Playa science #2: Dust storms.
By Ess Dot
Like it, or dread it, the sudden blinding disorientation of a playa storm is unavoidable part of life in Black Rock City. How does this work? Here is your latest uninvited dose of playa geek science.
You may have noticed that in a white out, there is a buzz of sand particles that hit your feet and ankles but only dust aloft. The interaction of these large particles and the finer dust is key to the puzzle. The guy who first wrote at length on Aeolian particle dynamics wasn’t a scientist, but a military man. Brigadier Ralph Bagold spent most of his career travelling in the Sahara. Bagnold was obsessed with the desert, dunes and sand dynamics. From all accounts he was much more interested in desert physics than ambushing lost German troops. In 1941, Bagnold published his observations in the classic “The Physics of Blown Sand and Desert Dunes”. The book has been cited over a 1000 times in the past 70 years, including NASA who used his work for modelling dune dynamics on Mars (It’s even been cited by this here lowly graduate student, who wrote a defence of the work for a term paper, lol)
Bagnold’s sharp insight was that airborne particle dynamics can be approximately divided into three phases. When the wind reaches a critical speed it disturbs a first phase of particles called ‘saltators’. When these guys get airborne, they bounce along disturbing myriads of other particles every time they hit the ground. Most of the newly disturbed particles simply creep along at a low level. These creepers are called ‘reptators’. However, a small fraction of particles become properly airborne to become tiny flying missiles and if the wind is strong enough, they add to the existing fleet of saltators. Some of the saltators get caught up, like in stuff around your camp. The newly sedimented, or ‘landed’ fraction, eventually balances out the the influx of saltators creating a steady-state of critters in the air. All this dramarama of flying saltators result in that satisfying tiny buzz on your ankles. The reptators creep along harmlessly eventually forming playa serpents (but that’s for another post!). Unfortunately, the saltators disturb a third fraction specific to the playa; the very fine mud dust that our bikes and art cars release during daily meanderings. When the saltators kick up this dust, along with the added shear force of ground gusts, these tiny 10-100 micron particles stay airborne for much longer periods of time. They are so fine that the sedimentation time is in tens of minutes, instead of ~0.5-10 seconds for the saltators. White out!
Bagnold went on to investigate very rare desert phenomena, such as the ‘singing sands’ which are a natural sound phenomenon sometimes heard near Barchan dunes. This ‘singing’ has been measured at a rumbling 105 decibels. I’ll bet Bagnold would have liked the rumblings of Black Rock City. Wait for the drop, Brigadier Bagnold, wait for the drop.
Through the sandglass we go…
Note that most deserts have a much higher sand – dust fraction than the playa. This is why the sand serpents never get to any great size and why saltators never really get above knee height. That along with the annual winter flood.