Published: 2007-12-17 12:38:35
Updated: 2007-12-17 12:38:35
Posted December 17, 2007
By Kenneth Monroe
MIKE MOSS SAYS: Kenneth, I'm no expert about trees and falling leaves, for sure, but I don't think there is probably a temperature trigger that would result in such a sudden shower of leaves all at once (if a tree biologist out there knows different, please chime in with a comment). My guess would be that a lot of leaves were ready to fall, and a fairly sudden surge of wind through the treetops sent them tumbling. You didn't mentione specifically whether you could hear the wind pic up at that time, but sometimes the wind doesn't have to be all that strong to knock a bunch of leaves off and noise associated with the wind itself may have been secondary to the noise made by all the leaves coming down. Also, you might easily not have felt any wind because a fairly solid tree canopy can effectively create an elevated "surface" below which even a moderate wind may not reach down to the ground below.
It isn't unusual at night to have a temperature inversion form that leads to a density gradient a few hundred or couple thousand feet above the ground, below which the wind goes calm and just above which the wind actually accelerates because it thus becomes "decoupled" from the frictional influence of the ground. Once this occurs, the speed difference between the enhanced wind above the inversion and the calm air below can sometimes build up to a critical level at which the boundary between them breaks down, vertical mixing (an "overturning") occurs and the wind near the surface surges for a few moments before the inversion re-establishes itself and a decoupled scenario resumes. This kind of overturning/mixing scenario would be my best guess about what you observed, but in this case the "surface" would have been at the level of the treetops.