My thanks to Tim Crook for his very interesting emails and for permission to share here.
You suggested getting in touch if we spot anything, and your podcast has certainly inspired me to pay closer attention to the places I walk and run (I’m a passionate trail runner and always run in barefoot shoes). Which leads me to my ‘clue’ that I wanted to tell you about.
When out running, I occasionally have to run on gravel or farm tracks as well as roads. I minimise the impact of these harder surfaces by keeping to the grassy verges where possible, as I prefer the softer grassier surfaces.
I was running around Combe Valley CP today, and I moved to the grass verge of a farmer’s gravel track. Just before my feet felt the grass, they felt a really soft surface, and I noticed that at the edge of the track I was running on, between the verge and the gravel, there was a matted buildup of straw or hay, about 10-12 inches in width. I looked over to the opposite verge and noticed that whilst there was straw there, it was a strip of only around 3-4 inches. Being intrigued, and inspired by your podcasts, I checked the compass direction of the path and my position. I know the area well so it took me just a moment to determine that the verge where the straw was greater (where I was running) was on the edge of the track, most exposed to the south west.
So I ‘make up’ that the larger hay buildup would have been caused by the prevailing wind blowing hay off farmers’ carts, the track and possibly (to a lesser extent), from the adjoining field, leading it to build up more on the south western edge.
Could that be right? If so, at certain times of year, a relatively useful potential navigational clue.
Thanks for listening and your email.
Great to read that you are observing patterns already. Yes, what you are possibly spotting is the accumulation of airborne cut grasses in a ‘wind shadow’. Loose light particles flow with the wind until they touch a still patch and then drop out and accumulate there. It is very common with sand, snow, dust etc., but can be seen with dry leaves, straw also.
I don’t suppose you have a photo do you? (Probably not if you’re running, but I thought I’d ask.)
I ran my Combe Valley loop today and attach four photos showing the ‘wind shadow’.
The first two photos are on the farm track where I first noticed it. The track runs broadly East/West and I’ve marked the more precise direction on each of the photos. The extent of the straw lying on the more northerly edge of the track is around 45cm (the extent varies, but broadly that’s the average extent all the way along the path). The extent of the straw lying on the southerly edge of the track never exceeds 20cm, and is usually significantly less.
I took the last two photos, as I noticed that when the path opened out (still heading east) and turned slightly (I didn’t take a compass reading at that point, but you will see from the orientation that the path has turned very slightly northwards), the build up of straw was less noticeable / significant. Could the difference be attributable to the position of the hedge in the first two photos, creating a wind break and resulting in the straw depositing close to it on the windward side? Whereas when the path opens out, the straw is more generally and broadly deposited?
Yes, you’re right. It is very noticeable that the asymmetry in straw is greatest where the asymmetry in plant growth either side is greatest also.
We are likely looking at a combination of wind shadow and eddy effects, especially near the hedge. Wind eddies are complex beasts and hard to map accurately at times, but they do lead to asymmetries around all obstacles.