More urban green keeps the city cooler.

Garden roof in Stuttgart, Germany © Optigroen


The increasing urbanisation, ongoing density and paving of ground surfaces affect not only the water mission, but more and more are causing higher temperatures and heat stress in towns and cities. This effect is reinforced by climate change and the resulting increases in temperature. Recent studies show that in the Haaglanden and Rotterdam regions the difference in surface temperatures between rural areas and highly urbanised areas can be as much as 10°C on hot days. Duyzer et al., 2011
Higher temperatures cause a higher mortality rate and affect the health, wellbeing and productivity of people, but also of flora and fauna. Higher temperatures are a direct cause of greater energy consumption for cooling and greater water consumption for cooling and for irrigating green areas. Higher temperatures have a direct impact on the quality of water. Urban planning solutions that incorporate more vegetation, more trees, green roofs, fewer impervious surfaces and the use of materials with high albedos (a material’s albedo is its reflection factor) can cause temperatures to drop in towns and cities. Other factors such as the sky view factor (the portion of the firmament that is visible from a particular location) and the emissivity of surfaces (the amount of heat radiating from a surface) also play important roles in causing heat stress. Duyzer et al., 2011

Beach at the river Maas in Rotterdam, The Netherlands © Steven Barnhard

On a larger scale, proper green interconnections between urban and rural areas for urban ventilation is important for creating air flows between the town or city and the countryside, for realising easily accessible recreation areas and for achieving cool routes for slow traffic.

Urban heat islands and heat stress

Wherever urban conglomerates arise, natural and open permeability and vegetation are largely replaced by impervious surfaces in the form of streets, squares, car parks and buildings. In practice, impervious areas in towns and cities prove to be substantially hotter on hot summery days than rural parts. This phenomenon is commonly known as ‘Urban Heat Islands’.

De temperatuur in de stad kan meer dan 8 graden oplopen ten opzichte van de buitenwijken en het omringende platteland  en
Urban heat island © EPA, UHI Basics, 2008
Nighttime irradiance balance shows ground heat emission. the dew, frost and condensation heat precipitates part of the emission is absorbed and in the air. The irradiance balance on a summer day shows that there is 16 per cent diffuse and 30 per cent direct solar radiation. some is distributed in upper air layers and some is absorbed in the atmosphere the ground heat heats up and emits heat radiation, reflection and heated air and some of the ground heat evaporates.
Radiation balance during the night and on a summer day - Source: atelier GROENBLAUW, based on Krusche et al., 1982

A distinction is made between Surface Urban Heat Islands (SHIs) and Urban Heat Islands (UHIs). The term used in the United States for the latter makes the distinction clearer: Atmospheric Urban Heat Islands. SHI refers to surfaces temperatures, while UHI concerns the temperature of the air above the ground, up to the level of buildings and tree tops. Naturally the two are connected.
As the day progresses, significant differences arise between the surface and air temperatures above surfaces with different designs. EPA, 2008


Air and surface temperature during the day and night © EPA, UHI Basics, 2008

This image shows a cross-section of the city. This city has a highly compacted and petrified city centre, less petrified suburbs and a green outer area. In doing so, the image shows four temperature curves showing temperatures across the city cross-section: surface and air temperatures during the day and night. 

Temperatures are higher during the day than at night. What is striking is that the surface and air temperatures are almost the same at night, with them being higher in petrified areas than in green areas. In contrast, these two temperatures are different during the day. The air temperature is the same everywhere in the city regardless of the degree of petrification. The surface temperature is considerably higher as petrification increases is almost always considerably higher than the air temperature. Only above water are these almost the same.

The lack of vegetation

In natural surroundings with vegetation and trees, the trees offer shade and keep surface and air temperatures down. In the shade the perceived temperature is 10-15°C lower than in the sun. A larger percentage of buildings and other impervious surfaces also diminishes the evaporation from the ground (and plants). Evaporation processes have a cooling effect on air temperatures and surface temperatures.

In general, increasing the green percentage by 10 percentage points leads to a decrease in air temperature by roughly 0.5°C. Kluck et al., 2020

55 per cent of rainwater runs off, 10 per cent infiltrates into the topsoil, 5 per cent infiltrates into the subsoil and 30 per cent evaporates.
City water balance © EPA, UHI Basics, 2008
Natural water balance © EPA, UHI Basics, 2008

Albedo and emissivity

The qualities of the surface materials in towns and cities, in terms of their capacity to reflect sunlight, their emissivity (which determines the amount of long-wave radiation emitted by the surface, and as such the surface temperature), as well as their capacity to absorb heat, affect the surface temperature and consequently the air temperature in the town or city. The capacity to reflect sunlight is commonly known as albedo. As a rule, the visible portion of the spectrum reflects better from light-coloured materials. However, darker materials also exist with special pigments that better reflect the infrared in sunlight. This is still the subject of a great deal of research and development.
Albedo is expressed as a value between 0 and 1 to indicate what proportion of sunlight is reflected.

Table Albedo value of different (roof)materials:

Materials Albedo value
Black built-up roof 0.04 – 0.05
Built-up roof (white gravel) 0.30 – 0.50
Asphalt 0.04 – 0.15
Light gray asphalt 0.25 – 0.27
Dark concrete tiles 0.05 – 0.35
White concrete tiles 0.70
Metal roof unpainted, corrugated 0.30 – 0.50


Paving and building materials used in urban areas generally have a lower albedo than areas with vegetation. The materials used in urban areas reflect less and absorb more sunlight, which naturally results in higher surface and air temperatures.
Most building materials, with the exception of metals, have high emissivity coefficients, meaning that those materials start radiating heat if the temperature rises even slightly. Metals do not start radiating heat until they reach much higher temperatures; compared with stony materials, for example, metals heat up to higher temperatures. Emissivity is determined by the material’s surface (or finishing). Painted metal has the emissivity value of the paint. EPA, 2008


Table emissivity values of a few materials:

Materials Emissivity values
Aluminium, polished 0.05
Aluminium, oxidized 0.25
Brick 0.85
Concrete 0.54
Glass 0.92
Black varnish, mat 0.97
Black varnish, gloss 0.87
White varnish 0.87
Paint, silver color 0.31
Water 0.98
Zinc 0.20

Heat absorption

Most urban building materials also have a greater heat absorption capacity than natural surroundings, which similarly contributes to higher temperatures in towns and cities. As a rule, urban environments absorb twice as much heat as rural areas EPA, UHI Basics, 2008.

Sky view factor and internal reflection

Urban planning and the geometry of the buildings also affect the emergence of Urban Heat Islands. Closely-built high-rises that form urban canyons offer shade during the day but do not allow the area to cool down at night. This is described using the concept of the sky view factor, which indicates what percentage of the firmament is visible from a given location on the surface. A high sky view factor, for example in an open field, enables reflection of short-wave sunlight radiation because of the absence of internal reflection. In urban canyons, the low sky view factor interferes with the night-time emission of long-wave radiation (the heat coming off buildings at night). EPA, 2008

Sky view factor and internal reflection © EPA, UHI Basics, 2008

Anthropogenic activity

Of course anthropogenic activities such as traffic, air conditioning and production processes produce heat and as such certainly play a part in the Urban Heat Island effect. Their contribution is difficult to quantify, and in the Dutch situation is estimated to be relatively minor. Duyzer et al., 2011

Geographic factors

The location of a town or city, for example in a valley or on a mountain or on the shores of a large body of water, as well as the direction of the prevailing wind and the orientation of the buildings in relation to those factors all affect the emergence of heat islands.

Higher emissions and energy consumption for cooling purposes

Hot spells during the summer also affect the amount of energy consumed: the peak load is in fact extremely high on summery days. Naturally the higher energy consumption for cooling purposes also influences the CO2 emissions of power stations and other emissions. As a consequence, smog tends to form during hot spells in the summer.

Example of the city New Orleans (USA) shows the graduate increase in the need of energy at the moment the air temperature is above 20°C - 25°C © EPA, UHI Basics, 2008

In the Netherlands, non-residential buildings already use more energy for cooling in summer than for heating in winter. Residential buildings are also increasingly using energy for cooling.

Temperature and the formation of smog

A direct connection also exists between the formation of smog and the air temperature. Hot air carries pollutants and particulate matter to higher altitudes, creating a smog bubble. Cooler air from outside the town or city heats up on the periphery and ceases to penetrate into the city centre. At night, the cushion of hot air can cause an inversion, preventing the town or city from cooling down and stopping the air from mixing with clean air from the countryside. Krusche et al., 1982

The formation of smog during day and night impact on air temperature in the city © Krusche et al., 1982

Water quality

Urban heating also affects the quality of water. Rainwater running off from hot roofs and roads on hot summer days can be heat up from a few degrees to as much as 17°C. The surface water into which it drains also heats up. The temperature of the water impacts every process in the water. The higher temperatures cause undesirable algae to increase in number, kill fish and lead to botulism. Botulism is poisoning caused by bacteria that is particularly lethal to fish and birds. Certain substances also present a danger to humans. EPA, 2008

Green space in London © Peter Herring

Labour productivity

The increasing number of tropical days with extreme temperatures in excess of 30°C can cause labour productivity to drop and the number of health problems and accidents to increase. The website of trade union FNV Bondgenoten,, offers the following summary, as well as a heat stress calculator.
‘Heat stress can cause the following problems:

  • Skin conditions, such as itching and blister rashes
  • Headaches
  • Heat cramps and heat edema
  • Heat exhaustion
  • Heat syncope, fainting, headaches, nausea and diarrhoea
  • Heat strokes
  • Reduced concentration, with a greater possibility of accidents.

This presents particular risks for certain professions, for example bus drivers and people working at altitude.’ FNV, 2011

All these risks apply particularly to vulnerable groups, such as people already suffering from ailments such as high blood pressure, lung conditions or diabetes, people recovering from illness, pregnant women, overweight people, people using certain medicines and people with sensitive skin.
The effect of the heat is calculated using the ‘Wet Bulb Globe Temperature’ (WBGT) method and is referred to as the WBGT index. That index incorporates temperature, humidity, physical activity and clothing. The WBGT was developed by the US Marine Corps to help reduce the number of heat heat-stress-related accidents. US Navy NAVAIR, 2011
For a hot summer’s day with a temperature of 30°C and a relative humidity of 60%, summer clothes and direct sunlight, very little wind, no acclimatisation and intense physical activity, the heat stress calculator based on this method gives a WGBT index of 29.34. According to this standard, this means that a construction worker, for example, should stop working. If the temperature is 25°C and the other parameters remain the same, the index is 24.79: the appropriate response is to work 50% of the time and rest 50% of the time FNV, 2011.
Neither the WBGT index nor the heat stress calculator are official instruments in the Netherlands. However, they offer an indication of the possible implications of heat and work FNV, 2011.
The WWF and the Kieler Institut für Weltwirtschaft conducted a study researching the effects of climate change on people’s health and performances in relation to temperature and quantifying the results in economic terms. That study took 16 towns and cities in Germany as examples. In Germany, around 24,500 more patients per year are already hospitalised as a result of heat-related complications. Transposed to the Netherlands, this translates to around 5,000 individuals. The reason is the tremendous burden on the patients’ cardiorespiratory systems caused by the heat. Very different studies show the precise loss of labour productivity to vary from 3% to 50%. U.S. Navy NAVAIR, 2011

German research (WWF/KIW) shows that office workers still function to their full capabilities at 23°C. At 30°C, they only function at 70%. It is an established fact that at 30°C typing speeds drop by half compared with 20°C. For physical labour, a 50% drop in performance is given for temperatures of more than 30°C. The German studies justifiably qualify these percentages, as it is unclear whether they represent a reduction the maximum performance rates of the individuals studied or to what extent that level of performance should be achieved in day-to-day working situations. Hübler, 2007 However, it is safe to conclude that both mental and physical labour productivity drop if temperatures exceed 25°C. The rates of decline given in the literature are particularly high for physical labour.
During the heat wave in 2003, sick leave rose from 3.4% to 11.5%. Drunen et al., 2007


The Dutch medical journal Nederlands Tijdschrift voor Geneeskunde identifies the following health implications of heat:

Excess mortality rates

Mortality rates rise during hot spells. The elderly, people with heart conditions or respiratory conditions and other vulnerable populations become more susceptible. At the same time, relatively fewer people die during the warmer winters, and some of the excess mortality concerns people dying days before they otherwise would have. The link between air pollution and summer smog is also a factor in excess mortality rates. Maud, 2009


The longer growing season means that the period during which people with allergies or asthma are affected is protracted. Around 15-20% of the Dutch population suffers from pollen allergies during the growing period, and the period in which pollen allergies occur is currently a month longer than it was previously. In addition, plants from southern Europe that are known for their allergenic properties are becoming more and more common in the Netherlands. Other causes of allergies such as house dust mites and processionary caterpillars are also becoming more and more common. Maud, 2009

Vector-borne infectious diseases

Vector-borne diseases such as Lyme’s, which is caused by the bite of infected ticks, are becoming increasingly common as the infection is spreading geographically, in part as a result of climate change. The starting moment and duration of the season are also expanding. Climate change is also affecting the ecosystem, causing changes that are beneficial to the propagation of hosts. In the Netherlands, the number of tick bites tripled between 1994 and 2005, as did the number of people infected. Very little data is available in the Netherlands about other vector-borne diseases. Maud, 2009

Water-borne and food-borne infectious diseases

The higher temperatures will cause an increase in water-borne and food-borne diseases, for example as caused by green-blue bacteria. The higher temperatures will also impact the drinking water distribution grid. For example, the reports of Legionnaire’s disease are increasing in number, particular during summer months. Maud, 2009

Measurements: Haaglanden and Rotterdam

In the Rotterdam-The Hague conurbation, research institute TNO studied the difference between surface temperatures in the urbanised area and the surrounding countryside. See the reproduced map below Duyzer, 2011.

Average surface temperature during the day in the research area based on the 15 Landsat images converted to a resolution of 30 m © Duyzer, 2011

The data shows that some parts of the urban area can be up to 10°C hotter than in the countryside. In particular, districts with large proportions of impervious surfaces and little vegetation, business estates and high-density residential areas with little vegetation heat up most. The TNO study revealed that Schildersbuurt and Transvaal in The Hague and Nieuw Mathenesse and Spaanse Polder in Rotterdam are the hottest. The hottest districts were shown to be in the biggest cities, i.e., that the size of the town or city also affects the risk of heat stress. This is an important finding in connection with spatial planning for the increasing urbanisation.

‘The district-average surface temperatures (SHI effects) prove to show a strong correlation with spatial planning parameters, such as the fraction of the area that is covered by grass (category: low vegetation), the fraction covered by woods (high vegetation), the built-up area (impervious construction), the paved area, such as streets and squares (paved).
Besides spatial planning parameters, physical parameters such as emissivity and albedo also prove to be important. Like high albedo factors, large proportions of vegetation result in relatively low temperatures. The fraction of low vegetation and the average albedo of a district are the parameters that best serve to explain the variation in surface temperatures. These spatial parameters therefore have the greatest impact on temperature and probably on the degree of thermal comfort too. In addition, more meteorological parameters such as wind, humidity, sunshine and shade are also important factors in determining thermal comfort.’
Duyzer, 2011

General solutions

Cool impervious materials

In the United States, research has been going on for years into heat stress and measures to reduce it. For example, ‘cool paving materials’ and ‘cool roofs’ have been developed. Some cities (including Chicago, New York and Houston) are putting a great deal of effort into preserving and expanding green surfaces.

Albedo of conventional asphalt and concrete pavement changing over the years © EPA, UHI Cool Pavements, 2008

Conventional paving materials such as concrete and tarmac reflect 5% to 40% of the sunlight, meaning that they absorb 60% to 95%. It should be noted, though, that these values change over time as a result of aging and pollution. Paving materials have been developed in the United States that can reflect 75% of the sunlight. EPA, UHI Cool Pavements, 2008
Research shows that albedo and emissivity are the factors with the greatest influence on the surface temperatures of materials. Permeable materials cool down faster as a result of evaporation. In dry conditions, the surfaces of permeable materials heat up faster because less heat is transferred through the material. The benefit, however, is that night-time emissions are less because these materials accumulate less heat. The factors to be considered when deciding on the type of paving material are complex, and further study is required before more nuanced pronouncements can be made. Pilot projects are being conducted in Osaka and Tokyo using permeable paving materials, where the ground below is kept moist through drainage or water is sprayed onto the paving using sprinklers. The first results of these pilots are promising EPA, 2008.

The thermal properties of paving materials also depend on other factors, which are only mentioned briefly here. Concrete and other building materials often have twice the heat retention capacity of natural materials such as dry soil and sand. This means that the night-time emissions from concrete are significantly higher than those from natural materials and surfaces with vegetation.
In recent years, research has been conducted in the United States into the thermal properties of paving materials, and new and modified materials have been developed, such as permeable concrete and tarmac, as well as special additives and finishing layers to increase albedo and emissivity EPA, UHI Cool Pavements, 2008.
Cities such as Houston have developed policy and implementation programmes for using cool paving materials. Paving materials have not yet been considered in the same fashion in Europe.

The role of vegetation

The most effective measure for reducing surface temperatures and keeping the city cool is to limit the percentage of impervious surfaces. Besides reducing heat, green unpaved surfaces also offer advantages for the water mission while also presenting increased potential in terms of biodiversity and quality of life.

Providing shade for impervious surfaces by planting trees on squares, in car parks, along roads and in private areas also helps to reduce surface temperatures and the perceived temperature in the city.

Green areas are still under a great deal of pressure in and around towns and cities. More trees are still being chopped down than planted, and more green areas are being replaced by buildings and paving than are being added. This is true not only of the increasing urban density in the Groene Hart zone of the Netherlands, but also of small oases in towns and cities. A movement has recently emerged that is calling for more green in towns and cities. That movement is beginning to affect the policies and planning of some municipalities. This is taking the shape of green roofs and green facades. More important than green roofs and green facades, however, are the retention of existing green areas at ground level and preferably also the creation of green corridors and grids in order to have an essential impact on the urban and living climate.

Trees and vegetation have a cooling effect on the climate of the town or city: the shade they provide means that less sunlight hits the ground and the moisture evaporating from their leaves absorbs heat. In general, 10 percentage points more green in the city or a neighborhood (40% green instead of 30% green) leads to a reduction of the air temperature by roughly 0.5°C. Kluck et al., 2020

Reflection of 100% sunlight on a leaf provides: 5 to 20% photosynthesis, 20 to 40% evaporation, 5 to 30% filtered green light, 10 to 50% Heat, 5 to 30% reflection.
Energy balance of a leaf - Source: atelier GROENBLAUW, based on Krusche et al., 1982

Evapotranspiration – evaporation of water caused by vegetation and the ground below and around it – also helps to reduce heat stress. This effect can result in air temperatures that are 2-5°C lower in the immediate vicinity.

During extreme heat this is not enough. The other option to keep the city livable is by creating enough cool places. In the shade of a tree, the perceived temperature is 3 to 19 °C lower than in the sun. Creating shade is therefore the most effective way to lower the perceived temperature. Trees and groups of trees are the most effective means of creating cool places and cooling the citizens on hot days, because they provide shade and cool the air through evaporation. Kluck et al., 2020

In the shade of a tree, only 10-30% (depending on the species of tree) of the sunlight reaches the ground in the summer. Various studies conducted in the United States have also measured that walls standing in the shade of trees heat up by an average of 20°C less. EPA, 2008


© NEXIT, Poelmans Reesink

Trees and other plants in towns and cities not only help to keep temperatures down but also help to clear the air of dust and other pollutants, retain rainwater and improve the overall quality of life. Not the least important consequence is that this increases the value of the living environment.

Creating shadow and sun shades

Creating shade is the most effective way to lower the windchill locally because it prevents the body from heating up due to sunlight. This can be done in many ways. Trees are most effective because they not only provide shade but also contribute to cooling through evaporation. But when there is little room for trees, shade sails, pergolas, umbrellas, arcades and narrow streets with high buildings such as in Mediterranean countries are also good ways to provide shade. An advantage of some shade facilities is that they can be removed at night, allowing the heat stored in the soil and buildings to escape and cooling the city more quickly. External solar shading is the most effective to keep buildings cool inside.