It is around 5:30pm on Wednesday 17 January 2024. I am at my mother-in-law’s house in Comitán, Chiapas, Mexico . I am listening to changes in air pressure through a two semi-modular synthesizers, by way of a module that I have created for this task. At this time of day, in most parts of the world, the pressure will rise as the air starts to cool. Today, in Comitán, is no exception. The trend line is unequivocally heading up.
However, despite what weather graphs seem to indicate, the pressure is not rising in a straight line. Instead I surf the waves of rising and falling pressure that sweep through the room. A relatively rapid rise in pressure is accompanied by a gust of wind that pulls lower pressure air in behind it. I see the effects of these pressure changes in the movement of banana leaves, catching the day’s last rays of sun, out the window in front of me.
You will find similar pressure changes elsewhere, however these specific waves of high and low pressure, and the motion of the air from which they are inseparable, have a particular character at this precise location in Comitán. The character of any given location’s air pressure waves, and their associated wind gusts, depends on many factors. Topography, proximity to the ocean, the amount of heat absorbed by the earth throughout the day in the location and its surrounds, itself a factor influenced by vegetation and the proportion of the sun’s energy that is reflected back into space by cloud, snow cover, or other reflective surfaces.
At 5:50pm I stand up and walk over to a south facing window. I look past the banana tree into the distance to the south to see if the mountains across the border in Guatemala are visible. I don’t expect to see them. More often than not they are hidden by clouds. Today is no exception. The mountains and the near perpetual cloud cover they attract are a factor in the movements of air that I am currently hearing through my synthesizer here in Comitán.
Behind me, over my right shoulder, a few kilometers from here, is the road to San Cristóbal. This road rises several hundred meters between here and there. Those higher mountains north-west of here also influence the shape of the air and the corresponding sounds coming from my synthesizer system. At this time of day cool air will fall down the slopes off the higher peaks around San Cristóbal towards Comitán. When cooler winds blow in from the north-west the same mountains act as wind break protecting Comitán from their full force. Mountain topography shaping the air.
There is a small car park on the other side of the wall to the left of me as I sit. When a car engine starts I listen carefully trying to discern any changes in air pressure that might be associated with the firing-up of an internal combustion engine.
The neighbors on the other side have dogs that frequently bark at this time of day. Are they responding to the rising air pressure, the increased traffic as people head out for the evening, or a combination of both? What about the barking itself? How might it influence the lower frequency air pressure changes that I hear. What is the smallest signal that might register in a meaningful way in an audible analogue of air pressure changes?
Sydenham
A bit over a month ago, in December 2023, I spent time listening to the air at Sydenham International, 81 Sydenham Road Marrickville. At the time it was the location of the exhibition Expanded Synthesis. As I listened to the air at this location, over a number of days, I was struck by the transitory effects on air pressure caused by cars and trucks driving along Sydenham Road and the planes flying low overhead on their approach to Sydney airport. Vehicular movement of any kind will produce corresponding air pressure changes and their associated movements of air. A speeding car generates pressure waves that are analogous to the wake of a speeding boat.
What about someone riding their bike along the road only 3 or 4 meters from the gallery door. If you were sitting inside the gallery with the pressure sensor, and the door was open, would it be possible to hear the pressure waves from this slower form of human powered vehicular motion?
If, on a particular day in December 2023 at 81 Sydenham Road, the answer was yes, would you also be able to discern the changes in air pressure that come from the movement of humans walking along the footpath immediately in front of the door?
If, again, the answer was yes, would it be possible to hear and discern the difference between the way the air was displaced by someone looking down at their phone as they walked, and someone walking fully upright and aware of their surroundings?
If, once more, the answer was yes, we might ask about the conditions on that day that made the subtlest movements of air discernible. No doubt a certain level of stability is required for such information to propagate. You would never be able to discern such subtleties on a windy day. What else might contribute to make the air a medium capable of registering these tiny fluctuations?
Cottonwoods
On the Cooks River, near Tempe Station, there is a grove of cottonwoods. Over the last 8 months of listening to barometric pressure changes I have developed a deeper appreciation for the way the leaves of these trees shape the air. There is a sweet spot of wind direction and speed that causes the leaves ripple at a particularly pleasing frequency. As I ride my bike through this groove I sense the ripple as one tree passes its exquisite wake to the next. I sense the transmission of the wake between trees, later, I imagine a large forest of cotton woods and the beautiful shapes of the forest air.
A quick search for ‘Cottonwood’. Populus deltoides, this looks like the right tree. Wikipedia tells me it is
a species of cottonwood poplar native to North America, growing throughout the eastern, central, and southwestern United States as well as the southern Canadian prairies, the southernmost part of eastern Canada, and northeastern Mexico… The leaf is very coarsely toothed, the teeth are curved and gland tipped, and the petiole is flat; they are dark green in the summer and turn yellow in the fall… Due to the flat stem of the leaf, the leaf has the tendency to shake from even the slightest breeze. This is one of the identifying characteristics.
Wikipedia
The frequency of the shaking cottonwood leaf and how it affects both humans and the air is a subject worthy of study. Based on my own experience and sensibilities, I have noticed certain sweet spots in the frequencies of leaf and air motion, frequencies that no doubt reflect something about the structure of the leaves and stems.
Changes in the weather and seasons exert their own influence on the leaves and their motion. Dry weather can lead the cottonwood to drop its leaves earlier in the year. Before that happens the stem and leaves will move differently as they dry out. Changes in barometric pressure, even those that don’t produce strong winds, may also influence the precise timing of particular leaves falling from the cottonwood.
Leaves of trees are a particular form the earth air interface that all plants have. The leaves of trees have a special relationship with the air because the growth of the tree’s trunk and branches lifts them higher off the ground (20-30m in the case of the cottonwood). They are in the air. The size and shape of the cottonwood leaves and stems have evolved in relation to moving air.
Clifftops
The wind speed and soil types of certain locations preclude the growth of trees. Near Mount Hay, around the clifftops overlooking the Gross Valley, in upper Blue Mountains, there is a windswept escarpment where the tallest plants are no more than half a meter high. Such locations and their surrounds show the habits of the air. They show how specific topography and geology contribute to evolutionary processes. The plants become maps of soil types and the pressure gradients that sweep through the areas where they grow. Barometric pressure written into their DNA in the form of adaptations that allow them to grow in high-wind environments where the soil cannot anchor trees. This is a form of atmospheric sensing, in relation to topography and geology, that occurs on evolutionary timescales.
Questions
What sense do plants make of more transitory changes in barometric pressure? If I were a tree and could sense a rapid change in pressure I would have advanced warning of the strongest wind-gusts that might break my branches or knock me over. If, as a tree, I could sense these things, what might I do with that information? Could I change something in the chemical or physical structure of my body so that my limbs would be less likely to break in the strong winds that rapid pressure changes foretell?