On a summer evening, a city center can be several degrees warmer than the countryside around it. This is called the urban heat island. and buildings heat during the and it slowly at . Cities have fewer , cool the air by giving off water .

Cities can reduce the heat island effect in several ways. One solution is to roofs white or colors, so that they sunlight of absorbing it. trees along streets shade cools the air. Parks and green roofs work in a way.

Cities are often warmer than the countryside around them, a pattern known as the urban heat island. The difference is usually greatest at night. During the day, concrete, asphalt and brick soak up energy from the sun. After sunset, these materials release the stored heat slowly, keeping streets warm long after the surrounding fields have cooled.

Several other factors add to the effect. Cities contain fewer plants, and plants cool their surroundings by releasing water vapor through their leaves, much as sweat cools the human body. Tall buildings can block wind and trap warm air between them. Cars, factories and air conditioners also release waste heat directly into the street.

The heat island is more than an inconvenience. During heat waves, higher night-time temperatures prevent people’s bodies from recovering, which increases the risk of heat-related illness, especially for older people and those without air conditioning. Higher temperatures also raise demand for electricity, which can increase pollution from power plants.

Large tropical cities such as Bangkok face a double challenge: they are already hot, and they are growing rapidly.

Because dark surfaces absorb most of the sunlight that hits them, one of the simplest ways to cool a city is to make surfaces lighter. A white or specially coated “cool roof” can reflect much of the sun’s energy back into the sky. Studies of cool roofs have found that they can lower the temperature of the roof surface dramatically on sunny days and reduce the energy needed for air conditioning in the rooms below.

Trees offer a different kind of benefit. Their shade keeps pavements and walls from heating up in the first place, and the water they release cools the air. A street lined with mature trees can feel several degrees cooler than a bare street nearby.

Each approach has limits. Cool roofs help the buildings they cover but do little for people walking at street level. Trees need space, water and many years to grow, and their roots can damage pipes and sidewalks if they are poorly planned.

For this reason, many planners now recommend combining strategies, from reflective roofs and pavements to parks, street trees and green walls.