In many cities the heat is unbearable in summer – especially in the heart of the urban fabric, where there is concrete everywhere. Could cold air corridors be the answer?

In the summers, cities tend to overheat. Cement and asphalt store the sun’s heat for a long time – and so metropolises are very hot even during the night.

Today it is becoming more and more important to find ways to “drop” the temperature in cities. And this is where the so-called “cold air corridors” come in.

What are cold air corridors?

Despite the heat of the metropolis the surrounding countryside remains considerably cooler, especially at night – on a cloudless summer afternoon the temperature can be as much as 15 degrees lower than in the city centre.

Green spaces, rivers, lakes, railway tracks and long tree-lined avenues can act as corridors through which the cooler air of the suburbs can blow towards the heart of the city.

But since cool air is heavier than warm air and thus blows only a few centimeters above the ground, then it can be blocked very easily, for example by buildings. Thus, when urban planning is done, measures should be taken so that these corridors remain as free as possible.

Why is it cooler at night in the suburbs?

Beyond the boundaries of the city center there is less cement and asphalt and more earth, trees and grass, which store much less of the sun’s heat. In addition, through the evaporation of water from the leaves of the many plants that these areas have, the air becomes cooler.

Topography is also very important. Heavier warm air that collects just above the ground can blow very hard down a mountainside, for example, and create a cooling wind.

Cities surrounded by mountains or located in a valley can benefit greatly from cool winds, as they can blow through urban centers to push warm air upwards and reduce the temperature of streets and buildings. Stuttgart, for example, is located in a valley and seeks to maintain its cool air corridors. These corridors must therefore be taken into account when planning urban development and new buildings in a city.

What can cities themselves do?

The importance of cold air from surrounding areas for cities is increasingly recognized in German urban centers, but also in several other major international cities such as New Delhi, Lima and Seoul.

These metropolises commission meteorologists to measure temperatures and winds in various areas in and around the city in order to create a detailed climate atlas. Based on this data, cities can stop “closing” fresh air corridors by building on them.

Cities can also take other measures to significantly reduce extreme heat and improve quality of life – such as planting trees along streets, reducing the use of combustion engines or building more parks.

Edited by: Giorgos Passas