Islands in the sun: Heatwave gives cities that sinking feeling | 4/5/2019 | Staff
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Boffins call it a heat sink—a passive exchanger designed to dissipate heat—but when the sink is an actual city, its concrete and asphalt sweltering in the heat, it feels more like an oven to those who live and work there.

The phenomenon where cities are hotter than the surrounding countryside is actually known as an urban heat island, and while the effect exists year-round, it is most acutely felt at exactly the worst moment—a heatwave.

Countryside - Vegetation - Uses - Water - Soil

In the countryside, vegetation uses sunlight and water from the soil for photosynthesis which in addition to converting carbon dioxide into oxygen, also releases water into the air.

This helps disperse solar energy and cool the surrounding area.

Cities - Vegetation - Heat

Meanwhile, in cities, there is not nearly as much vegetation to disperse heat.

Moreover, asphalt and cement absorb solar energy during the day and release it during the night.

Result - City - Countryside - Buildings - Streets

The result is the city is hotter than the surrounding countryside, as buildings and streets act as a giant heat sink, and this is most noticeable during heatwaves.

France's national meteorological service has found an average annual difference between Paris and surrounding rural areas on the order of 2 to 3 degrees Celsius (4 - 5 degrees Fahrenheit).

Heatwave - Difference - Degrees - Celsius - Meteo-France

During a heatwave, the difference "can reach close to 10 degrees Celsius", said Meteo-France.

During a 2003 heatwave, when the daytime temperature hit 40 degrees C it fell to between 23 and 26 degrees during the night in the areas of the city that are the greenest or got the most breeze. But in the city centre it fell only to 28 degrees.

Microclimate - Effects - Night - Period - Body

This urban microclimate "aggravates the effects felt, in particular during the night, a critical period when normally the human body recuperates," said Aude Lemonsu, who heads up Meteo-France's research centre.

These urban heat islands thus magnify the effects of climate change which is expected to increase the number...
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