KNYSNA NEWS – Not unlike Table Mountain's table cloth, Knysna has its own occasional veil that obscures the famous Heads and residents are used to occasionally living without their view of the landmark when it is covered in a thick layer of fog. Or is it mist?
The first thing KPH realised when foraging about last week to find out more about the phenomenon was that there is a singular difference between fog and mist, which causes many people to mistake one for the other.
According to South African Weather Service (SAWS) liaison officer Garth Sampson, this difference lies in visibility. "It is called fog when visibility is 1km or less and called mist when visibility remains above 1km. In driving terms it tends to be referred to as fog when visibility is 100m or less," he says.
According to Sampson, the fog that formed along the Southern Cape coastline last week is called advection fog. "This happens when the moist air moves over a cold surface, which in this case means the water along the coast was cooler than the air temperature," Sampson explains.
"Some days prior to the fog there was an upper high-pressure system which resulted in the warm temperatures over the cooler Atlantic Ocean. When we have a situation of fairly light onshore* or near onshore breeze, air crosses the warmer Agulhas Current situated further out to sea and becomes moisture laden due to evaporation over the warmer current. The air then moves over the cooler inshore* water, cools, condenses, and forms fog."
Which explains why the Heads and the estuary were covered in fog. But, it drifted further inland as well.
"The fog then moves in over the land. In places where the inshore water is not that cool, then the air moving in from the sea will only condense and form fog when forced to rise up against mountains such as the Tsitsikamma Range," Sampson enlightens us. "This is then called orographic or adiabatic fog. In this case the coast itself may be clear, but fog will persist along the mountain ranges."
Hopefully this information will serve to slightly lighten the fog surrounding this phenomenon along our shore.
*Onshore is moving from the sea towards the land, while inshore is close to (especially in sight of) a shore.
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