Psithurism

That silent 'p' is one of those giveaways that a word is of Greek derivation.  Psithurism sounds like it should be condition that effects Star Wars evil-doers.  Instead, it refers to the wind in the trees.  

Air as it moves past an object exerts pressure, causes vortices, and ultimately makes vibrations that we detect as sound.  Leaves that touch one another can also create different layers of sonic vibrations.  It is a form of white noise, a sound comprised of many different frequencies similar to the sound of running water or the hiss of static on a radio*.  

Some trees are designed to enhance this effect.  The quaking aspen, Populus tremuloides, has the root of the word 'tremble' in its Latin binomial.  The tree was named by André Michaux as part of his botanical explorations of North America in the late 18th century.  He and his son deserve a separate essay.  

Because the petiole of the leaf is flattened perpendicularly to the blade, the slightest air movement causes an oscillation... and psithurism in a grove of aspen.  Aspen often form large clones, resulting in entire mountainsides being covered with the trees and their pleasant sound can carry with the hillwalker as one passes through.  

The closely related Populus deltoides (photo above) is also known for the sounds it makes.  This cottonwood has the additional trait of holding onto most of its leaves during winter.  Known as marcescence, this means that even in the dead of winter, the gentle sigh of the wind in the canopy of our bosque may be heard. 

Alas, with a storm bearing down on the state this weekend, I don't expect to spend much time enjoying the relaxing sounds of the breeze in the cottonwoods up the street.  Instead, it will likely be a near gale, and when that is past, a couple inches of snow.  With the snow, will come its own sonic wonder:  the muffled stillness of new snow on the landscape. 

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* Does anyone listen to a real radio anymore?  In my car, I mostly listen to Sirius XM digital music.  Now that I'm not commuting to and from work daily, I don't listen to too much NPR.  But recently I discovered http://radio.garden/, a wonderful global map of radio stations.  It deserves it's own essay in the days ahead.