Boundary Layers Walk

v3.1 Draft

vortex behind a tree visualized with ribbons

introduction

Time: about one hour

The Boundary Layers walk is an outdoor education activity introducing atmospheric turbulence and stratification. The goal is to build an awareness of wind’s movement through the local environment of buildings and trees. The activities involve using ribbon wands to visualize wind, movement exercises to understand how plants cope with wind force. A limited set of the the Beaufort Scale is used to estimate wind force.

Mathew Lippincott developed the original exercise through a Guerilla Science residency at Pratt. This version of the exercise was revised and illustrated with assistance of the AREN Project.

This is a personal archive.

materials

  • ribbon wands
  • optional but recommended:
  • mossy logs/rocks (one per 5 attendees)
  • magnifying glasses or hand lenses for each log or rock
  • please put mosses back after your workshop

locations

  • buildings, especially in fields
  • an open field
  • tall grasses or bushes near a field
  • a lone tree or stand of trees

reading this exercise

This exercise is written as a script. Instructor text is in plain text. Instructor actions are in italics

Boundary Layers Walk

Today we’re going to explore how trees reshape the atmosphere and observe the signs of the wind moving around us.

A boundary layer is the space between an object and a free flowing stream of air. In a boundary layer, air speed increases from nothing at the surface of the object to the speed of the stream.

There is a boundary layer between between you and the moss you’re looking at. There is also a boundary layer between the earth and the atmosphere, called the Atmospheric Boundary Layer.

Distribute hand lenses and moss

moss growing on a partially collapsed wall

A smooth object will have a smooth boundary layer, with still air only at its surface. The air stream is slowed around the object, slipping in layers of decreasing speed to still air at the surface. A smooth boundary layer is named a laminar boundary layer after these slipping layers.

Moss is not smooth, it is a tiny forest. Each tendril is its own plant, and all the tendrils work together to hold in moisture. If you look carefully, a whole world of microscopic creatures lives in that forest. Life can thrive in the moss because moss has slowed the air down and captured water in a protected boundary layer.

moss up close

Moss’s rough surface creates a turbulent boundary layer. As air moves over its surface, the flow of air over the moss separates from the main stream of air and begins to reverse and tumble. Turbulence dissipates the wind’s energy more quickly than the slipping layers of air in a laminar boundary layer, and turbulent boundary layers are thicker and slower than laminar boundary layers.

From mosses to grasses and trees, plant communities make their own protective boundary layers because boundary layers make pleasant, safe places to grow.

Moss and boundary layers

Trees create the thickest boundary layer, lifting the wind above the surface of earth to create a big pocket of still air that retains moisture. Trees can make their own weather by working in together in forests.

visualizing wind in the boundary layer with ribbons

A lightweight ribbon will become caught in an air current, allowing us to see the wind moving around obstacles.

Hand out ribbon wands

When we use a ribbon to visualize the wind, we can call it an anemoscope, from the Greek for wind seeing. Scientific instruments don’t have to be complicated. Sometimes they can just be ribbon wands.

Other Anemoscopes:

  • weathervane
  • windsock
  • bubbles

Allow open exploration, encourage groups to stand in different positions down wind from trees and large obstacles.

What are the effects of buildings and other large obstacles?

Split into groups of 4-8 to explore wind moving around trees and other large obstacles such as buildings.

buildings and ribbon wands

Bring the group together to discuss findings.

the boundary layer in grasses

This exercise takes place in medium grasses or near bushes. Transitioning from laying down to standing, participants will move through the grass’s boundary layer.

  • Form a circle

The wind is exponentially stronger at the top of a plant community than at the base. The best way to experience the transition in wind force still air at the ground to a full speed stream is to move through a plant’s boundary layer.

Encourage participants to engage to their comfort level. It is ok to leave your eyes open or not want to lay down

Take an appropriate pause at each of these levels with eyes closed.

  • lay down
  • sit up
  • stand

lay, sit, stand

Pay attention to the sounds of the wind, changes in temperature, the feeling of wind on your skin and especially on your face.

trees’ effects on the boundary layer

Now that we have a sense for what a boundary layer feels like, lets explore how plants make one.

I’m going to be a lone conifer tree in a field, an evergreen that has needles.

me as a tree

being a conifer tree

resonating like a tree

Musical instruments are made from wood because it resonates. Wood holds a note. We love wood for resonating, but resonating is a problem for a tree. Trees want to shed the wind’s force quickly so they can readily absorb the next gust.

The movement of a trees’ many branches help them absorb the wind because each branch resonates at a different frequency, and their clashing rhythms dampen motion.

Encourage everyone to put their hands out and be trees

Branches each sway at a slightly different pace, and those clashing rhythms cancel each other out.

  1. Wave one arm up and down at about once per second. Move vigorously, then pause.

  2. Wave the other arm up and down at about three times per second.

  3. Attempt to wave both arms at the two different speeds.

how trees shape the wind

As the air moves around me, it sticks to my rough surface and starts to spin.

As the air spins it expands to fill the space behind me and detaches from my leaves, creating a tree-sized spinning air bubble, or air mass. Pulling at the air around it, this air mass becomes a vortex. The trail left in the wind is called a vortex street. The vortex street continues about 8-10 times the height of the tree, and extends about 2.5 tree heights above a tree.

vortex behind a tree, an overhead view

Flags flap because vortex streets form behind a flagpoles. Each vortex peels off alternating sides of the pole, inducing the waving seen in a flag.

practicing the Beafort wind force scale for conifers

handout conifer beaufort

A light breeze is a Force 2 wind, the kind you can just barely feel on your cheek. My twigs and needles are going to begin waving. (4-7mph, Force 2)

A moderate breeze is a Force 4 wind where dried leaves and dust get kicked up (13-18mph) My small branches are in constant motion, and I’m starting to sway at the top.

In a Force 6 strong breeze (25-31mph) my trunk is swaying and my large branches are in constant motion.

As a Force 8 gale (32-45mph) picks up, my whole trunk is in constant motion and the clashing of my branches breaks needles and twigs.

In gale and storm force winds trees fight for survival. If the wind hits 60mph I am hit by 200 tons of force. I’m dissipating that energy by moving and straining everything I’ve got, from my roots up to my needles. Movement can dissipate about a 1/3 of the wind’s force.

Break into teams and try out being trees at different wind force levels.

Just as branches swaying at different frequencies can dissipate energy, tree trunks swaying at different frequencies in a forest can dampen each others’ swaying as they bump into each other. These collisions can diffuse up to a 1/3 of the wind’s energy. We can expect a stand of trees to move less than a single tree, and a forest to move less than a stand of trees.

Spread out to be trees again.

being a deciduous tree

Now we’re going to be deciduous trees. Trees with broad leaves that fall in preparation for winter.

Hold your arms up like a field goal.

Deciduous trees have branching trunks that go up and run into each other, much like a forest of conifers. By spreading their canopy out, deciduous trees are somewhat better at dissipating the winds energy than conifers.

handout deciduous beaufort

Practice three levels of the Beaufort scale together.

In a Force 2 light breeze (4-7mph)

  • leaves rustle

In a Force 4 moderate breeze (13-18mph)

  • small branches move
  • small leaves turn over

In a Force 6 strong breeze (25-31mph)

  • large branches in motion
  • large leaves turned over

systematic ribbon wand deployment

Return to the ribbon wands and spread out across a field at even intervals behind a tree.

vortex behind a tree visualized with ribbons

Observe differences in wind effects and share observations.

Throw ribbon wands in the air and celebrate!