Article by Adrian Reid

7 November, 2020

Cities get hot, much hotter than the surrounding countryside. Luckily there are all sorts of measures that the authorities can take to make cities cooler and safer for everyone. In fact, steps such as improving shade, trees, access to water and cooling breezes work so well, cities can be made comfortably cool, even in one of the hottest places on earth, Masdar City, UAE.

Article by Adrian Reid

7 November, 2020

Cities can be hot places to be in

The larger the city is, the warmer it gets. Satellite measurements show that New York, Mexico City, Kuala Lumpur and Tokyo are all about 10 °C (18 °F) warmer than the surrounding countryside.

That gets worse if humidity is taken into account. Wuhan, Karachi, Shanghai, Manila, Asunción and Tokyo all have a 'feels-like' temperature that is 20 °C (36 °F) higher than the surrounding countryside.

To make cities more liveable, especially as temperatures climb, we need to start changing our cities now.

There's a lot that can be done.

Cities are hot places to be in. 5033181, Pixabay

White paint

One of the oldest solutions to the heat has been to paint the town white. For countries like Greece and Turkey a splash of whitewash on the walls and roofs has been the time-tested method to avoid the Mediterranean heat. The white works so well that it reflects 85% of the sunlight hitting the roof.

This simple effect translates into dramatic reductions in temperature.

NASA research shows white roofs could cool a building by 23 °C (42 °F). If white roofs were introduced across a city, it could reduce the urban heat island effect by a third.

White paint keeps Santorini cool. Tamal Mukhopadhyay, Unsplash

If the idea spread world-wide, it would be equivalent to removing 44 gigatonnes of CO2 from the atmosphere. This is roughly the amount of CO2 that is added to the atmosphere each year. .

White roofs could be a cheap, low-tech way to cool the planet and give us more time to find a real solution to climate change.

Green cities

Or if you don't like white, make your city green. Planting trees and plants brings the temperature down. The greenery replaces hot concrete, and cools the air with moisture.

The result — a 2 °C (3.6 °F) drop in average temperatures.

Trees and plants don't have to be limited to parks and the roadside. Greenery goes anywhere..

In fact, by planting up the sides of buildings, as well as on the roofs, an area many times larger than the actual city could be planted in greenery.

In Singapore they call it Gardens in the Sky, with the goal of covering 200 hectares of the Singaporean cityscape in cool greenery by 2030. They are already more than halfway there.

Shade

Greenery also provides shade, and there's no more shady spot than under a tree on a hot day. But how many trees do you need to make it really cool? In 2016, Canadian scientist Carly Zeiter decided to answer that question. For most of the summer she went biking around the U.S. city of Madison, Wisconsin, measuring the temperature and the amount of trees covering each block.

What she and her team discovered is that it doesn't take a lot of trees to bring down the temperature. The scientists estimate that with just 40% tree cover over a typical city block could be enough "to trigger the large cooling effects that trees have to offer."

Trees provide shade in hot Barcelona. Nicolas Vigier, Public domain

Shade can also come from buildings and overhead cover — something often used in older, traditional towns in hot regions. In hot countries many streets are narrow and often covered to provide shade from the extreme heat. Behind the walls facing the street, people live around a courtyard, which is shaded on all sides.

Shade is an obvious way to keep cities cool. But in a crowded, congested city, where space is at a premium, wouldn't all the trees and narrow streets simply cause more traffic jams? They might, or you could just redirect the traffic.

Cars

Cars raise the temperature of the city. Exhausts and hot engines pump out heat. Dark-coloured streets made of tar and asphalt absorb the heat, keeping cities hot all night long.

In Barcelona, city planners decided that cars had to go. Their solution was to completely reorganise the city's traffic system.

Barcelona's revamp was based around "superblocks", or "superrillas" as it's known in Spanish. A superblock is a group of city blocks that cars can't enter. They can either stop and park in an underground carpark, or continue driving around the superblock, but at a reduced speed. The closed streets have been turned over to more people-friendly uses, plus more room for plants and trees.

Scientists believe the superblocks have resulted in cooler temperatures, and that each year this has saved 117 Barcelonians from a premature death, as well as the additional lives saved from air and noise pollution. Thanks superblock.

Wind

Sweating on a hot day is your body's way to cool down. All you need is a breeze, or at least a fan. Cities work the same way.

If the wind can get past the high buildings, and through city streets, it could work to cool down the city.

The problem is how to do this.

Stuttgart, located in the middle of Germany, is surrounded by hills which block the wind. To get a breeze working, Stuttgart's city planners have engineered "an entire system of urban air circulation." Their idea was to use tramlines as funnels for the wind.

Tramway tracks are wind corridors, Stuttgart, Germany. European Environment Agency (EEA), Use permitted

Water

On a hot day your go-to response more than likely involves water — whether it's to get a cool drink or dive in the pool. Water cools people down, and it cools cities down. Rivers, lakes, fountains, even paddling pools evaporate water and cool down the surrounding temperature, by as much as 3-8 °C (5.4-14.4 °F)

Where would all that water come from? Rain is free.

Most cities do their best to get rainwater out of the city as quickly as possible, down stormwater drains and off to rivers and canals. But some cities are now thinking about doing the opposite. They want to use rainwater as a way of greenifying the city.

The idea is called "sponge cities", and cities from Berlin to Beijing are adopting the idea. Where possible on rooftops or otherwise unused land, green areas are designed to absorb rainwater, and keep temperatures cool.

Masdar City

Masdar City lies in the United Arab Emirates (UAE), one of the hottest countries in the world, surrounded by the Arabian Desert. In this unlikely location, city planners have reversed the urban heat island effect, and created what must be the world's first cool city. They claim to have made the city 10 °C (18 °F) cooler than neigbouring Abu Dhabi.

The cool temperatures were the result of careful design. The entire city is oriented north-east to south-west, to benefit from cool winds at night and to minimise the incoming heat during the day.

A central feature of Masdar is the windcatcher. This is a traditional feature in Arabian courtyards, which Masdar planners have innovated on. The windcatcher funnels breezes from 45 metres (50 yards) high, down to the street level. As the wind is channeled downwards, water spray cools the air, turning hot desert gusts into a cool breeze.

The city also banned cars and replaced them with a personal rapid transport system (PRT), which transports people from carparks outside the city.

Without cars, Masdar was able to return to the traditional, narrower streets which both direct the breeze and allow more shade for residents.

Masdar City shows the potential of heatwave proofing cities. If Masdar's residents can stay cool in the hottest region on the planet, then other cities should also have a chance of staying cool in a mega-heatwave, with the right preparation.