Trees and other plants help cool the environment, making vegetation a simple and effective way to reduce heat.
The most obvious way trees cool the air is by shading. Reducing the amount of sunlight striking buildings and pavement reduces the amount of energy that is absorbed and re-radiated into the air.
Trees also cool the air by a process known as transpiration cooling. As trees release water into the atmosphere from their leaves via transpiration, the surrounding air is cooled as water goes from liquid to a vapor. This process is similar to evaporation pads used to cool greenhouses — except in trees, water moves into the tree’s roots from the soil and travels through the tree’s water-conducting system, eventually being transpired from the leaves. The water that is released in its gas-vapor-form has a cooling effect on the surrounding air. Shaded surfaces, for example, may be 20–45°F cooler than the peak temperatures of unshaded materials. Transpiration cooling, alone or in combination with shading, can help reduce peak summer temperatures by 2–9°F.
Trees and vegetation are most useful when planted in strategic locations around buildings or to shade pavement in parking lots and on streets. Researchers have found that planting deciduous trees or vines to the west is typically most effective for cooling a building, especially if they shade windows and part of the building’s roof.