Urban heat islands

The urban heat island effect is the phenomenon where urban areas have higher temperatures than surrounding rural areas with more greenery.

What causes heat islands?

Urban heat islands are caused by several factors, including:

• the types of materials we use in cities for buildings, pavement, and roads that can absorb more heat than natural surfaces such as leaves or grass
• human additions to the sun's heat, such as air conditioning, vehicles, cooking, or machinery
• the shapes of our cities that may restrict airflow to carry heat away
• the mass of our buildings that can absorb heat and release it gradually, even through the night

In contrast, more natural, often rural areas with greenery may reflect more light, release moisture and provide shade.

The urban heat island effect is usually strongest on hot, calm days and can be especially noticeable at night, when buildings and roads slowly release the heat they stored during the day.

How can we reduce the urban heat island effect?

There are ways to help reduce the urban heat island effect, such as planting more urban trees or using green roofs.

Deciduous trees have the handy feature of providing shade for a house in the summer while letting light through in the winter when the leaves fall.

A study using 2015 data found that "doubling tree cover in European cities could cut the number of heat-related deaths during summer months by nearly 40 per cent." In Columbia's Medellín, creating 30 Green Corridors helped to reduce temperatures by 2°C in the first three years of the program.

Learn more about the urban heat island effect on the EPA site.