Cooling down hot cities: What are cold air corridors?

Cities get especially hot in the summer. The concrete and asphalt absorb and retain the sun's heat, so even the nights are extremely warm. So-called tropical nights with temperatures above 20 degrees disturb sound sleep. How to cool cities? Are the so-called cold air corridors helpful?

What are cold air corridors?

In the vicinity of the cities, the nights are much cooler. Even on the hottest days, it is up to 15 degrees at night. Cities could cool down somewhat, if that air could flow into them.

Corridors of cold air are green areas, rivers, lakes, railways and very wide streets with greenery. Cold air flows only a few centimeters above the ground, so embankments and buildings prevent this flow. That is why, when designing cities, care should be taken to keep such corridors free.

Why are the nights cooler in the area?

Quite simply, there is no concrete and asphalt there, but earth, sand and meadows prevail. There, the sun's heat is not absorbed in such a way that it stays until late at night. In addition, fresh air is created by the evaporation of water above the leaves, and in suburban areas there is usually a lot of greenery. That is why the air in them cools down faster after sunset. It is especially fresh in meadows and orchards, as well as over clay, sandy and peaty soil.

How does fresh air get to the city?

Topography is also very important. Cold air is heavier than warm air and collects near the ground, say in meadows. If the meadow is on a slope or on a hill, then the cold air flows down strongly. This creates a cooling wind.

Cities that are in valleys, on slopes or on hillsides can profit from that cool air. Through the corridor, the cold air can reach all the way to the center, pushing the warm air, which rises, and the fresh air cools the streets and buildings.

For example, Stuttgart lies in a valley and wants to maintain a local ventilation system based on its cold air corridors. Urban plans and construction are coordinated with this goal.

What can cities do?

All over the world, more and more attention is being paid to the possibility of cooling cities with the inflow of fresh air from the surroundings. Meanwhile, in Germany, numerous cities and regions have conducted appropriate climate analyses. They did the same around the world, in cities such as New Delhi, Lima, Lagos, Seoul, Melbourne and Portland.

Meteorologists first measure temperatures and air currents in different places in the city and its surroundings, enter the data into special computer programs and thus create detailed climate atlases of cities. On the basis of these atlases, cities can decide not to rebuild the corridors through which fresh air flows, to improve them and to protect and strengthen the natural cooling of the environment.

In addition, cities can introduce a number of measures for significant cooling, reducing the number of tropical nights and improving the quality of life. These measures include the removal of surfaces sealed with concrete and asphalt, planting trees along the streets, fewer fossil fuel engines, greening of roofs and facades, more parks and water bodies, brighter facades and buildings, increasing areas under shade.