Changing the built environment to incorporate more tree canopy and greenery to lower Urban Heat
Transformation of an impervious parking lot site in Lawrenceburg, Indiana to a public green space providing a desirable amenity to the community, while also reducing UHI and stormwater run-off.

What is an Urban Heat Island?

graphic of the urban heat island effect

An urban heat island occurs when a city experiences much warmer temperatures than in nearby rural areas. The difference in temperature between urban and less-developed rural areas has to do with how well the surfaces in each environment absorb and hold heat.

How Does an Urban Heat Island happen?

The primary driver of UHIs is human activity. Urban areas have a higher concentration of buildings, roads, and other infrastructure that absorb and retain heat from the sun, particularly materials like asphalt and concrete. These surfaces, often referred to as “heat sinks,” can absorb a significant amount of heat during the day and release it slowly at night, keeping urban areas warmer.

Urban Heat Island effect (UHI) is a very real challenge in cities -it’s recorded and documented that cities are heating up, becoming far hotter in some cases than any modern recording statistics available. Urban climates are distinguished by the balance between solar gain and heat lost from walls, roofs and ground; by heat exchange via air movement between ground, buildings and atmosphere; and by the generation of heat within the city itself (Earl et al, 2016).

The nature of urban materials and surfaces (hard, paved), land cover (lacking vegetation) and metabolism (waste heat from transport, industry and air conditioning) creates a significant temperature discrepancy between city and country –the abovementioned urban heat island (UHI) effect (Akbari et al, 2008).

Quality of Urban Life at Risk

The combination of a warming climate, Urban Heat Island effect (UHI) and longer, hotter and more frequent heatwaves is having a growing impact on the quality of urban life, from simple low-grade misery to increased risk of death. Lack of vegetation cover is a defining feature of built-up urban areas.

It is also a major contributor to the Urban Heat Island effect (UHI) effect through decreased evapotranspiration in cities. Vegetation facilitates Urban Heat Island effect (UHI) mitigation via evapotranspiration, shading and providing cooler surfaces to reduce mean radiant temperature (the ‘averaged’ effect of heat radiating from surrounding surfaces).

Suitable species selection and planting design with taller vegetation – shrubs and trees – can also help channel cooling breezes to where they are needed. In addition, urban vegetation supports the combat of climate change by more effective stormwater management, improved air quality, biodiversity, urban ambience and energy-saving, often referred to as ‘co-benefits’.

How to reduce the urban heat island effect

  • Build green infrastructure improvements into regular street upgrades and capital improvement projects to ensure continued investment in heat-reducing practices throughout your community.
  • Planting trees and other vegetation—Space in urban areas might be limited, but integrating small green infrastructure practices into grassy or barren areas, vacant lots, and street rights-of-way can be easily done
  • City officials like the city of Cockburn in Western Australia, are starting to implement Urban Forest Plans to increase the tree canopy and help the city use trees to address urban heat, stormwater management, and other concerns. Read the article ‘The city of Cockburn is aiming to to increase streetscape shade with urban forest plan’ here.
    • Making traditional water quality practices serve double duty by adding trees in or around roadside planters and other green infiltration-based practices to boost roadside cooling and shading.
    • Transforming communities by planting native, drought-tolerant shade trees and smaller plants such as shrubs, grasses, and groundcover wherever possible.
  • Building green roofs and living walls —Green roofs and living walls are an ideal heat island reduction strategy, providing both direct and ambient cooling effects.  In addition, green roofs improve air quality by reducing the heat island effect and absorbing pollutants.

How a Connected Tree Canopy lowers Temperature

Connected tree canopy to lower urban heat

In a healthy forest, there are different layers of plants besides just the trees. These layers include the mid-canopy, understory, and ground cover plants. This layering creates a temperature difference, with the top of the trees being warmer and the ground cover being cooler.

Because cooler air is denser, it causes air to flow downward through these different layers, eventually reaching the ground. As the air cools, it releases moisture, which condenses on the leaves of plants in the lower layers. This process helps capture more water than what occurs in the upper canopy of trees.

Even below the ground, a network of tree roots and fungi can absorb humidity from the air if the soil allows air and water to pass through. This means that not only do water droplets form on leaves, but invisible water vapor can also enter the soil.

This is important because when you compare a diverse, layered forest to a single-species forest without these layers, the diverse forest captures more moisture through this process. In monoculture forests like pine or eucalyptus plantations without diverse plant layers, there is less downward airflow, leading to a decrease in moisture in the soil.

Trees act as nature’s water factories, capturing moisture and nutrients from the atmosphere. The vast surface area of a tree, including its leaves, branches, and trunk, allows it to intercept humidity, dust, pollen, and more from the air.

As moisture condenses on a tree’s surfaces, it eventually drips down to the soil, bringing valuable nutrients with it. This process contributes significantly to the total precipitation that falls in a forest. When it rains, trees intercept raindrops and guide them down to the soil, where the water is absorbed by the tree’s roots. This natural process creates a water storage system in the forest.

Trees not only store water but also release tiny particles into the atmosphere, serving as nuclei for raindrop formation. This means that trees contribute to rainfall, not just in wet tropical regions but everywhere. They help maintain a balance in our water cycle, ensuring consistent and steady water flow.

However, when we remove trees through deforestation, we disrupt this delicate balance. Less rainfall occurs on barren land, leading to droughts, while downstream areas experience floods due to the absence of tree-covered hillsides that would have moderated the flow of water.

How to Plant More Trees in Urban Areas?

See our ‘Ultimate Guide on How to Plant Trees in Urban Areas’ article to get a holistic view of the full urban tree planting process.