A COOL DESERT IN AFRICA
Fog creeps inland from the sea = Amy Schoeman

The Namib straddles the Tropic of Capricorn, but its seas are uncommonly cold, as polar water flows offshore in the Benguela Current. Consequently the climate of the coastal Namib is quite different from that of the rest of the country. Much drier and far windier, it is a cool desert in a hot country, frequently blanketed in fog.

Surface temperatures of the sea in the inshore waters, although not nearly as cold as the depths of the Benguela Current, average only 13-15ºC in winter and 15-17ºC in summer.

The central coast is the foggiest part of the Namib. On 146 days of the year fog reduces visibility on the ground to 1 000m or less. It tends to be thickest early in the day and gradually clears as the sun climbs higher.

Namib fogs form when moisture-laden air from the sea is cooled such that the moisture condenses into fog banks or low clouds. Cold water cools the air offshore and cold ground does so onshore. Inland the air is further cooled as it flows from sea level to higher ground.

Although offshore and onshore fogs are blown inland, sometimes as far as 100km, fog is unusual deep in the Namib. It only reaches the central desert, around 40-80km from the coast, on about 40 days of the year. In general its incidence, density and duration decrease with altitude above 600m.

Ground fog condenses into droplets of water on plants, rocks, hillocks, dunes and other obstacles. Terrain and plants with irregular surfaces "comb out" or catch the most water as fog passes over them. The larger the surface area, the heavier the precipitation. In the greater part of the coastal desert, precipitation of this kind actually exceeds rainfall. Fog precipitation is fairly constant, whereas rainfall is highly variable.

Even so the Namib is hyper-arid, one of the driest places on earth. The average rainfall is well below 50 mm per year in most of the coastal Namib. In the inner Namib it is 50-100mm. Little enough even in absolute terms, this is next to nothing when measured against potential evaporation, a theoretical average of 1 680-2 240mm per year.

Whereas years sometimes pass without any rain in a particular place, a single rainstorm on occasion delivers -- in a matter of hours -- two or three times the annual average. It usually rains on only 5-15 days per year. When rain does fall it is mostly scattered, with showers in some places and none elsewhere.

For much of the year masses of dry air from the equator flow at high altitude to southern latitudes where they settle and form highpressure zones. One such zone is located more or less permanently off the Namib coast and blocks inflows of moist air from the tropical north-east. The rain winds simply cannot get through to the coastal Namib because they run up against onshore winds from the Atlantic.

In addition a climatic condition known as temperature inversion comes into play. While the layer of air just above the surface of the sea is moist, it is cooler and heavier than the dry air massed above it and therefore unable to rise, with a consequent reduction in the turbulence necessary for cloud formation and rain.

In most of the Namib rain falls in the summer months, carried across the subcontinent on north-east winds. South of Lüderitz Bay rainbearing winds mostly come from the opposite direction in winter when cold fronts reach far inland from the Atlantic.

The average annual temperature on the Namib coast is below 16ºC. Calculated from daily averages over a year, it is among the lowest anywhere in a tropical latitude.

In high summer the coast is the coolest part of the country, with average maximums -- in the hottest month -- that are generally less than 22ºC. At a remove from the ocean, the inner Namib gets much hotter. Maximum temperatures in the hottest month average around 30ºC. They sometimes rise into the 40s.

The tables are turned in the coolest months, when the coastal desert is a bit warmer, with average minimums of 10-12ºC compared with 8-10ºC in the inner desert.

Cool southerly winds blow steadily for most of the year on the coast, while in winter the berg wind or Ostwind (east wind) -- a hot wind laden with sand -- sporadically roars out of the interior for two or three days at a time. Calm days are rare. Winds blow 84% of the time on the central coast and 92% of the time on the south coast.

The south coast is much the windiest part of the Namib, with winds that exceed 40km/h on summer afternoons, compared with less than 30km/h on the central coast.

In the southernmost part of the coastal Namib winds are strong enough to blow barchan dunes northward at a rate of 50 m per year. The average velocity at Pomona, a ghost town on the coast between Lüderitz Bay and the Orange River mouth, is the highest in southern Africa. Here winds of 40-80km/h blast the coast in summer.

In the winter months the strength and frequency of winds drop along the entire coast. Even as the winds retreat, however, the fogs advance. In the same season the incidence and density of fogs increase to their highest levels.

A season unto the grasses ..........

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