Time and Trace

During a private course yesterday we spent some time looking at the effect of the wind on trees and grass. We also looked at the lee effect, when leaves and other natural drifting materials accumulate on the lee side of obstacles.

This is something that I am both more sensitive to and wary of since my trip to the Sahara in March. The lee effect there puzzled me for several days until a sandstorm blew in and blotted out the sky. Ironically it clarified things mentally. There is a difference between a prevailing wind and wind that has a huge short-term impact. The sand that was deposited in some areas that I crossed in Libya indicated a wind direction that clashed with the direction that both the trees and the dunes were suggesting. It became clear that this was caused by a sandstorm blowing in from a different direction to the prevailing wind and leaving its own traces.

There is nothing too mysterious in this picture however. It is late afternoon, near the equinox and we are looking east. A northerly wind has combed the sand and left undulations, but note that the undulations vary subtly in angle showing how locally the wind direction can vary due to terrain. There is a small build of sand in the southern lee of the stunted clumps of grass. The rocks also show signs of weathering, but not equally on all sides.