North Africa

The dominance of high-pressure conditions in the Sahara is marked by the low average precipitation in this region. Over most of the central Sahara, the mean annual precipitation is less than 25 mm, although the high plateaux of the Ahaggar and Tibesti receive over 100 mm. Parts of western Algeria have gone at least two years without more than 0.1 mm of rain in any twenty-four-hour period, and most of southwest Egypt as much as five years. However, twenty-four-hour storm rainfalls approaching 50 mm (more than 75 mm over the high plateaux) may be expected in scattered localities. During a thirty-five-year period, excessive short-period

What The Weather Climate Egypt

Figure 10.29 Seasons of maximum precipitation for Europe and North Africa, together with average monthly and annual figures (mm) for twenty-eight stations.

Sources: Thorn (1965) and Huttary (1950). Reprinted from D. Martyn (1992) Climates of the World, with kind permission from Elsevier Science NL, Sara Burgerhartstraat 25, 1055 KV Amsterdam, The Netherlands.

Figure 10.29 Seasons of maximum precipitation for Europe and North Africa, together with average monthly and annual figures (mm) for twenty-eight stations.

Sources: Thorn (1965) and Huttary (1950). Reprinted from D. Martyn (1992) Climates of the World, with kind permission from Elsevier Science NL, Sara Burgerhartstraat 25, 1055 KV Amsterdam, The Netherlands.

rainfall intensities occurred in the vicinity of west-facing slopes in Algeria, such as at Tamanrasset (46 mm in sixty-three minutes) (Figure 10.30), El Golea (8.7 mm in three minutes) and Beni Abes (38.5 mm in twenty-five minutes). During the summer, rainfall variability is introduced into the southern Sahara by the variable northward penetration of the monsoon trough (see Figure 11.2B), which on occasion allows tongues of moist southwesterly air to penetrate far north and produce short-lived low-pressure centres. Study of these Saharan depressions has permitted a clearer picture to emerge of the region. In the upper troposphere at about 200 mb (12 km), the westerlies overlie the poleward flanks of the subtropical high-pressure belt. Occasionally, the individual high-pressure cells contract away from one another as meanders develop in the westerlies between them. These may extend equator-ward to interact with the low-level tropical easterlies (Figure 10.31). This interaction may lead to the development of lows, which then move northeast along the meander trough associated with rain and thunder. By the time they reach the central Sahara, they are frequently 'rained out' and give rise to dust storms, but they can be reactivated further north by the entrainment of moist Mediterranean air. The interaction of westerly and easterly circulation is most likely to occur around the equinoxes or sometimes in winter if the otherwise dominant Azores high-pressure cell contracts westward. The westerlies may also affect the region through the penetration of cold fronts south from the Mediterranean, bringing heavy rain to localized desert areas. In December 1976, such a depression produced up to 40 mm of rain during two days in southern Mauretania.

5 Australasia

The subtropical anticyclones of the South Atlantic and Indian Ocean tend to generate high-pressure cells which move eastward, intensifying southeast of South Africa and west of Australia. These are warm-core systems formed by descending air and extending through the troposphere. The continental intensification of the constant eastward progression of such cells causes pressure maps to give the impression of the existence of

North Africa Map Ahaggar Mountains

Figure 10.30 Track of a storm and the associated three-hour rainfall (mm) during September 1950 around Tamanrasset in the vicinity of the Ahaggar Mountains, southern Algeria.

Source: Partly after Goudie and Wilkinson (1977).

Figure 10.30 Track of a storm and the associated three-hour rainfall (mm) during September 1950 around Tamanrasset in the vicinity of the Ahaggar Mountains, southern Algeria.

Source: Partly after Goudie and Wilkinson (1977).

Ahaggar Mountains

Figure 10.31 Interaction between the westerlies and the tropical easterlies leading to the production of Saharan depressions (D), which move northeastward along a trough axis.

Source: After Nicholson and Flohn (1980), copyright © 1980/1982 D. Reidel Publishing Company. Reprinted by permission.

Figure 10.31 Interaction between the westerlies and the tropical easterlies leading to the production of Saharan depressions (D), which move northeastward along a trough axis.

Source: After Nicholson and Flohn (1980), copyright © 1980/1982 D. Reidel Publishing Company. Reprinted by permission.

a stable anticyclone over Australia (Figure 10.32). About forty anticyclones traverse Australia annually, being somewhat more numerous in spring and summer than in autumn and winter. Over both oceans, the frequency of anticyclonic centres is greatest in a belt around 30°S in winter and 35-40°S in summer; they rarely occur south of 45°S.

Between successive anticyclones are low-pressure troughs containing inter-anticyclonic fronts (sometimes termed 'polar') (Figure 10.33). Within these troughs, the subtropical jet stream meanders equatorward, accelerates (particularly in winter, when it reaches an average velocity of 60 ms-1 compared with a mean annual value of 39 ms-1) and generates upper-air depressions, which move southeastward along the front (analogous to the systems in North Africa). The variation in strength of the continental anticyclones and the passage of inter-anticyclonic fronts cause periodic inflows of surrounding maritime tropical airmasses from the Pacific (mTp) and the Indian (mTi) oceans. In

High Pressure System Inflow Jet Stream

Figure 10.32 Airmass frequencies, source areas, wind directions and dominance of the cT high-pressure cell over Australia in summer (A) and winter (B).

Source: After Gentilli (1971).

Figure 10.32 Airmass frequencies, source areas, wind directions and dominance of the cT high-pressure cell over Australia in summer (A) and winter (B).

Source: After Gentilli (1971).

addition, there are incursions of maritime polar air (mP) from the south, and variations in strength of the local source of continental tropical (cT) airmasses (see Figure 10.32).

The high-pressure conditions over Australia promote especially high temperatures over central and western parts of the continent, towards which there is a major heat transport in summer. These pressures keep average rainfall amounts low; these normally total less than 250 mm annually over 37 per cent of Australia. In winter, upper-air depressions along the inter-anticyclonic fronts bring rain to southeastern regions and also, in conjunction with mTi incursions, to southwest Australia. In summer, the southward movement of the intertropical convergence zone and its transformation into a monsoon trough brings on the wetter season in northern Australia (see Chapter 11D), and the onshore southeast trades bring rain along the eastern seaboard.

New Zealand is subject to climatic controls similar to those of southern Australia (Figure 10.33). Anticyclones, separated by troughs associated with cold fronts often deformed into wave depressions, cross the region on average once a week. Their most southerly track (38.5°S) is taken in February. The eastward rate of anticyclonic movement averages about 570 km/day in May to July and 780 km/day in October to December. Anticyclones occur some 7 per cent of the time and are associated with settled weather, light winds, sea breezes and some fog. On the eastern (leading) edge of the high-pressure cell the airflow is usually cool, maritime and southwesterly, interspersed with south or southeasterly flow producing drizzle. On the western side of the cell, the airflow is commonly north or northwesterly, bringing mild and humid conditions. In autumn, high-pressure conditions increase in frequency up to 22 per cent, giving a drier season.

Simple troughs with undeformed cold fronts and relatively simple interactions between the trailing and leading edge conditions of the anticyclones persist in about 44 per cent of the time during winter, spring and summer, compared with only 34 per cent in autumn. Wave depressions occur with about the same frequency. If a wave depression forms on the cold front to the west of New Zealand, it usually moves southeastward along the front, passing to the south of the country. In contrast, a depression forming over New Zealand may take thirty-six to forty-eight hours to clear the country, bringing prolonged rainy conditions (e.g. Figure 10.34). Relief, especially the Southern Alps, predominantly controls rainfall amounts. West- or northwest-facing mountains receive an average annual precipitation in excess of 2500 mm, with some parts of South Island exceeding 10 000 mm (see Figure 5.15). The eastern lee areas have much lower amounts, with less than 500 mm in some parts. North Island has a winter precipitation maximum, but South Island, under the influence of depressions in the southern westerlies, has a more variable seasonal maximum.

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  • lidya
    What are the weather conditions and climate condition in africa?
    7 years ago
  • annukka
    Where is the ahaggar Mountains in north africa?
    7 years ago
  • rufus
    What is the weather climate in egypt?
    6 years ago
  • milo
    Why do high pressure cells move southward in Africa during winter?
    6 years ago
  • susanne
    Where is the lowest average precipitation in north africa?
    3 years ago
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  • courtney
    How anticyclones affects south african climate in summer and winter?
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    How does africa affect weather for north africa?
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    How wouldb discribe the climate of north africa?
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  • AZIZA
    What is the yearly climate for north africa?
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  • annett
    What is the mean average average precipitation for northern africa?
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    What is the weather in the northern region of africa tomorrow?
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    What is the average temp in the sahara of north africa?
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    Which month is winter in north africa?
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    Which jet stream affect weather if south africa?
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  • georgina
    Why regions in the North of Africa receive less than 500 mm of rain a year?
    6 months ago
  • ASMERET
    Why does regions in the north of Africa receive less than 500mm of rain a year?
    5 months ago

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