temperatures.

Trees and other plants, for instance, can lower air temperatures by providing shade. This vegetation, along with soil and water, also helps cool nearby air through “evaporative cooling,” which is a natural process by which evaporating water absorbs heat (much like sweat cools the human body).