How Trees Make Rain

If you’ve taken high school geography you probably remember learning the basics of weather. For instance, that coastal regions tend to bear the brunt of rain blowing in from the ocean. And that those systems gradually dissipate as they move further inland until the clouds get rained out. That’s why a lot of interior landlocked regions of continents tend to be drier, seeing far less precipitation.

I’ve been reading Peter Wohllben’s The Hidden Life of Trees. The book is an exposé of the little-known, and even less seen, living processes of trees. The “hows” and the “whys” of all things tree-related. One remarkable nugget of information in the book (which I never learned in school) is that trees and forests are additionally important because they help extend the reach of rainfall coming off the sea into the interior. I’ll let him explain how this works, below:

“Of all the plants, trees have the largest surface area covered in leaves. For every square yard of forest, 27 square yards of leaves and needles blanket the crowns. Part of every rainfall is intercepted in the canopy and immediately evaporates again. In addition, each summer, trees use up to 8,500 cubic yards of water per square mile, which they release into the air through transpiration. This water vapour creates new clouds that travel farther inland and release their rain. As the cycle continues, water reaches even the most remote areas. This water pump works so well that the downpours in some large areas of the world, such as the Amazon Basin, are almost as heavy thousands of miles inland as they are on the coast.”

transpiration evapotranspiration trees forests British Columbia deforestation
Photo © John Zada

Wohllben goes on to say that in order for this water transference phenomenon, this hopscotching of rain, to take place there needs to be a forest – including a coastal forest to kick off the process. If coastal forests are cut down by humans the chain reaction collapses.

With this knowledge one can start to see how large-scale deforestation, such as that happening on Canada’s West Coast, should not just be the concern of environmentalists and activists – but perhaps everyone. Including those with no vested interest in ecology or wilderness protection. People living in the landlocked outer edge of forest ecosystems, especially farmers, and those who eat their food, even if they don’t identify with such issues, are inextricably bound to them.