Urban Heat Island

An urban heat island is a city or district that is warmer than the rural land around it. The difference arises because buildings and pavement replace vegetation and moist soil, absorb more solar energy, store heat during the day and release it at night. Roofs make up a large share of urban surface area, so roof colour and type have a significant influence on urban temperatures.

Discovery

The effect was first described by Luke Howard in the 1810s, who found that central London was about 2.1 °C (3.7 °F) warmer at night than the surrounding countryside.

Magnitude

The US EPA cites a review finding that, in the United States, urban daytime temperatures run about 0.6-3.9 °C (1-7 °F) higher than outlying areas and night-time temperatures about 1.1-2.8 °C (2-5 °F) higher. Differences tend to be largest in humid regions and in larger, denser cities, and are often greater at night.

Causes

• Less vegetation and water, which normally cool through shade and evaporation;
• Building materials such as dark roofing and pavement that absorb and re-emit heat;
• Urban geometry, where tall buildings and narrow streets trap heat;
• Waste heat from vehicles, air conditioning and industry;
• Weather, with calm, clear conditions intensifying the effect.

The EPA notes that conventional roofing can become as much as 37 °C (66 °F) warmer than the air. Hot roofs also warm stormwater runoff, which can affect streams.

Health and energy effects

Heat islands raise cooling demand and electricity peaks and worsen heat-related illness and mortality, particularly among older adults, children, people with chronic illness and low-income households. Warm nights reduce the relief people would otherwise get after hot days. Higher temperatures also increase ground-level ozone formation.

Roof-based mitigation

• Cool roofs reflect more sunlight. A simulation of a 2018 heatwave in London found that cool roofs could reduce average outdoor temperatures by up to 1.2 °C, more than added trees or solar panels in the same study.
• Green roofs cool through shade and evapotranspiration and also manage stormwater.
• Solar panels shade the roof and convert part of the energy to electricity.

Research groups such as Lawrence Berkeley National Laboratory's Heat Island Group have studied these measures for decades, and many cities now include roof requirements in heat action plans.