Shade trees for urban cooling

It takes 540 calories of heat energy to evaporate a gram of water.

Trees drink and evaporate many kilograms of water on a sunny day. This means they are pulling thousands and thousands of heat calories from the air. From the tree’s perspective, this trick allows it to stay relatively cool while being exposed to all the sun’s heat.

I used to think it was cool near a tree because of the shade it cast, but that was only part of the story. Transpirational cooling – removing heat energy from the air to help with evaporation – means it feels cooler than the shade cast by a building. 

Leaves are outside air conditioning units for a plant during hot weather. BUT that the cooling effect only works if there is enough water for the tree to use. If it runs low on water, it closes the stomates on its leaves and transpiration stops which means it starts heating up. Not only can the tree then begin to suffer heat shock, it also reduces some of the pleasures we enjoy while being in our garden.

So water your trees for both your pleasure and for the health of your trees.