John Tyman's Cultures in Context Series EGYPT and the SAHARA www.johntyman.com/sahara |
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1.2 Climate : 021- 048 |
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John Tyman's Cultures in Context Series EGYPT and the SAHARA www.johntyman.com/sahara |
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1.2 Climate : 021- 048 |
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| .021. The Sahara is actually the western end of a zone of extreme aridity which extends from the Atlantic to beyond the Red Sea to include the deserts of Arabia, the coastal deserts of Iran and Pakistan, and the Sind Desert in India. |
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| .022. It is a zone characterized year-round by high pressures and descending air masses -- which are warmed adiabatically by the compression caused by the increase in pressure as air descends. Their relative humidity is reduced in the process, so hot dry “tropical continental” air masses are dominant here for most of the year. |
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| .023. The bareness of the land is, of course, a reflection of its climate -- which is characterized by extreme aridity, high temperatures and violent winds. The average annual rainfall will almost everywhere total less than one hundred millimeters, and many places average only 3 or 4 days with rain annually. In some areas it has not rained for years! (Erg Occidental near El Golea) |
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| .024. The strong winds of the Sahara raise both dust and sand grains and drastically reduce visibility. Particles of dust can be carried to great heights and produce brilliant sunsets: but sand grains remain within a metre or two of the surface, forming a moving carpet that is a health hazard. (Sunset at Assekrem) |
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| .025. The number of days with sandstorms varies from year to year and from place to place. In Salah averages 55 days with sandstorms per year, but some places get a lot more. (Start of sandstorm at In Salah) |
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| .026. Dust and sand particles driven by strong winds will penetrate your clothing, your luggage and your tent; but they can also have direct and harmful impacts of your skin, eyes, and respiratory system -- which explains (in part) the facial coverings worn by those who travel here. (In the market at Tozeur) |
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| .027. Sandstorms almost always occur during the day, because there is little wind during the night. The still air then, and the low humidity, is reflected in clear skies and spectacular moonrises. (Moonrise near In Salah) |
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| .028. And because the atmosphere high above deserts is usually free of cloud, it does not filter out damaging ultraviolet rays to the same degree as in a humid environment. With little or no cloud year-round, solar radiation is both prolonged and intense. (Midday radiation near Ain Khudra in the Sinai) |
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| .029. Many places register more than 10 hours of sunshine per day. Tamanrasset averages 3686 hours per year. In addition bare land surfaces in deserts reflect more of the sun’s rays, so that protection from solar radiation is essential. (My guide near Ain Khudra in the Sinai) |
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| .030. The fine, hot and dry weather of the central Sahara is broken from time to time by atmospheric disturbances from either the north or the south. The southern fringe gets most of its rain in summer (between July and September), when more is lost by evaporation. (Remains of roadside pool south of Illizi) |
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| .031. The Sahara’s northern fringe displays a Mediterranean rhythm, with rain in winter (between October and February). This is welcomed by nomads as well as cultivators, but it can disrupt transportation. After a storm the wet heavy sand sticks to the tyres of motor vehicles. (Flooded road north of Tozeur in southern Tunisia) |
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| .032. The rainfall is unreliable as well as low. It comes in sudden storms at irregular intervals. One year no rain at all may fall: then, without warning, there will be a violent downpour which will destroy the camps of those foolish enough to camp in dry river beds and also cut roads ... which justifies expensive causeways across normally dry channels. (Dry riverbed at Biskra) |
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| .033. Over-grazing reduces the beneficial effects of rain, because, instead of percolating downwards, most of the water runs off the bare surface of the ground ... reducing the agricultural value of the moisture and increasing its destructive power. (Herding near El Oued) |
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| .034. The building materials used in the construction of houses in the Sahara are also susceptible to storm damage. In light of the prevailing aridity, most are built of clay and will be damaged by heavy rain. At Siwa, on the eastern edge of the Libyan Desert, houses which had stood for 600 years were destroyed when it rained for 3 days in 1926! (The Shali section of Siwa) |
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| .035. Tamanrasset received 160mm of rain one year but only 6mm a year or two later. In many places significant showers may well occur only once every 10 years. And the variability of the rainfall is paralleled by a limited number of rainy days. In Salah averages 17mm a year, with only 5 days receiving a measurable amount of precipitation. Cairo (shown here) averages 29mm annually, with 7 rainy days. (The al-Urman gardens in Cairo in winter ... its wet season) |
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| .036. The climatic consideration of greatest biological and cultural significance, therefore, is not the annual total rainfall but the average length of the period between storms of sufficient magnitude for some of the water to remain long enough in favoured localities for seeds to germinate, grow into mature plants, and re-seed themselves. On such plants nomadic animals, and indirectly their owners, depend for food. (Pool of rainwater dammed by road in the Tassili-n-Ajjer) |
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| .037. Low lying areas of fine-textured and impermeable soils may fill with water following rain, but such lakes are typically less than a meter in depth and soon evaporate, to produce glistening salt lakes or playas. (Near Touggourt) |
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| .038. In places, oases lying below sea level (the result in part of eolian defoliation) are fed by relatively fresh groundwater, which supports areas of cultivation. But they are saline around their margins, where water flow is insufficient to maintain an adequate movement of water downwards through the soil. (On the outskirts of Siwa) |
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| .039. In addition to being one of the driest places on earth, the Sahara is also one of the hottest, with average annual mean temperatures exceeding 30 degrees Celsius. Clothing is loose to allow air to circulate and evaporate perspiration. (Women in El Oued) |
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| .040. The hottest months in central and northern areas are June, July and August. Relative humidities below 20% are normal then and sometimes drop to 5%. To be free of dehydration under these conditions a person needs between 8 and 15 litres of water each day. (Cafe in Tozeur) |
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| .041. But the Sahara is also notorious for wide annual ranges in temperature. Shade temperatures in the 50s are common in summer but at the very same place it may freeze in winter. (Winter clothing at Kairouan in December) |
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| .042. Also, throughout the year, with clear skies, temperatures drop off rapidly after sunset and the nights are cool; so tourists warm themselves around campfires in winter. Most places in the Sahara will experience temperatures below zero at least once a year, and daily ranges in excess of 35 degrees are common. (Below zero temperatures at Fort Serouenout on Christmas Day) |
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| .043. Because of such wide ranges in temperature between summer and winter and between day and night, what matters most to plants regarding temperature is not the mean but the incidence of high temperatures likely to cause heat damage and low temperatures which may also impact on plant growth. (Sun-scorched plant near Tozeur) |
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| .044. Since there is little if any shade, official temperature readings taken in shaded instrument enclosures are less representative of the actual conditions experienced during the heat of the day than they might be elsewhere. Similarly, due to the paucity of both vegetation and soil moisture, soil temperatures will be higher also. In summer the temperature of the sand may reach 70 degrees Celsius -- so it is best not to go barefoot then! (At Ain Khudra in winter) |
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| .045. The walls of houses can be just as hot, and temperatures inside may exceed 40 degrees. So walls are usually thick, with few windows, and many people spend the night in the open on their rooftops. [See frames 350-354] The white and pink walls of houses in Ghardaia are made of sand, clay and gypsum to reflect at least some of the sun’s heat rays. (Ghardaia) |
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| .046. Temperatures are moderated slightly by altitude ... especially daily maxima, both mean and extreme. The annual precipitation is also likely to be higher in mountain areas. North of the Sahara the Atlas Mountains are covered by snow in winter but all the streams that flow south from there are intermittent. (Atlas Mountains north of Biskra) |
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| .047. Assekrem in the central Sahara averages164 mm of precipitation per year. Snow may fall here in winter, and waterholes freeze over. The vegetation of damper sections is different here also, and includes Mediterranean species like the olive and the fig, together with stunted cypresses and myrtles. Assekrem was for 11 years the home of the famous French writer and hermit Father Charles de Foucauld, murdered nearby at Tamanrasset in 1916. (Assekrem Mountains of the Hoggar) |
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| .048. The climatic implication of which tourists are reminded most forcefully, though, is that with so little rain and such dry air, nothing decays here and the use of toilet paper is actively discouraged ... since it can blow around for decades. (Garbage outside Illizi) |
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