Editing Kalahari Desert (section)

==Climate==
The climate is sub-humid rather than semi-arid in the north and east, where the dry forests, savannahs, and salt lakes prevail. South and west, where the vegetation is predominantly [[Deserts and xeric shrublands|xeric]] savanna or even a semi-desert, the climate is "Kalaharian" [[semi-arid]]. The Kalaharian climate is subtropical (average annual temperature greater than or equal to 18&nbsp;°C, at peaks reaching 40&nbsp;°C and above, with mean monthly temperature of the coldest month strictly below 18&nbsp;°C), and is semi-arid with the dry season from April to September, the coldest six months of the year. It is the southern tropical equivalent of the [[Sahel]]ian climate with the wet season during summer. The altitude has been adduced as the explanation why the Kalaharian climate is not tropical; its altitude ranges from 600 to 1600 meters (and generally from 800 to 1200 meters), resulting in a cooler climate than that of the Sahel or [[Sahara]]. For example, winter frost is common from June to August, rarely seen in the warmer Sahelian regions.<ref>{{in lang|fr}} ''Les milieux désertiques'', Jean Demangeot, Edmond Bernus, 2001. Editor: Armand Colin. {{ISBN|9782200251970}}, page 20 in particular.</ref> For the same reason, summer temperatures certainly can be very hot, but not in comparison to regions of low altitude in the Sahel or Sahara, where some stations record average temperatures of the warmest month around 38&nbsp;°C, whereas the average temperature of the warmest month in any region in the Kalahari never exceeds 29&nbsp;°C, though daily temperatures occasionally reach up to close to {{convert|45|°C|°F|0|abbr=on}} (44.8&nbsp;°C at Twee Rivieren Rest Camp in 2012).<ref>{{cite web|url=http://www.mherrera.org/records.htm|title=World Record Temperatures -Highest Lowest Hottest Coldest temperatures-|website=www.mherrera.org}}</ref>

[[File:Gewitter in der Kalahari.jpg|thumb|Heavy [[thunderstorm]] near [[Stampriet]]]]
The dry season lasts eight months or more, and the wet season typically from less than one month to four months, depending on location. The southwestern Kalahari is the driest area, particularly a small region toward the west-southwest of Tsaraxaibis (Southeast of Namibia). The average annual rainfall ranges from around 110&nbsp;mm (close to aridity) to more than 500&nbsp;mm in some north and east areas. During summertime in all regions, rainfall may go with heavy thunderstorms. In the driest and sunniest parts of the Kalahari, over 4,000 hours of sunshine are recorded annually on average.

In the Kalahari, there are three main mechanisms of atmospheric circulation, dominated by the [[Kalahari High]] [[anticyclone]]<ref>{{in lang|fr}} Tropicalité Jean Demangeot Géographie physique intertropicale, pages 44–45, Figure 19, source: Leroux 1989.</ref> in winter, and by the Kalahari Heat Low in summer:<ref> P. D. Tyson FRSSAf & S. J. Crimp (1998) THE CLIMATE OF THE KALAHARI TRANSECT, Transactions of the Royal Society of South Africa, 53:2, 93–112, DOI: 10.1080/00359199809520380 </ref>

* The North and Northwest of the Kalahari are subject to the alternation "[[Intertropical Convergence Zone]] (ITCZ)/"Continental [[Trade winds]]". The ITCZ is the meeting area of the boreal trade winds with their austral counterparts what meteorologists call "Meteorological equator" and the sailors "Doldrum" or "Pot-au-noir" : the ITCZ generates rains in the wet season, whereas the continental trade winds cause the dry season;
* The rest of the Kalahari is subject to the maritime trade winds that largely shed their moisture as they cross up and over the [[Great Escarpment, Southern Africa|Southern African Great Escarpment]] before arriving over the Kalahari.
*In the southern hemisphere summer (from December to February), a low-pressure system develops over the Kalahari, which is driven by strong surface heating (known as a [[Thermal low|Thermal Low]]). The low-pressure system helps to control the moisture-bearing easterly wind emanating from the Indian Ocean,<ref> P. D. Tyson FRSSAf & S. J. Crimp (1998) THE CLIMATE OF THE KALAHARI TRANSECT, Transactions of the Royal Society of South Africa, 53:2, 93–112, DOI: 10.1080/00359199809520380 </ref> thus influencing rainfall across southern Africa.<ref>{{Cite book| url=https://oxfordre.com/climatescience/display/10.1093/acrefore/9780190228620.001.0001/acrefore-9780190228620-e-513 | doi=10.1093/acrefore/9780190228620.013.513 | chapter=Climate of Southern Africa | title=Oxford Research Encyclopedia of Climate Science | year=2017 | last1=Reason | first1=C.J.C. | isbn=978-0-19-022862-0 }}</ref>

There are huge [[Subterranean river|subterranean]] water reserves beneath parts of the Kalahari; the [[Dragon's Breath Cave]], for example, is the largest documented non-subglacial underground lake. Such reserves may partly be the residues of ancient lakes; the Kalahari Desert was once a much wetter place. The ancient [[Lake Makgadikgadi]] dominated the area, covering the Makgadikgadi Pan and surrounding areas, but it drained or dried out some 10,000 years ago. It may have once covered as much as {{convert|120000|km2}}.<ref>{{cite book|last1=Goudie|first1=Andrew|title=Great Warm Deserts of the World: Landscapes and Evolution|date=2002|publisher=Oxford University Press|page=204}}</ref> In ancient times, there was sufficient moisture for farming, with dikes and dams collecting the water. These are now filled with sediment, breached, or no longer in use, though they can be readily seen via Google Earth.<ref>{{Cite web|url=http://www.sciforums.com/threads/ancient-dikes-dams-and-reservoirs-of-the-kalahari-region.159050/|title=Ancient Dikes, Dams and Reservoirs of the Kalahari Region|website=www.sciforums.com}}</ref>

The Kalahari has had a complex climatic history over the past million or so years, in line with major global changes. Changes in the last 250,000 years have been reconstructed from various data sources, providing evidence of former extensive lakes and drier periods. During the latter, the area of the Kalahari has expanded to include parts of western Zimbabwe, Zambia, and Angola.<ref>Thomas, D.S.G. and Shaw, P.A. 1991 'The Kalahari Environment'. Cambridge University Press, Cambridge</ref>