Nature Notes

by Amy Schoeman

The Namib’s life-giving coastal fog

On most days a dense belt of sea fog hangs over the ocean along Namibia’s coast. Generated by the cold Benguela Current, it is this fog that supports the Namib’s singular ecosystem and gives the coast its aura of mystery and impenetrability. The regular fogs make the atmosphere extremely damp for about 340 days of the year, causing the humidity to be more than 90 per cent on most days, often reaching 100 per cent.

The Benguela Current flows from Antarctica towards the southernmost tip of Africa, from where its icy waters are directed up the continent’s Atlantic coast as far north as Mossamedes in Angola. An influential feature of the Benguela is that the temperatures of the water next to the coast are always lower than further out in the ocean, in the region of 12ºC to 15ºC as opposed to between 18ºC and 20ºC. This causes the evaporation rate and air temperatures on the open sea to be higher than closer to the shore. When this relatively warmer and moister air flows towards the coast and rises because of the elevation of the land, the resultant mixture of the two air masses with different temperatures causes condensation and advective fog is formed.

During the night, when the desert surface cools, the humid sea air of the day is transformed into visible clouds of mist because of condensation, or, as it is often formulated, the fog moves towards the interior. On about two days out of three, it penetrates inland for about 50 kilometres, on some days even further, up to 100 km.

In the low-lying river courses the penetration is much deeper than on the more elevated gravel plains. Here and on the beaches the fog often lies suspended as a low cloud between 200 and 400 metres above the ground, whereas on higher ground it rests on the surface, sometimes leading to a fine early-morning drizzle. This explains why the lichen fields on the more elevated plains bloom so luxuriantly on certain days. This early morning mist creates an eerie atmosphere in which sounds are muffled and hushed, making dunes and bushes appear disproportionately large and out of focus.

As the sun ascends in the sky, the fog blanket warms up, heat rays penetrate and warm the ground surface, which in turn radiates heat back and causes the mist to lift and disperse. This ‘invisible’ vapour can often be seen hanging as a hazy belt along the coast when approaching in an aircraft from the interior. On certain days it causes the sun at the coast to seem less potent than further inland. As the day progresses, a sea breeze begins to blow, gradually increasing in strength to disperse the remaining moisture and vapour over the desert.

This article appeared in the Feb/March ‘05 edition of Travel News Namibia.