Trees are cool

With the heat of summer well and truly upon us, it is worth noting that researchers point out that ground temperatures on a hot day can be as different as 10⁰C in some localities due to the presence or lack of tree cover (ABC News link).

Credit: US.Forest Service

Trees are natural air conditioners that help keep us cooler and healthier.

When the sun’s rays reach a hard surface such as a building, a bitumen road or concrete footpath, heat is absorbed into the object and radiated into the surrounding air. This is one of the reasons that overnight temperatures in cities are always higher than the countryside – the heat collected through the day is being radiated back into the air.

Not only do they provide shade from the sun’s direct radiation on a hot day, trees can also actually cool the air by transpiration. Moisture in the ground is absorbed by the roots and transported throughout the tree and out into the air through pores in the leaves.

Transpiration at the surface of the leaves occurs when the moisture in the leaves changes to water vapour by absorbing heat from the air, thus cooling the air around the tree – pretty much like the evaporative coolers on the roofs of houses.

A large gum tree on a hot summer’s day can transpire as much as 200L of moisture into the air, taking around 118,000 calories of energy from the air to do so.

Provided there is enough moisture in the soil, the amount of transpiration is directly proportional to the leaf area, hence leafy tree and understorey plus mulched ground surface to retain soil moisture are important factors that make trees nature’s air conditioners.

For our own health and wellbeing as our climate warms up, well-designed streetscapes and developments that include trees, should be an important consideration of urban planning.