domingo, 23 de março de 2014

Berg winds

The sheltering desert - p.63

The next morning there was a strong east wind and sun was being swept along in clouds and hurled, against the size of the canyon like pending rain. It was not surprising that all the walls and rockface just above the dried out bed were smooth and polish. A gust of wind got stuck in the corner of our shelter whirling up dust and  mica flakes which rain on us and our possesions. When we woke up our hair, ears, eyelids were full of sand and there were little dunes on our rather grubby pillows. 

Our frypan, with fat in it, was now half full of sand and mica dust, and because the top of our jar hadn´t been closed properly a layer of dust that formed on the jam  too.

When we went out into the riverbed the full force of the wind lashed us with sand and schist. Through half-closed eyes we watched curtains of it being hurled up the rock walls. Looking up canyon as though expecting something different, our are eyes met the glistening white bastion where our cave was. The sky itself seemed dusty. Thei east wind of Namib was no stranger to us and we knew that it would probably last for three days.


Bergwinds Demystified

Over the last week, you may have heard the eNCA weather presenters mention “bergwinds” on more than one occasion. Although they tend to bring unusually high maximum temperatures for this time of year, many people find them incredibly uncomfortable, as they can bring unpleasantly hot, dry and windy conditions. But what exactly constitutes and causes a bergwind? What makes it so hot and dry? And why does it not occur more regularly?Bergwinds derive their name from the direct Afrikaans translation of “mountain wind” and is often compared to a föhn/foehn wind, where dry air warms up as it flows down the lee side of a mountain. Similarly, in South Africa, the lee side of the “mountain” is represented by the altitude difference between the plateau (average height of 1.2km above sea level) and the coast (sea level).
Berg winds occur when a strong persistent high pressure dominates over the interior and another high is situated to the south or south east of the country. The high over the interior results in a large amount of subsidence (or stable, sinking air) and is associated with fine and dry weather. The dry air over the interior starts to move within the circulation of the high (anti-clockwise), resulting in a north easterly offshore wind along the west coast. As the air from the plateau moves towards the coast it flows down the escarpment and warms by 1ºC for every 100m it descends. In simple terms, the dry air from the interior (higher altitude) warms up as it moves towards the coast (sea level), resulting in a spike in the temperature. The stronger the berg winds, the more rapid the temperature increase. Bergwinds occur in all coastal areas in South Africa (northerly winds along the south coast and north westerly along the east coast), but are most prevalent along the west coast.Often a coastal low develops along the coast with bergwind conditions. It’s a bit of a "chicken or egg question”- does the low form because of the decrease in pressure as the warm air descends from the plateau, or does the air move from the high over the interior towards the area of low pressure along the coast? Either way, the bergwinds precede the coastal low, rapidly increasing the temperature. As soon as the low moves through an area, the temperature can drop just as quickly, with a change in wind direction and often, an increase in cloud cover, or even fog, both enhancing the cooling effect. The low will then travel along the coast, with bergwinds preceding it, and cooling behind. In winter the coastal low is then followed by a frontal system, bringing in rain, after the hot dry conditions.Some enjoy the break from the cold, enjoy the warm and brave the wind, whilst others are just irritated by the hot, dry and windy weather. Either way, bergwinds form part of our unique weather pattern in South Africa between Autumn and Spring, and they are here to stay.

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