Could we call this a natural experiment? Nature sets up the conditions and in 4 cases the outcome is almost identical. Excellent repeatability, so good science.
Of course Marcel Minnaert has noticed this many years ago. And has also found a plausible explanation.
Autumn is the time par excellence for studying air currents and the dry leaves that - in their millions whirl all along the roads, are very sensitive wind-indicators. In every ditch, every slight depression of the terrain - eve if it's only an undulation of a few centimeters deep! - the withered leaves accumulate. At such points, the wind has to be significantly weaker.
Other accumulations occur where the air rises temporarily, so that the horizontal component of the airflow is small. Thus one finds sometimes a big pile of leaves in the recessed corner of a building, caused by the whirling ascent of air there.
And look at ribbons of autumnleaves that you can find along any sidewalk and along each wall: as the wind comes closer to the wall the horizontal component of the airspeed decreases and the wind will first drop the sand and only ater that the withered leaves.
Id just like to say thank you for writing this blog...I find it deeply fascinating and I cannot find anyone else who writes about this kind of thing...Keep up the good work.
ReplyDeleteAdam - London.