Why is it so…
Why is it hotter in the city?
Pavement, roofs and buildings soak up the sun's heat and release it slowly, and there are few plants to cool the air. That makes a city an urban heat island, often several degrees warmer than the countryside, and warmest compared with it at night.
1–7°Fwarmer by day (EPA)
2–5°Fwarmer at night
Treescool the block
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What makes the island
- Dark surfaces like asphalt and roofs absorb most of the sunlight that hits them.
- Few plants means little evaporation, which is nature's air conditioning.
- Tall buildings trap heat between them and block the breeze.
- Waste heat from cars, AC units and industry adds a little more.
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Why it's worst at night
The US EPA estimates heat islands run about 1 to 7°F warmer than outlying areas by day and about 2 to 5°F warmer at night. At night the countryside cools off quickly while the city keeps releasing the heat it stored all day, so people in cities don't get the overnight relief their bodies need during heat waves.
Cooler blocks
Trees, parks, green roofs and lighter-colored pavement and roofs all cut the effect. Even within one city, a leafy neighborhood can be noticeably cooler than a paved one a mile away.
Questions people ask
- How much hotter are cities?
- Typically a few degrees on average, and sometimes much more on calm, clear nights. The difference varies by city, neighborhood and weather.
- Do parks really cool a city?
- Yes. Shade and evaporation from plants cool the air, and the effect spreads a short distance into nearby streets.