What is an urban heat island?

Cities like Paris, France can run several degrees hotter than the countryside on summer nights. Asphalt, concrete, and dark roofs store heat by day and release it after sunset. That pattern is called an urban heat island, and it is the reason your home may stay warm even when rural areas nearby have cooled.

Why cities hold heat

Four things stack up, and none of them are avoidable in a dense built environment.

Dark, dense surfaces. Asphalt and concrete absorb a lot of solar energy and hold onto it. A road surface can reach well over 50 °C in the afternoon and is still radiating that heat back at midnight.

Very little vegetation. Plants shade the ground and release water vapour as they transpire, which cools the air around them. A street of bare paving does neither.

Waste heat. Traffic, industry, buildings, and air conditioning units all discharge heat into the same streets. Air conditioning is a particularly awkward case: it makes one flat cooler by pushing that heat, plus the energy used to move it, out into the street where everyone else is walking.

Blocked airflow. Tall buildings on both sides of a narrow street trap radiated heat and slow the wind that would otherwise mix warm air upwards. Planners call it an urban canyon, and it is very effective at holding heat in place.

The night is where it hurts

Daytime temperature differences between city and countryside are often modest. The gap opens up after sunset, when open land radiates its heat away quickly to a clear sky and the city cannot, because its surfaces are still discharging what they absorbed.

That is exactly the wrong time for anyone relying on night ventilation. Your cooling strategy depends on outdoor air dropping below indoor air for a long enough stretch, and a heat island shortens that window from both ends: it starts later and ends earlier.

Where you feel it in your own home

• Balconies above sun-baked streets stay warm well into the evening, so the “cool” air you are pulling in through that window is not as cool as the forecast suggests
• Courtyards with greenery are usually several degrees cooler than the street façade of the same building, which makes them the better intake side
• Top floors and unshaded roofs take the full load of the day and radiate downwards into the rooms below all night
• Ground floors on shaded streets often stay noticeably cooler than the flats above them

The practical consequence: when you have windows on more than one side, the greener, shadier, more sheltered side is usually the right intake. See where to place a fan.

What cools a city, and your block

• More trees and planting along streets, squares, and schoolyards
• Less bare concrete: shade, permeable ground, and pocket parks
• Lighter surfaces and reflective or shaded façades on sunny sides
• Green roofs and planted courtyards instead of heat-trapping asphalt

This app helps inside your home. Your street and neighbourhood matter too, and those are decided elsewhere: see heatwaves and public choices.