We all know that standing under a tree canopy is cooler on hot sunny days. But few wonder why that is and how greenery is so effective in cooling the air. In theory, trees cool the surrounding air in two ways: by blocking sunlight and by producing cool air through the process of transpiration. This article will delve deeper into how these two processes work.
Blocking sunlight, or as many people commonly refer to it, providing shade.
Ground surfaces, roads, and buildings have a great capacity to absorb heat and reflect very little of it. This is why we should avoid walking barefoot on concrete. Some people are surprised to find that the soles of their shoes nearly melt after walking outside in the daytime for too long.
In fact, tree canopies can block up to 90% of sunlight and the accompanying heat.
Meanwhile, trees with their branches and leaves can prevent heat from transferring to these surfaces. In fact, canopies can block up to 90% of sunlight and the accompanying heat. Some species of trees with broader canopies, larger leaves, and higher leaf density are even more effective. For example, tropical trees with large leaves can block sunlight very effectively, allowing only 1-2% of sunlight to reach the ground.
When the amount of heat that passes through the canopy is low, the air temperature also decreases. Most of the cooling effect of trees primarily comes from the shade they create. However, essentially, shade does not cool the air; it merely prevents the air from warming up as much.
Transpiration
Trees absorb water from the roots and transport it to the leaves through various stages.
Trees absorb water from their roots and transport it to the leaves, where the photosynthesis process occurs. However, up to 99% of the water that reaches the leaves is released back into the atmosphere due to the opening and closing of stomata (tiny openings on the leaf surface) for gas exchange of CO2 and oxygen. This process is called “transpiration” and is a key factor in creating the cooling effect of trees. (1)
Trees can lose between 70-120 liters of water in a day.
Trees can lose between 70-120 liters of water in a day, depending on their canopy size and leaf area. For instance, the black willow (Salix nigra) or the white poplar (Populus alba) with canopy sizes up to 83.6 m2 can lose up to 82 liters of water daily through transpiration.
For the water on the leaves to change from liquid to vapor, it requires energy. Thus, energy from the warm surrounding air and sunlight is “gathered” to facilitate this (2). When water vaporizes through the transpiration process (process 1), the energy leaving the system is greater than the energy used to convert water into vapor on the leaves (process 2).
According to the law of conservation of energy, if the energy entering the system (to serve process 2) is less than the energy escaping (to serve process 1), the temperature will decrease to maintain equilibrium and conserve thermodynamic energy. If we consider the area around the tree as a “system,” this is how the process operates.
With trees, the cooling effect can also be felt in the area below.
Typically, the cooling effect is often limited to the surface from which the water evaporates. Similarly to humans, our sweat evaporates using heat from the skin, making us feel cooler. However, with trees, the cooling effect can also be felt in the area below, around the tree canopy.
- The first reason is that all the leaves form a larger area—the canopy. A massive amount of water evaporates from the canopy, from 70 to 120 liters a day, extracting thermal energy from all around the tree.
- The second reason is that there are more stomata on the underside of the leaves, which enhances the cooling effect.
- Finally, water evaporates from the ground beneath the tree, cooling the soil; this process is known as evaporative cooling. The two cooling processes of transpiration and evaporative cooling work together to make the surrounding air and ground cooler.
The Cooling Effect of Trees in Urban Areas
Vailshery, Jaganmohan, and Nagendra conducted a series of studies from 2009 on 10 major roads in Bengaluru, India. The research was based on assessments of ambient temperature and surface temperature throughout the day on sections with and without trees. For example, on Bellary Road, which connects the city to the airport, the air temperature at 2 PM was 34.5 degrees Celsius in areas without trees, compared to 29.8 degrees Celsius in shaded areas.
Surface temperatures showed significant differences. At 3 PM, under direct sunlight, the measured temperature was 51.5 degrees Celsius, while under the tree canopy it was 32.5 degrees Celsius, resulting in a difference of up to 19 degrees Celsius. Several other studies have concluded that trees can cool the air by 2 to 10 degrees Celsius.
Planting trees along streets can effectively cool the city.
If trees are planted close together in clusters, they can cool the temperature by about 5 to 10 degrees Celsius. Planting trees along streets or creating rooftop gardens can effectively cool the city and combat the urban heat island effect.
