A Fog Harvester Feasibility Study
Table Mountain could be SA’s next big fog farm.
Following last week’s In Touch newsletter topic of the week “Fog, but not as you know it!” which mentioned fog harvesting, CS Africa would love to share on the fog harvesting stations we supplied for Table Mountain.
We had three consecutive dry winters (2015–2017) in southwestern South Africa (SSA) resulting in the Cape Town “Day Zero” drought in early 2018. Cape Town’s water crisis got so bad that there were competitions to see who could wash their shirts the least. Restaurants and businesses were encouraging people not to flush after going to the toilet. The city was just 90 days away from turning off the taps.
A year on, the South African city’s parched dams we’re over 80% full. Water use restrictions have been relaxed. And Day Zero – the point at which Cape Town’s municipal water supply would be shut off – never came to pass.
Find more information in this article: https://www.weforum.org/agenda/2019/08/cape-town-was-90-days-away-from-running-out-of-water-heres-how-it-averted-the-crisis/
For this reason, the City of Cape Town has established a fog harvesting project on Table Mountain to measure the potential for fog harvesting to supplement the area’s primary water supply.
While still in its infancy, the programme hopes to determine the feasibility of constructing multiple fog catchers on the mountain. The feasibility study and research phase would be completed by December 2022.
As of date, two Campbell Scientific weather stations are installed on Table Mountain measuring basic weather parameters as well as the fog collected. Both weather stations are equipped with 1.5 m² screens specifically designed to capture or “harvest” the water from fog. These screens are manufactured from 40% shade cloth and 316 stainless steel respectively as the City of Cape Town is testing various available material options. As fog condensates on the surface of the screen, moisture accumulates on the surface and gathers in the gutter below. The droplets are fed through a tipping bucket rain gauge. Each drop is equivalent to 0.649 ml of water.
Looking at the data it is interesting to note the sequence of events creating potential for fog harvesting. In summer, our dry season, the wind will typically pick up in the afternoon and move from the South more towards South East. As the wind accelerates the temperature on Table Mountain drops significantly and the relative humidity drastically increases. As the relative humidity reaches 100% you will notice the solar irradiance drops from 1000 W/m² to less than 100 W/m². Minutes later the first tips will be recorded on the “fog gauge” whilst the rain gauge records no precipitation indicating that we are indeed collecting fog and not rainwater. When we first looked at the data back in 2018 (before we experienced two wet winters in 2020 & 2021) the above-mentioned sequence was quite exciting to witness in real time.
“The more we research and understand, the stronger and better prepared we become.” – Alderman Xanthea Limberg (Mayoral Committee Member for Water and Waste).
The first official studies concerning moisture density on Table Mountain were conducted over a century ago by German-born botanist Rudolf Marloth. Studies conducted since then have placed yearly fog precipitation at around 3,294 mm, triple the average yearly rainfall recorded in Cape Town. The mountain’s famous ‘tablecloth’ – scientifically referred to as an orographic cloud formation – is responsible for much of the high-altitude moisture and optimism surrounding the fog harvesting programme.
