question: The Sahara Desert and other large deserts around 30 degrees north are said to be caused by the persistent high pressure in the horse latitudes. However, some extremely fertile areas like the American South, India, etc. are also near the same latitude. How is this possible?
comment: I believe it has alot to do with winds, and the minerals blown from the deserts into these fertile lands
passages: Modulation of the precession driven insolation cycle is the primary impact of obliquity on the North African Monsoon. Evidence for the impact of obliquity on the intensity of the North African Monsoon has been found in records of dust deposits from ocean cores in the Eastern Mediterranean that occur as a result of Aeolian processes. This evidence requires complex feedback mechanisms to explain since the strongest impact of obliquity on insolation is found in the high latitudes. Two possible mechanisms for the existence of an obliquity tracer found in the Eastern Mediterranean Aeolian dust deposits have been proposed. The first of which suggests that at times of higher obliquity the temperature gradient between the poles and the equator in the southern hemisphere is greater during boreal summer (summer in the northern hemisphere). As a result of this gradient the strength of the North African Monsoon increases. A second theory that may explain the existence of an obliquity signature in the North African climate record suggests that obliquity maybe related to changes in the latitude of the tropics. The latitudinal extent of the tropics is roughly defined by the maximum wandering path of the thermal equator. An area that today is located between the Tropic of Capricorn and the Tropic of Cancer. However, as the obliquity changes, the overall wandering path of the thermal equator shifts between 22.2° and 24.5° north and south. This wandering may affect the positioning of the North African Summer Monsoon Front and thus impact the perceived strength of the North African Monsoon. Further confirmation of the impacts of obliquity on the North African Monsoonal have been provided through a global fully coupled atmosphere–ocean–sea ice climate model, which confirmed that precession and obliquity can combine to increase precipitation in North Africa through insolation feedbacks.
   Another key piece of evidence for a processional control on the North African Monsoon can be found in the deposits of freshwater diatoms in the tropical Atlantic. Ocean cores from the tropical Atlantic have been found to have distinct layers of the freshwater diatom Aulacoseira Granulata also known as Melosira Granulata. These layers occur on a 23,000 year cycle that lags the maximum in precession insolation by roughly 5000 to 6000 years. To explain these cyclic freshwater diatom deposits we have to look inland at the Sahara region of Africa. Around the time of the insolation maximum in the precession cycle the North African Monsoon is at its strongest and the Sahara region becomes dominated by large monsoonal lakes. Then as time progress toward the insolation minima, these lakes begin to dry out due to weakening North African Monsoon. As the lakes dry up thin sediment deposits containing freshwater diatoms are exposed. Finally, when the prevailing northeasterly winds arrive during winter, the freshwater diatom deposits in the dried lake beds are picked up as dust and carried thousands of kilometers out into the tropical Atlantic. From this series of events the reason for 5000 to 6000-year delay in the freshwater diatom deposits is evident, since the North African Monsoon must become sufficiently weak before the monsoonal lakes in the Sahara begin to dry up and expose potential freshwater diatom sources. One key factor that must be noted with freshwater diatom deposits is species identification. For instance some ocean cores directly off the western coast of Africa show a mix of freshwater lake and river diatom species. So for a core to accurately represent the diatom cycle of the Sahara it must be recovered from a region of the tropical Atlantic that has sufficient distance from the coast such that the impacts of river outflows are minimized.
   Observed variations in the strength of the eastern equatorial Atlantic upwelling zone can also be used to support a cycle of the North African Monsoon that is regulated by the precession cycle. When insolation in North Africa is at its peak during the precession cycle the easterly trade winds over the equatorial Atlantic are strongly diverted toward the Sahara. This diversion weakens the equatorial upwelling zone in the eastern equatorial Atlantic, resulting in warmer waters in the pelagic. On the other end of the spectrum when insolation in North Africa is at a minimum due to the precession cycle, the diversion of the easterly trade winds is relatively weak. Due to this the region of upwelling in the eastern equatorial Atlantic remains strong and the waters in the pelagic zone are cooler. The proof that this pattern of periodic weakening of the eastern equatorial Atlantic upwelling exists is found in deposits of surface dwelling planktic organisms in ocean sediment cores. Such cores show that the relative abundance of warm and cold water planktic species vary with a consistent beat of 23,000 years, matching the 23,000 year precession insolation cycle.
   The consistently warm, dry, and sunny conditions of the horse latitudes are the main cause for the existence of the world's major non-polar deserts, such as the Sahara Desert in Africa, the Arabian and Syrian deserts in the Middle East, the Mojave and Sonoran deserts in the southwestern United States and northern Mexico, all in the Northern Hemisphere; and the Atacama Desert, the Kalahari Desert, and the Australian Desert in the Southern Hemisphere.
  
  Section::::Etymology.
   The Saharan Air Layer (SAL) is an extremely hot, dry and sometimes dust-laden layer of the atmosphere that often overlies the cooler, more-humid surface air of the Atlantic Ocean. In the Sahara Desert region of North Africa, where it originates, it is the prevalent atmosphere, extending from the surface upwards several kilometers. As it drives, or is driven, out over the ocean, it is lifted above the denser marine air. This arrangement is an inversion where the temperature increases with height. The boundary between the SAL and the marine layer suppresses or "caps" any convection originating in the marine layer. Since it is dry air, the lapse rate within the SAL itself is steep, that is, the temperature falls rapidly with height.
   An example of the Saharan pump has occurred after the Last Glacial Maximum (LGM). During the Last Glacial Maximum the Sahara desert was more extensive than it is now with the extent of the tropical forests being greatly reduced. During this period, the lower temperatures reduced the strength of the Hadley Cell whereby rising tropical air of the Inter-Tropical Convergence Zone (ITCZ) brings rain to the tropics, while dry descending air, at about 20 degrees north, flows back to the equator and brings desert conditions to this region. This phase is associated with high rates of wind-blown mineral dust, found in marine cores that come from the north tropical Atlantic.
   Many of these variations in the early Holocene appear to be caused by the discharge of meltwater from the Laurentide Ice Sheet into the Atlantic, which weakens the Atlantic meridional overturning circulation and shift storm tracks north away from the Mediterranean. Some dry periods in marine cores in the Gulf of Guinea appear to coincide with events recorded in Greenland ice cores. Other variations in precipitation observed in records have been attributed to solar activity changes, water levels of Lake Turkana for example appear to reflect the 11-year solar cycle.
   Insolation, which is simply a measure of the amount of solar radiation received on a given surface area in a given time period, is the fundamental factor behind the Orbital Monsoon Hypothesis. Due to variations in heat capacity, continents heat up faster than surrounding oceans during summer months when insolation is at its strongest and cool off faster than the surrounding oceans during winter months when insolation is at its weakest. The wind pattern that results from the continent/ocean insolation temperature gradient is known as a monsoon. Values of summer insolation are more important for a region's climate than winter values. This is because the winter phase of a monsoon is always dry. Thus the flora and fauna of a monsoonal climate are determined by the amount of rain that falls during the summer phase of the monsoon. Over periods of tens to hundreds of thousands of years the amount of insolation changes in a highly complex cycle that is based on orbital parameters. The result of this cycle of insolation is a waxing and waning in the strength of the monsoonal climates across the globe. A wide range of geologic evidence has shown that the North African Monsoon is particularly susceptible to insolation cycles, and long term trends in monsoonal strength can be linked to slow variations in insolation. However, the abrupt shifts back and forth from the "green Sahara" to the "desert Sahara" are not entirely explained by long term changes in the insolation cycle.
   In the mountains of Ethiopia, many areas receive over 2,000 mm (80 in) per year, and even Asmara receives an average of 570 mm (23 in). This rainfall is the sole source of water for many areas outside Ethiopia, including Egypt. In the winter, the northeasterly trade winds do not provide any moisture except in mountainous areas of northern Somalia, where rainfall in late autumn can produce annual totals as high as 500 mm (20 in). On the eastern coast, a strong upwelling and the fact that the winds blow parallel to the coast means annual rainfall can be as low as 50 mm (2 in).
   Dallol in the Danakil Depression is one of the hottest places year-round anywhere on Earth. There is no rain for most of the year; the yearly rainfall averages range from 100 to 200 millimetres (4 to 7 in), with even less rain falling closer to the coast. The climate varies from around during the very short rainy season (November–February) to during the dry season (February–November); "see" climate of the lowlands of the Danakil Depression.
   BULLET::::- In northern summer, an enormous low pressure system known as the "monsoon" forms over the huge Africa-Asia-Europe landmass. Centred over approximately Rajasthan and prevented by the huge barrier of the Himalayas from moving further west, it extends a strong trough over West Africa. This trough sends warm and saturated westerly winds over West Africa, creating a wet season peaking in June on the coast (as the trough moves north) and in August inland.
   Section::::Orbital Monsoon Hypothesis.:Insolation.
   In the Kalahari, there are two main mechanisms of atmospheric circulation, dominated by the Kalahari High anticyclone:
  
  BULLET::::- The North and North-west of the Kalahari is 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;
   BULLET::::- Two high elevation winds, the African Easterly Jet and the Tropical Easterly Jet modulate atmospheric air flows over Africa and thus also the amount of precipitation; the Tropical Easterly Jet comes from India and is powered by temperature gradients between the tropics and the subtropics while the African Easterly Jet is powered by temperature gradients in the Sahel. A stronger West African Monsoon resulted in a weaker African Easterly Jet and thus decreased transport of moisture out of Africa.
  
  BULLET::::- Increased latent heat fluxes play a role in explaining the moistening in coupled climate models.
   BULLET::::- Increased precipitation during winter is correlated with a larger extent of Mediterranean precipitation and might have aided in the establishment of the African humid period, especially in North Africa, around the northern Red Sea, in the Tibesti and in northern Arabia and generally at higher latitudes where the monsoon did not arrive. It has also been proposed for the Tibesti region. This precipitation may have extended to other parts of the Sahara; such would have led to the areas of summer and winter precipitation overlapping. Such changes in Mediterranean-derived precipitation may correlate with changes in the North Atlantic and Arctic Oscillations.
   BULLET::::- Gobi Desert to Korea, Japan, Taiwan (at times) and even Western USA (blowing east). See also Asian dust.
  
  BULLET::::- Thar Desert pre-monsoon towards Delhi, Uttar Pradesh, Indo-Gangetic Plain. See also 2018 Indian dust storms.
  
  BULLET::::- Shamal June–July winds blowing dust in primarily north to south in Saudi Arabia, Iran, Iraq, UAE, and parts of Pakistan.
  
  BULLET::::- Haboob dust storms in Sudan, Australia, Arizona associated with monsoon.
  
  BULLET::::- Khamsin dust from Libya, Egypt and Levant in Spring associated with extratropical cyclones.
  
  BULLET::::- Dust Bowl event in USA, carried sand eastward. 5500 tons were deposited in Chicago area.
   Orographic precipitation is well known on oceanic islands, such as the Hawaiian Islands or New Zealand, where much of the rainfall received on an island is on the windward side, and the leeward side tends to be quite dry, almost desert-like, by comparison. This phenomenon results in substantial local gradients of average rainfall, with coastal areas receiving on the order of per year, and interior uplands receiving over per year. Leeward coastal areas are especially dry—less than per year at Waikiki—and the tops of moderately high uplands are especially wet—about per year at Wai'ale'ale on Kaua'i.
   A great percentage of the world's population lives in the equatorial regions, but for the most part, these regions do not experience weather as it is understood by this definition. The Sahara in northern Africa, for instance, is almost uniformly hot, sunny and dry all year long especially due to the non-stop presence of high atmospheric pressure aloft, whereas weather trends on the Indian subcontinent and in the western Pacific, for instance, the monsoonal belt, occur gradually over the very long term, and the diurnal weather patterns remain constant.
   In the north, along the Mediterranean shore, cool onshore winds blow inland, while further south, hot, dry winds, known as "Ghibli", blow from the interior, creating blinding sand-storms.
  
  Section::::History.
  
  Section::::History.:Historical desert.
   BULLET::::- "Multidecadal variability in East African hydroclimate controlled by the Indian Ocean", Nature, 2013
  
  BULLET::::- "Late -twenthieth-century warming in Lake Tanganyika unprecedented since AD 500", Nature Geoscience, 2010
  
  BULLET::::- "Late Quaternary behavior of the East African monsoon and the importance of the Congo Air Boundary", Quaternary Science Reviews, 2011
  
  BULLET::::- "Abrupt Shifts in Horn of Africa Hydroclimate Since the Last Glacial Maximum", Science, 2013
  
  BULLET::::- "Coordinated hydrological regimes in the Indo- Pacific region during the past two millennia", Paleoceanography, 2010
   The African humid period that took place in East Africa on the other hand appears to have been caused by different mechanisms, namely decreased seasonality of precipitation, increased precipitation or shorter dry seasons and increased inflow of moisture from the Atlantic and Indian Oceans. The Atlantic moisture inflow was in part triggered by a stronger West African and Indian monsoon, perhaps explaining why the effects of the African humid period extended into the Southern Hemisphere. The effect of the easterly trade winds is unclear; increased moisture transport by easterly trade winds may have aided in the development of the African humid period but alternatively a stronger Indian Monsoon that draws easterly winds away from East Africa may have occurred.
   Whether some parts of the desert such as highlands like the Red Sea Hills were reached by the westerlies or weather systems associated with the subtropical jet stream and thus received precipitation is contentious and only clearly supported for the Maghreb in northwestern Africa, river flow/terrace formation and lake development in the Tibesti and Jebel Marra mountains as well as residual Nile flow may be explained in this way. The highlands of Africa appear to have been less affected by drought during the last glacial maximum. Finally, glaciers were active in the Bale and Semien Mountains of Ethiopia during the last glacial maximum.
   The idea that changes in insolation (solar heating) caused by long-term changes in the Earth's orbit are a controlling factor for the long-term variations in the strength of monsoon patterns across the globe was first suggested by Rudolf Spitaler in the late nineteenth century, The hypothesis was later formally proposed and tested by the meteorologist John Kutzbach in 1981. Kutzbach's ideas about the impacts of insolation on global monsoonal patterns have become widely accepted today as the underlying driver of long term monsoonal cycles. Kutzbach never formally named his hypothesis and as such it is referred to here as the "Orbital Monsoon Hypothesis" as suggested by Ruddiman in 2001.
   A similar class of winds, the Sirocco, the Bora and Santa Ana winds, are examples where orographic lifting has limited effect since there is limited moisture to remove in the Saharan or other air masses; the Sirocco, Bora and Santa Ana are driven primarily by (adiabatic) compression heating.
  
  Section::::Effects of orographic lifting.:Associated clouds.
  
  As air flows over mountain barriers, orographic lift can create a variety of cloud effects.
   Section::::Orbital Monsoon Hypothesis.:Eccentricity.
  