Temperature

Thermal Comfort

At what temperature does a person feel comfortable? That depends on all kinds of things. Not only from the actual air temperature, but also from the radiation of the sun, the wind and the humidity. In addition, personal preference, age and behaviour play a role.

In the countryside it cools down in the evening, in the city it stays warm at night. Green interweaving of city and countryside is important for the ventilation of the city. Surveys show that people feel more comfortable in a green environment during warm periods. Green areas get less hot during the day and cool down faster in the evening than the built areas in the city and therefore reduce heat stress.

City design should make sure that there are open corridors that allow warm air to flow out of the city and cooler air from the surrounding area or from cooler parts of the city can enter the city. Note that heat in the city is not about one place. It’s about reducing the number of hot spots and increasing the number of cool spots.

Applying smaller green elements within a network of green streets and squares in combination with larger green elements such as parks and urban forests is probably the most effective approach.

Softening

Concrete, stone and asphalt heat up easily in the sun and release stored heat easily. The night time impact of paved areas is much higher than that of natural materials and vegetated soils. An effective method against heating is therefore to limit paved surfaces.

When it comes to maintenance of roads and sewers, it should be considered whether the existing dimensions of pavements are necessary. Little-used footpaths with a lot of weeds can be replaced by green areas. This increases the visual quality and cost-neutrally, and old paving tiles can be reused elsewhere.

For design of new areas, it is advised is to opt for green parking spaces, green squares and green streets and to limit pavement to only where necessary. Larger trees that are at least fifteen years old have more effect at lower management costs. It is also useful to use materials with a high albedo (reflection factor of sunlight from a material). White or light colours and natural materials often reflect sunlight better and absorb less heat.

Shading

Greenery provides shade and evaporative cooling and thus cools the air. Shading is particularly effective. Stony surfaces heat up less. Increasing the ‘canopy’ of the soil through the canopy of trees can significantly reduce the heat island effect. Healthy trees with many leaves and a large, wide, dense crown work best. Trees that are not healthy make a limited contribution. Good tree care (e.g. location, water supply, etc.) is therefore important for an effective contribution to a better climate.

Evaporation

Evaporation prevents solar radiation from being converted into tangible heat. In addition to shade, greenery also provides local cooling of the air through the evaporation of water from the leaves. This happens mainly in the afternoon, evening and early night, and is more profound from plants with a high leaf mass.

The cooling effect of evaporation only works if the plants receive sufficient water, even in drier periods.

All types of greenery contribute to reducing the heat island effect. Trees have the greatest effect due to their size and volume. And especially healthy mature trees with a good growing site and water supply. That’s why street trees, urban forests and parks are so important.

Due to increasing heat stress, urban green spaces need to be partly adapted, and supplemented with species that are more resistant to heat and drought. However, not every drought-resistant tree species cools down equally well. The cooling capacity strongly depends on the tree species and environmental growing conditions. Research into the ‘tree morphology’ of optimal climate trees provides important guidance.

The shape of the leaves and crowns, the permeability of the sun/shade, air circulation and how the branches are oriented are all examined.

Choose perennial plants with a deep root system, such as prairie plants. These colourful plants are more tolerant to dry summers and can be low maintenance. They include vigorous perennials and ornamental grasses that are highly suited to sunny places. They are commonly planted in a layer of lava rock or other inert medium. This layer retains water, up to a third of its weight in dry periods. The lava layer is also a hostile environment that limits weed recruitment.

Succulents, such as sedum, store water and evaporate it overnight. Sedum is therefore very suitable as ground cover for sunny and warm places. Watering is often not necessary. Most sedums can withstand long periods of drought.

More greenery at, on and around buildings is a natural measure to limit temperature rise in the built environment. Deciduous trees, creepers, and espaliers are suitable as natural sun protection for buildings. In the summer there is shade, in the winter the sun can warm up the building.

It is cooler in neighbourhoods with a relatively large area of green facades. Green façades have a major effect locally on peak temperatures that could arise between tall buildings. They also contribute to cooling through evaporation, especially at night. Because of the planting, the temperature of facades rises less during the day, so that they radiate less heat at night and the city can cool down better. In the vicinity of green roofs, the external temperature can remain two degrees cooler.

Streets

Research shows that in streets with or without greenery, the air temperature differs little. In streets with trees, the radiation temperature, however, is as much as four degrees cooler. Residents appear to have a preference for smaller urban trees and shrubs, interspersed with green front gardens. Above all, strive for variation in layering, colour and leaf shape of the greenery.

There is no single solution for design of green streets. After all, there are differences in the height and width of the buildings. Place trees where they have the most effect. Provide shade in places during the hottest time of the day (between twelve and four o’clock). And give people the choice to be able to stay in the sun or shade.

Also take the wind and air movement into account. Look further into the behaviour and use of public spaces by the residents.

Where people want to motivate slow traffic, attention should also be paid to (partially) cool walking and cycling routes.

Parking Spaces

The planting of trees along traffic roads and in parking lots keeps surface temperatures and wind chill lower. Paved industrial estates and parking lots in particular form large heat islands.

Due to shade (trees) in parking lots, less fuel evaporates from the tanks. Cars also heat up less.

Squares and Shopping Areas

The layout of squares and shopping areas is often constructed to accommodate large flows of people. Heat can be a major problem, because the perceived temperature can reach up to 47.5 °C during hot summers (Breda 2019).

With a wind chill above 35 °C, hardly anyone wants to stay there because standing still is too hot. This has consequences for the number of visitors and consumer spending. In addition to paying attention to heat stress among the elderly, children and the sick, it is good to look at which intensively used places in the city suffer from heat stress and to use greenery to do so.

View the Squares and Shopping Areas chapter

Parks

During warm periods when it gets (too) hot in homes and buildings, parks can be a relatively cool haven, especially for the elderly. To do this, these green areas must be within easy walking distance. That is why it is good to create a network of smaller and larger green areas throughout the city.

The layout of a park determines the local microclimate. Is there a good balance between sunbathing areas and large trees? Ten percent more tree cover results in a reduction of more than three degrees in the radiant temperature.

Research shows that it is important to design and furnish a park with a variety of spaces: 40% sun, 20% partial shade (border area with legs in sun and head in shade) and 40% shade. This ratio is based on preferences of park visitors at different times of the day and under different conditions (summer and tropical).

View the Parks and Cemeteries chapter

Cool Neighbourhood

To make optimal use of the cooling effect of parks and other green areas on the environment, cool air from a green space must be able to flow into the surrounding neighbourhoods. The green space should therefore not be enclosed, but have openings that connect to corridors through which the cool air can flow into surrounding parts of the city.

The cooling effect of parks and urban forests can extend to a distance of up to 1.5 km. Studies show that the effect is between one and four degrees, depending on the size of the park, the local conditions, and the distance to the park. Two smaller interconnected parks can have a greater cooling effect on the environment than a single park.

View the Neighbourhood and Community Green Spaces chapter

Water

The contribution  presence and appearance of water in the form of, for example, a pond mainly contributes to a subjective feeling of cooling. Shallow, stagnant water heats up and is a heat source during hot periods by retaining heat longer. Running water, such as a waterfall, water wall or fountain, does provide extra cooling locally, because water-in-motion evaporates more easily.

Along banks and quays, water provides space for a cooling layout combined with trees. Choose solutions that retain water for irrigation of greenery.

View the Water Theme

Wind

Wind can be cooling, but too much wind can be a nuisance for cyclists and pedestrians. Wind nuisance often occurs around tall buildings. This makes the use of walking routes and terraces less attractive. Well-placed trees can act as windbreaks to protect against strong winds.