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Born on this day
Wallace Berman
7th week in year
18 February 2024

Important eventsBack

Snow in the Sahara Desert18.2.1979

Wikipedia (15 Feb 2013)

The Sahara (Arabic: الصحراء الكبرى‎, aṣ-Ṣaḥrāʾ al-Kubrā , 'the Greatest Desert') is the world's hottest desert, the third largest desert after Antarctica and the Arctic. At over 9,400,000 square kilometres (3,600,000 sq mi), it covers most of North Africa, making it almost as large as China or the United States. The Sahara stretches from the Red Sea, including parts of the Mediterranean coasts, to the outskirts of the Atlantic Ocean. To the south, it is delimited by the Sahel, a belt of semi-arid tropical savanna that composes the northern region of central and western Sub-Saharan Africa.

Some of the sand dunes can reach 180 metres (590 ft) in height. The name comes from the colloquial Arabic word for desert, (صحرا ṣáḥara  [ˈsˤɑħɑrɑ]; Literary Arabic: صحراء ṣaḥrāʾ  [sˤɑħˈrɑːʔ]).

Climate

The climate of the Sahara has undergone enormous variations between wet and dry over the last few hundred thousand years. During the last glacial period, the Sahara was even bigger than it is today, extending south beyond its current boundaries. The end of the glacial period brought more rain to the Sahara, from about 8000 BC to 6000 BC, perhaps because of low pressure areas over the collapsing ice sheets to the north.

Once the ice sheets were gone, the northern Sahara dried out. In the southern Sahara though, the drying trend was soon counteracted by the monsoon, which brought rain further north than it does today. The monsoon season is caused by heating of air over the land during summer. The hot air rises and pulls in cool, wet air from the ocean, which causes rain. Thus, though it seems counterintuitive, the Sahara was wetter when it received more insolation in the summer. This was caused by a stronger tilt in Earth's axis of orbit than today, and perihelion occurred at the end of July around 7000 BC.

By around 4200 BC, the monsoon retreated south to approximately where it is today, leading to the gradual desertification of the Sahara. The Sahara is now as dry as it was about 13,000 years ago. These conditions are responsible for what has been called the Sahara pump theory.

The Sahara has one of the harshest climates in the world. The prevailing north-easterly wind often causes sand storms and dust devils. When this wind reaches the Mediterranean, it is known as sirocco and often reaches hurricane speeds in North Africa and southern Europe. Half of the Sahara receives less than 20 mm (0.79 in) of rain per year, and the rest receives up to 100 mm (3.9 in) per year. The rainfall happens very rarely, but when it does it is usually torrential when it occurs after long dry periods.

The southern boundary of the Sahara, as measured by rainfall, was observed to both advance and retreat between 1980 and 1990. As a result of drought in the Sahel, the southern boundary moved south 130 kilometers (81 mi) overall during that period.

Recent signals indicate that the Sahara and surrounding regions are greening because of increased rainfall. Satellite imaging shows extensive regreening of the Sahel between 1982 and 2002, and in both Eastern and Western Sahara a more than 20-year-long trend of increased grazing areas and flourishing trees and shrubs has been observed by climate scientist Stefan Kröpelin.

Snow and ice

On February 18, 1979, snow fell in several places in southern Algeria, including a half-hour snowstorm that stopped traffic in Ghardaïa, and was reported as being "for the first time in living memory". The snow was gone within hours. Several Saharan mountain ranges, however, receive snow on a more regular basis. Although relative humidity is low in the arid environment, the absolute humidity is sufficiently high for moisture to condense when driven up a mountain range. In winter, temperatures drop low enough on the Tahat summit to cause snow on average every three years; the Tibesti Mountains receive snow on peaks over 2,500 meters (8,200 ft) once every seven years on average.

On January 18, 2012, snow fell in several places in western Algeria. Strong winds blew the snow across roads and buildings in Béchar Province.

   
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