Bringing people into contact with water is a very effective way to reduce heat stress. But contrary to popular belief, water only cools when it evaporates (on the skin) or through a moist air flow that has been cooled. Standing and even moving surface water has hardly any cooling effect.
For example, due to evaporation of water from ponds, canals and rivers, the air temperature only drops very slightly: up to a maximum of 1°C. Due to evaporation of water from fountains and misting systems, the local air temperature can drop somewhat more sharply: up to a maximum of 3°C Jacobs, C. et al. 2020,Völker, S. et al. 2013.
Application and design
Water that is suitable for swimming, playing or dipping your feet in can provide cooling. Water playgrounds, swimming water, fountains, cheats and misting systems are therefore measures that can be effective against heat in the city. Canals, canals and ponds in an urban environment usually do not help to create local coolness in the city. During hot days, this type of small urban surface water does not lower the air or perceived temperature on the street, even when the water is flowing. Yet these types of urban water elements often provide the opportunity to create green, shady or windy places where the perceived temperature is lower. It is therefore good to design urban water in combination with shade, green elements, fountains and ventilation strategies around urban water. In addition, the water naturally offers more cooling if it is possible to swim, paddle or play in it. This includes water play areas, swimming water, fountains, tricks and misting systems.
Ways of cooling with water
- Fountain: Fountains provide cooling through evaporation. Because many small water droplets have a relatively larger contact surface with the air than one flat water surface, there is more evaporation. The smaller the droplets, the more evaporation and the more efficient the cooling of the air.
- Nebulization: Nebulization of water is an effective cooling measure. Misting installations can provide cooling in two ways. Firstly, the many small water droplets ensure that the contact surface with the air is much larger than one flat water surface. In addition, the water mist also immediately cools the skin because the water droplets moisten people’s skin and extract heat from it through evaporation. Water atomization is especially effective in combination with a fan, because wind accelerates the heat exchange between wet skin and the environment, and thus cooling.
- Wetting streets: Wet streets can cool the air temperature and the surface temperature of the streets themselves for a few hours. To achieve this, it is necessary to regularly wet the streets. The question is whether this is a useful measure, because there is often a shortage of water during periods of heat.
- Urban water with a large surface area: if the water surface area in or near an urban area is large, this can have an effect on a larger scale and in the longer term. The air temperature in the city will then become somewhat lowerRonda, R.J. et al. 2017.
In addition to the cooling effect, certain water measures can provide the following benefits:
- Water spray has the advantage of being visible. This allows people to choose whether or not to be cooled by the mist.
- Fountains can have a positive effect on the oxygen balance of surface water.
The cooling effect of water depends greatly on the form in which the water is used.
Fountains can lower the air temperature by 1 to 4.7°C. The perceived temperature (PET) therefore decreases by 1 to 5°C. Model studies for the Dutch climate show that evaporating droplets from smaller fountains can have a local cooling effect on the air temperature. At a height of 1.5 m, a fountain can cool the air temperature by up to 1°C. For the perceived temperature, this local effect can be approximately 3°C (PET). In warmer climates, fountains can reduce air temperature and wind chill by up to 5°C. Usually the cooling effect of a fountain is very local and has little effect at square level. But the extent to which the cooling effect extends also depends on wind speed and direction. The cooling effect of fountains is greater on the leeward side: that is where the wind has blown through the fountain. In practice, especially very large fountains can also provide cooling for the environment. This can happen when the spraying water draws the cooled air around it, thus creating a cooling air flow.
Misting installations reduce the air temperature between 0.7 and 30°C . Water fogging is especially effective in combination with a fan or wind. Wind accelerates cooling due to the faster heat exchange between wet skin and the environment. An experiment was carried out in Japan with a misting installation that was combined with a fan. This combination caused the skin temperature to drop by 1 to 3°C within 10 seconds.
Wet the streets
By wetting streets, the air temperature can drop by 0.8 to 3°C. Experiments in the Netherlands show that the air temperature at a height of 2 meters on a wet street can drop by a maximum of 2°C. And this can reduce the surface temperature of the street by up to 13°C. Wetting the street also ensures that less solar radiation is reflected (the albedo decreases). This also reduces the perceived temperature for pedestrians and cyclists on the road. A side note is that the evaporation of the water can increase the humidity. Wetting streets can also have a negative effect on the perceived temperature, but this effect is negligible for the Dutch climate.
Ponds, canals and rivers (small area)
Surface water from ponds, canals and rivers in the city usually offers only very limited cooling: a maximum of 1°C. The energy required for evaporation is extracted from the water, causing the water temperature to drop only to a limited extent. Whether the evaporation ultimately leads to noticeably cooler air depends on the temperature difference between the water and the air and the size of the water surfaceKluck, J. et al. 2020.
- Surface water from ponds reduces the air temperature by 0.5 to 0.7°C and the perceived temperature (PET) by 0.6 to 3.6°
- A lake reduces the air temperature by 0.5 to 1.6°
- Canals and canals reduce the air temperature by 0.1 to 0.8°C and the perceived temperature (PET) by 0.2 to 2°
Large areas of water
Large areas of water in or near the city can noticeably cool the air temperature, up to 4°C. For example, the cool water of the IJmeer during a hot day can ensure that it is a few degrees cooler up to a few hundred meters inland in the east of Amsterdam.
- Jacobs, C., Klok, L., Bruse, M., Cortesão, J., Lenzholzer, S. & Kluck, J. (2020). Are urban water bodies really cooling. Urban Climate, 32, 100607
- Kluck, J., Klok, L., Solcerová, A., Kleerekoper, L., Wilschut, L., Jacobs, C., & Loeve, R. De hittebestendige stad, Een koele kijk op de inrichting van de buitenruimte, Hogeschool van Amsterdam, 2020
- Ronda, R.J., Steeneveld, G.J., Heusinkveld, B.G., Attema, J.J. & Holtslag, A.A.M. (2017). Urban Finescale Forecasting Reveals Weather Conditions with Unprecedented Detail. Bull. Meteorol. Soc., 98, 2675-2688
- Völker, S., Baumeister, H., Classen, T., Hornberg, C. & Kistemann, T. (2013). Evidence for the temperature-mitigating capacity of urban blue space – A health geographic perspective. Erdkunde, 67(4), 355-371