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  • Writer's pictureMartin Ford

A Cooling Trend

Our Top 20 List of how trees positively affect our cities continues with a very timely post given the heat and humidity that attends an Ontario summer after a long, and particularly cool spring.

A popular Chinese proverb states: The best time to plant a tree was 20 years ago. The second best time is now.” Not only do trees provide shade from the sunlight, they also absorb the direct rays of the sun, then cool the air as it passes by the leaves and through the canopy. It is particularly effective when there are many canopies joined together like those of a forest. If each city in the world had the 50% tree surface coverage that Vienna, Austria, has over its streets and homes, the hot summer days in our “urban heat islands” would be a lot more comfortable with daytime temperatures lowered by as much as 12C in very well-treed areas!

An established tree draws over 100 litres of water each day from deep in the soil up into its leaves in a process we still only partially understand. As the water evaporates from the leaf in a process known as transpiration, energy is required for that change of state. This is called the evaporation latent heat factor. The energy is drawn from the surrounding air as the water on the leaf surface turns to gas and evaporates. This process is similar to how refrigerators are able to cool our food and how perspiration cools our bodies.

On very hot days, only 10% of the water drawn from the soil is actually used for a tree’s food production in the photosynthesis process. The other 90% is simply for cooling the leaf surface which in turn, cools the surrounding air. This cooling benefit is passive, requiring no electrical energy, pollution nor emission of greenhouses gases from power grids. With an extensive tree canopy of 40% or greater to shelter our cities, we would not only have a more pleasant climate, but we would also contribute to an environmental healing process.


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