4 min read

Living in the Boundary Layer

Coming to a full stop
Living in the Boundary Layer
Wind, water, and the earth's surface in conversation. Photo by David Lukas

Today's topic is on my mind because I recently read a book called Boundary Layer. In this book, author Kem Luther explores zones where different spheres of history and science overlap. While it's interesting to think of these layers as metaphors, as Luther does, I thought it would be far more fascinating if we could use this week's newsletter to explore real-world examples of boundary layers.

And boy, oh boy, did I open a Pandora's box!!! This topic turns out to be far bigger, far more important, and far more technical than I ever imagined. I found myself wading through a voluminous literature full of complex equations and highly specialized jargon. And searching the internet for simple explanations got me nowhere.

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In the simplest terms, boundary layers are what happens when fluids—either gases or liquids—flow over solids, or conversely, when solids move through fluids. At first glance, this concept might seem obscure and unimportant, but if you think about it, this is what's happening for every bird, fish, flying or swimming insect, flowing river, blowing wind, or fluttering leaf. In essence, every living organism and most physical objects are impacted by boundary layer forces in some way.

flying bald eagle
Flying in the boundary layer. Photo by David Lukas

The paradox of boundary layers is that moving fluids have a particular velocity as they flow in a direction (their freestream velocity), but as fluids get closer to the surface of a solid object, they begin to slow down due to drag. Moving even closer to a surface, velocity continues to decrease until, at the surface of the object, there is so much drag that fluids stop flowing altogether!

flow diagram
Fluid velocity will always decrease to zero as it approaches a surface. Diagram by David Lukas

You can experience this yourself if you lay on the ground on a windy day, which effectively puts you in the boundary layer along the ground where air currents are slower and warmer. And this same effect of air currents coming to a stop explains why fan blades collect dust and grime even if they're constantly spinning.

sleeping cat
Laying down is a great way to get out of the wind. Photo by JackieLou DL from Pixabay

Boundary layers are one of the most important forces in the natural world and every plant and animal has to deal with them on scales that are both micro and macro. For example, let's consider leaves on a tree.

wind along river
Flow of wind over leaves, and flow of water over rocks. Photo by David Lukas

Trees can only move water and nutrients from their roots to their branches by evaporating water from their leaves and pulling water upwards with capillary action (my newsletter here explains this in more detail). But if every leaf is wrapped in a boundary layer of still air there can be no evaporation and no movement of water or movement of nutrients through a tree.

lush vegetation
Over 90% of the water taken up by plant roots is evaporated from their leaves, creating unique microclimates wherever there's lots of vegetation. Photo by David Lukas

This means that every feature on a leaf, whether it's teeth, lobes, spines, curved surfaces, or ruffled edges, is a different solution for fine-tuning the thickness of the boundary layer. (Leaves that are lower in the canopy need thicker boundary layers in order to warm up and be more photosynthetically active, while leaves at the tops of trees need thinner boundary layers to dissipate excessive heat that will stop photosynthesis entirely.)

alder leaves
Characteristic leaf features help shape the boundary layer. Photo by David Lukas

At the same time, the tops of trees in a forest form another boundary layer on a larger scale because trees stick up into the wind and slow air flowing over the earth's surface. This specific boundary layer has extremely important implications for local climate, evaporation of moisture from the ground, and whether nutrients being carried in the wind are dropped on the soil or not.

trees and sky
The contours of trees and forests shapes wind, clouds, and climate. Photo by David Lukas

On the largest scale, there is what we call the planetary or atmospheric boundary layer. This is the lowest part of the earth's atmosphere—from 300 feet at night, to half a mile during the day—where moving air is directly impacted by contact with the earth's surface.

smog and planetary boundary layer
The planetary boundary layer matters because it traps and concentrates emissions in the zone where we all live. Photo by Foto-RaBe from Pixabay

All of us, all organisms, live our lives inside this planetary boundary layer, so it might be the most important boundary layer of all.