Ever wonder how you can help cool the urban environment, either by doing planting in the yard or volunteering in a local environmental group? Wonder no more. This post will help explain how you can do your part in cooling down urban environments, with special focus on how the process actually works.

According to the Environmental Protection Agency (EPA), shade trees and other small plants like shrubs can help cool the urban environment. Unfortunately, many US communities have lost trees and green spaces as we’ve continued to develop. Many of these communities can take advantage of existing space, like grassy or barren areas, to increase their vegetative cover and reap the benefits. But first, how do trees and vegetation help cool urban climates?

Shading

The leaves and branches of a tree can reduce the amount of solar radiation that reaches the area below the canopy of a tree or plant. Although the  amount of sunlight transmitted through the canopy can vary depending on the species, most in the summertime can block out 90 to 70 percent of the sub’s energy. The tree benefits from these solar rays by absorbing the light and using it for photosynthesis. A lot of times they even send this energy back into the atmosphere. This is a different story in the winter.

The range of sunlight transmitted in the summer is wider – 10 to 80 percent – because of evergreen and deciduous trees that have different wintertime foliage. Of course, deciduous trees lose their leaves and let more sunlight through.

Evapotranspiration

Trees and vegetation absorb water through their roots and emit it through their leaves – this is called transpiration. For example, a large oak tree can transpire about 40,000 gallons of water per year, while an acre of corn can transpire about 3,000 to 4,000 gallons a day.

Evaporation happens during the conversion of water from a liquid to a gas. This also occurs from the soil around vegetation and from trees and vegetation as they intercept rainfall of leaves and other surfaces. Transpiration and evaporation – evapotranspiration – cools the air by using the heat from the air to evaporate water.

Evapotranspiration in combination with shading can help reduce peak summer air temperatures depending on climate. Trees and other large vegetation can serve as windbreaks or wind shields can reduce wind speed in the vicinity of buildings. These impacts can be positive or negative in the summertime. In contrast, they can provide substantial energy benefits in the wintertime.