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Problems of Vertical and Horizontal Arrangement in the High Mountains of the Extreme Arid Zone (Central Sahara)

B. Messerli
Arctic and Alpine Research
Vol. 5, No. 3, Proceedings of the Symposium of the International Geographical Union Commission on High Altitude Geoecology (Late Summer, 1973), pp. A139-A147
Stable URL: http://www.jstor.org/stable/1550163
Page Count: 9
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Problems of Vertical and Horizontal Arrangement in the High Mountains of the Extreme Arid Zone (Central Sahara)
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Abstract

Three geomorphological-ecological altitude stages in the mountains of the extreme arid zone are described: glacial or nival forms, solifluction or "periglacial appearances," and the problem of the upper tree line. The present altitude levels are not thermally dependent in the alpine, Mediterranean, or fully tropical sense, but are primarily influenced by availability of moisture. They are not comparable with high mountains of other climatic zones and are subject to their own laws. In the fully arid mountain areas of the tropics an extratropical thermal seasonal climate clashes with a tropical monsoon precipitation distribution. As the winter temperature regime at higher levels is adequate to induce frost-climate processes in winter, not a greater height, but a higher moisture supply, is required for initiation of snow and frost processes. Direction of wind and precipitation, as well as a sheltered radiation position, are more important than altitude. The summer precipitation with some heavy rainfall is insufficient at the upper tree line. Humidity does not increase linearly with elevation. Temperature conditions are suitable for tree growth in the high areas of these mountains, but moisture supply is the limiting factor, as is the case for the formation of nival forms and patterned ground. Temperatures of the free air show only a slight curvature in summer over the dry belt and in winter a regular increase towards the equator. Missing altitude levels cannot, therefore, be thermally explained and extended theoretically across the arid zone. The high mountains of the arid belt develop a zone of thermal readiness with punctual appearances of nivation, patterned ground, and tree vegetation, dependent on episodic or local moisture supply but independent of altitude and latitude. If aridity is ameliorated and temperature falls (e.g., as during the last cold period), there will be a double effect. The thermally prepared altitude level becomes active and is simultaneously extended to lower levels by the temperature depression. Therefore, the mountains in the arid zone of the tropics, on a borderline between northern and southern climatic influences, will become a most sensitive indicator for the very smallest climatic and circulatory oscillations. A very fine differentiation of cloud formation between different mountain ranges and even within the same mountain range is shown by satellite pictures. The currently accepted vertical and horizontal arrangement of the high mountains in the extreme arid zone may require significant revision as more data become available.

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