Air & surface cooling
maximised by SIMPaCT
You may have heard or read something about the Urban Heat Island (UHI) effect. But have you come across the Park Cool Island (PCI) effect?
Both effects are described in the scientific literature. SIMPaCT is maximising the PCI effect of Bicentennial Park in Sydney. Here is the explanation how it works.
Example of the surface cooling effect from parks and other urban green infrastructure. The left image shows the normal view of a residential neighbourhood in Sydney. The right image shows the infrared view of the same place where surface temperatures are represented by different colours. The coolest surface temperatures can be observed for shaded and for unshaded vegetation. Lawn in the park has a surface temperature around 30°C, like the ambient air temperature when the image was taken at 15:00 on 26 December 2020. Water transpired from shrubs and trees will cool ambient air. In contrast, all hard infrastructure, including roads and roofs have much higher surface temperatures. Solar energy stored in these surfaces is radiated back into the environment as sensible heat, whereby it warms ambient air. Image © Sebastian Pfautsch
The way you feel temperature in a city is the result of many different effects. These effects scale down many levels, from the geographic location – anything between polar and equatorial, from sea level to mountainous and costal to continental – down to what type and colour of clothes you are wearing. From the macro to the micro scale.
There is the temperature and relative humidity of the air surrounding you and the speed by which this air is moving around you. The level of direct and indirect solar radiation you are exposed to. The type of activity you are doing and how much you sweat. And then there are all the external sources of additional heat, like heat that is radiated from roads, carparks, buildings and other hard infrastructure and heat generated from human activity, like cars, air conditioning systems or factories.
In 2021, Working Group 1 of the Intergovernmental Panel on Climate Change (IPCC) published The Physical Science Basis Report. For the first time, the 234 scientists from 64 countries created a Fact Sheet about climate change and cities. Why? Because our world is rapidly urbanising, and cities are major sources of heat which impacts billions of people. Moreover, the scientists reported with ‘very high confidence’ that “urbanisation has exacerbated the effects of global warming in cities”. Cities are sources, and their communities are victims of increasing air temperatures.
The absorption, storage, and radiation of heat by grey infrastructure leads to urban warming. Plants and water bodies help cooling our cities. To increase the area in cities covered by parks, forests and gardens is the most effective way to tackle the issue of the UHI effect and urban overheating.
The warming and cooling effects of grey (top three), blue (Water) and green (Vegetation) infrastructure in cities. The figure was published in the Regional Fact Sheet – Urban Areas by the Intergovernmental Panel on Climate Change (IPCC) in 2021. Image © United Nations Environmental Program (UNEP) and World Meteorological Organisation (WMO)
Collectively termed urban green infrastructure, plants can reduce the air temperature across entire metropolitan regions by several degrees Celsius. Research at Western Sydney University has shown that during a heatwave, air temperatures in a in well-shaded residential street was nearly 8 °C lower compared to a nearby street that had barely any trees to shade it.
Of all urban green infrastructure elements, which also include green walls and vertical and rooftop gardens, trees are the most effective elements for cooling. Urban trees are the natural air conditioning systems of our cities where they provide cooling in two ways: 1. by shading grey infrastructure, especially ground surfaces and 2. by transpiring water from their leaves.
The best shade is provided by densely foliated, wide tree crowns that are low to the ground. These crowns belong to older trees that had time, water, nutrients, and space to grow tall and expand their branches freely. Dense tree shade can reduce the surface temperature of a road by more than 40°C. By blocking direct solar radiation, shade from a tree brings the temperature of any surface down to ambient air temperature. This has very important implications when we try to reduce the UHI effect. More tree shade in cities will lower surface temperatures, and cities with more trees are consequently cooler.
While the principle of shade-cooling is an easy concept to understand, transpiration cooling is a little more complicated. Soil usually contains some moisture. The atmosphere usually contains much less moisture (except when it rains), and the resulting difference between soil and atmospheric moisture creates a gradient of energy. Evolution has shaped trees into organisms that can utilise this energy gradient to grow into the tall (more than 100 meters), long-lived (more than 4,000 years) organisms we see around the world today.
Trees use the moisture gradient between the soil and atmosphere to allow water to be passively sucked through them. Losing water at the leaf level into the dry atmosphere initiates tension in the capillaries of water that stretch through the leaf-petiole-twig-branch-stem-root continuum. The capillaries run through a water transporting structure called xylem. By simply allowing water to escape from the leaf, the xylem will transport soil moisture from the root to the leaf to replace what was lost.
During the process of transpiration, water inside the leaf undergoes a phase change from being a liquid to becoming a gas. This process, where the water molecules are pulled apart, requires energy, which is provided by solar radiation. Using this energy leads cooling of the leaf itself and as the leaf is in contact with the surrounding air, it lowers the temperature of this air. Tree crowns can have many thousands of leaves and wind will mix the cool air from inside and around the tree crown with the warmer air from the city. Millions of leaves will provide enough cooling to reduce the air temperature inside an entire city.
Now we can get back to the PCI effect. Not only trees, but also grass, shrubs, and any other vegetation usually transpire water. Some plant types in parks are more effective in cooling surfaces, others are cooling the air or both. Take lawn: it is effective in cooling the surface temperature, but not very effective in cooling the air as it is not tall enough to be much affected by wind. Take a tall tree with a large crown: it can transpire 300-600 litres of water in a single summer day, providing ample air cooling and surface cooling through its shade. The PCI effect describes how the combination of all sources of cooling in a park lower the air temperature around the park.
Large free-standing, deciduous park tree. The large crown provides many square meters of shade whereby it reduces surface temperature. Water loss through transpiration from the leaves cools the ambient air. As shade trees provide cooler air and surface temperatures, while at the same time block harmful UV radiation, it is understandable that most park visitors enjoy spending time in the shade of trees where human thermal comfort is greatly improved. Image © Sebastian Pfautsch
Studies have found that the type, age and density of vegetation, the way the vegetation is arranged, and the overall size of a park influences the PCI effect. In addition, the time or day, season, park geometry and morphology as well as wind channels can have an effect.
Two parameters are usually used to describe the PCI effect: the Cooling Effect Intensity and the Cooling Effect Distance. The Cooling Effect Intensity varies due to the above-mentioned influence of vegetation, park size and management. Irrigated parks are usually cooler than those that are not irrigated. The relationship with size was found to be curvilinear, whereby the Cooling Effect Intensity increased rapidly from a pocket park to about 10-15 hectare by up to 3°C, after which the park size had only a very small effect. Increasing the Cooling Effect Intensity to 4°C would require a park size of 100 hectare.
Park size was found to have a positive and near linear relationship with the maximum Cooling Effect Distance. Although not measured empirically yet, for the size of Bicentennial Park, it is reasonable to expect this distance to be 250-500 meters downwind in summer. This will lower ambient air temperatures in adjacent residential and office precincts, which in turn lowers energy requirements for indoor cooling.
Bicentennial Park is fully irrigated. With SIMPaCT, we will use machine learning to fine-tune the delivery of the irrigation system in a way that leaves all plant types across the park optimally hydrated, especially in summer. This will support maximum transpiration rates throughout the day, keeping air and surface temperatures low, generate the largest possible Cooling Effect Intensity and Cooling Effect Distance. SIMPaCT will deliver the greatest possible Park Cool Island Effect of any park in Sydney, and possibly anywhere in Australia.
Come and visit the park and enjoy its coolth at any time and be amazed how much cooler its tree groves, shaded benches and playgrounds are next summer.