Definition
The urban heat island effect is an environmental phenomenon in which a city center packed with buildings and paved roads becomes noticeably warmer than the suburbs and rural areas around it. It gets its name because, on a temperature map, the city looks like a hot island rising out of a cooler sea.
Gray asphalt and concrete soak up the sun all day
Step barefoot onto a sandy beach or a blacktop road in midsummer and it feels like it could burn you. Step onto grass in the shade of a tree, and it feels cool. Natural soil and trees reflect sunlight and release moisture, and that evaporation carries heat away. Trees do this through their leaves, releasing water as vapor. It's called transpiration.
Cities, though, are full of asphalt and concrete instead of soil. These man-made materials absorb heat very fast and hold on to it for a long time. Rain doesn't soak into the ground either. It runs straight into the storm drains. With little moisture left to evaporate and cool things down, the city's surfaces practically boil.
They absorb more sunlight than they reflect, and they lose the cooling effect of evaporating water. So during the day, a city turns into one giant heat-absorbing plate.
Artificial heat and a forest of buildings keep nights hot
In the countryside or a forest, the heat built up during the day escapes into the sky soon after sunset. A city doesn't cool off so easily at night. The heat stored in buildings and roads leaks out slowly all night long. On top of that, the artificial heat we pour out constantly warms the air too.
Car engines, countless air conditioner units, factories, and lighting all release huge amounts of heat. An air conditioner pulls heat out of a room, then adds the heat from running its own machinery, and pushes all of it outside. The more people and buildings a city has, the more heat gets dumped outdoors.
Tall buildings packed close together make it worse. Narrow, deep streets act like canyons that trap the heat absorbed during the day so it can't escape into the night sky. The buildings also block the paths that wind would take, so cool air can't blow in and sweep the hot air away. The heat stays trapped in the city.
A closer look: surface and air temperatures are different
When we talk about heat islands, it helps to separate the "surface heat island" from the "atmospheric heat island." The surface heat island is when the ground itself, such as asphalt or building walls, gets superheated. The atmospheric heat island is when the air we actually breathe in the city gets warmer.
During the day, the surface heat island is strongest, because surfaces in direct sunlight reach scorching temperatures. The atmospheric heat island, on the other hand, shows its biggest gap in the middle of the night or just before dawn. The surrounding forests and suburbs cool down fast, while the city slowly releases the heat it stored all day. That's why city dwellers get far more tropical nights, which are nights when the temperature stays above 25°C (77°F) and makes it hard to sleep. In Korea, these are called yeoldaeya (열대야 in Korean).
To tackle this, urban planners now use green roofs, which put plants on top of buildings, and wind corridors, which are designed so breezes can flow freely through the city.
Common misconceptions
The urban heat island effect is the same thing as global warming.
Global warming is the rise in Earth's average temperature caused by greenhouse gases. The urban heat island effect is a local warming of specific city areas, caused by man-made structures and waste heat. The causes and the scale are different.
Where you meet it
On a summer night, the suburbs get a cool breeze, but in downtown Seoul, tropical nights go on and you need the air conditioner running all night to fall asleep.
To cool down road surfaces, water trucks spray water on downtown streets in the middle of a summer day.
In one sentence
Concrete buildings, asphalt, and the artificial heat people give off make a city much hotter than the natural land around it.