Why is it often warmer in the city than in the countryside?
A few weeks ago, I spent vacation with Mika on a farm and it was perfect summer weather. After our vacation, we drove back to the city to get ice cream. On the way, it was unbearably hot, and I almost considered heading home. I wondered if cities could actually be warmer than the countryside and why that might be. I grabbed my ice cream and headed to my friends at (AC)³ to ask.
They explained there’s something called the ‘urban heat island effect’ that often occurs in cities. It’s not a real island, but like one, it’s a small area of high temperature sticking out over a “sea” of cooler surroundings.
Linus, who studied urban heat islands before joining the (AC)³ project, explained that multiple factors combine to cause the effect of the hotter cities.
Our cities
Buildings and streets heat up in the sun. Cities have lots of stone, asphalt, and concrete, which store heat like a hot water bottle and slowly release it over a longer time. I recalled a story from earlier this winter:
“Hey, isn’t this Lukas’s snowman trick?”
“Exactly, the streets and parking lots store heat, so you find perfect snow for snowmen there when it’s too powdery elsewhere,” says Linus. In summer, parking lots and plazas stay warm even late in the evening. Countryside meadows and fields don’t store heat as well, so they stay cooler. More cars mean engine heat.
We also heat homes in winter and use air conditioners in summer to cool homes and stores, both of these warm the city air.
Plants
Another factor that heats the city are fewer plants, which is making the heat worse. Plants provide shade and evaporate water to cool themselves (and the air). Soil and grass hold water, but cities have paved surfaces where rain drains straight into sewers instead of soaking into the ground.
This cooling effect works because evaporating water requires energy. I remember that I know this from cooking and our cloud discussion. The heat pulled from the air to evaporate water cools it down, like a natural air conditioner.
During extreme heat, cities can get dangerously hot for people and animals (especially polar foxes like Mika). If it’s too sweltering, find a park, a garden or a tree, it’s usually pleasantly cool there.
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Text: Linus Andrae, Illustration: Patrizia Schoch