The Sahara is the world's largest low-latitude hot desert. It is located in the horse latitudes under the subtropical ridge,
a significant belt of semi-permanent subtropical warm-core high
pressure where the air from upper levels of the troposphere tends to
sink towards the ground. This steady descending airflow causes a warming
and a drying effect in the upper troposphere. The sinking air prevents
evaporating water from rising, and therefore prevents adiabatic cooling, which makes cloud formation extremely difficult to nearly impossible.
The permanent dissolution of clouds allows unhindered light and thermal radiation. The stability of the atmosphere
above the desert prevents any convective overturning, thus making
rainfall virtually non-existent. As a consequence, the weather tends to
be sunny, dry and stable with a minimal chance of rainfall. Subsiding,
diverging, dry air masses associated with subtropical high-pressure systems
are extremely unfavorable for the development of convectional showers.
The subtropical ridge is the predominant factor that explains the hot desert climate (Köppen climate classification BWh)
of this vast region. The descending airflow is the strongest and the
most effective over the eastern part of the Great Desert, in the Libyan
Desert: this is the sunniest, driest and the most nearly "rainless"
place on the planet, rivaling the Atacama Desert, lying in Chile and Peru.
The rainfall inhibition and the dissipation of cloud cover are
most accentuated over the eastern section of the Sahara rather than the
western. The prevailing air mass
lying above the Sahara is the continental tropical (cT) air mass, which
is hot and dry. Hot, dry air masses primarily form over the
North-African desert from the heating of the vast continental land area,
and it affects the whole desert during most of the year. Because of
this extreme heating process, a thermal low is usually noticed near the surface, and is the strongest and the most developed during the summertime. The Sahara High represents the eastern continental extension of the Azores High, centered over the North Atlantic Ocean.
The subsidence of the Sahara High nearly reaches the ground during the
coolest part of the year, while it is confined to the upper troposphere
during the hottest periods.
The effects of local surface low pressure are extremely limited
because upper-level subsidence still continues to block any form of air
ascent. Also, to be protected against rain-bearing weather systems by
the atmospheric circulation itself, the desert is made even drier by its
geographical configuration and location. Indeed, the extreme aridity of
the Sahara is not only explained by the subtropical high pressure: the Atlas Mountains
of Algeria, Morocco and Tunisia also help to enhance the aridity of the
northern part of the desert. These major mountain ranges act as a
barrier, causing a strong rain shadow effect on the leeward side by dropping much of the humidity brought by atmospheric disturbances along the polar front which affects the surrounding Mediterranean climates.
The primary source of rain in the Sahara is the Intertropical Convergence Zone, a continuous belt of low-pressure systems near the equator which bring the brief, short and irregular rainy season to the Sahel
and southern Sahara. Rainfall in this giant desert has to overcome the
physical and atmospheric barriers that normally prevent the production
of precipitation. The harsh climate of the Sahara is characterized by:
extremely low, unreliable, highly erratic rainfall; extremely high
sunshine duration values; high temperatures year-round; negligible rates
of relative humidity; a significant diurnal temperature variation; and extremely high levels of potential evaporation which are the highest recorded worldwide.
Temperature
The sky is usually clear above the desert, and the sunshine duration
is extremely high everywhere in the Sahara. Most of the desert has more
than 3,600 hours of bright sunshine per year (over 82% of daylight
hours), and a wide area in the eastern part has over 4,000 hours of
bright sunshine per year (over 91% of daylight hours). The highest
values are very close to the theoretical maximum value. A value of 4300
hours (98%) of the time would be recorded in Upper Egypt (Aswan, Luxor) and in the Nubian Desert (Wadi Halfa). The annual average direct solar irradiation is around 2,800 kWh/(m2 year) in the Great Desert. The Sahara has a huge potential for solar energy production.
Sand dunes in the Sahara
The high position of the Sun, the extremely low relative humidity, and the lack of vegetation
and rainfall make the Great Desert the hottest large region in the
world, and the hottest place on Earth during summer in some spots. The
average high temperature exceeds 38 to 40 °C or 100.4 to 104.0 °F during
the hottest month nearly everywhere in the desert except at very high
altitudes. The world's highest officially recorded average daily high
temperature was 47 °C or 116.6 °F in a remote desert town in the Algerian Desert called Bou Bernous, at an elevation of 378 metres (1,240 ft) above sea level, and only Death Valley, California rivals it. Other hot spots in Algeria such as Adrar, Timimoun, In Salah, Ouallene, Aoulef, Reggane
with an elevation between 200 and 400 metres (660 and 1,310 ft) above
sea level get slightly lower summer average highs, around 46 °C or
114.8 °F during the hottest months of the year. Salah, well known in
Algeria for its extreme heat, has average high temperatures of 43.8 °C
or 110.8 °F, 46.4 °C or 115.5 °F, 45.5 °C or 113.9 °F and 41.9 °C or
107.4 °F in June, July, August and September respectively. There are
even hotter spots in the Sahara, but they are located in extremely
remote areas, especially in the Azalai, lying in northern Mali. The major part of the desert experiences around three to five months when the average high strictly
exceeds 40 °C or 104 °F; while in the southern central part of the
desert, there are up to six or seven months when the average high
temperature strictly exceeds 40 °C or 104 °F. Some examples of this are Bilma,
Niger and Faya-Largeau, Chad. The annual average daily temperature
exceeds 20 °C or 68 °F everywhere and can approach 30 °C or 86 °F in the
hottest regions year-round. However, most of the desert has a value in
excess of 25 °C or 77 °F.
Sunset in Sahara
Sand and ground temperatures are even more extreme. During daytime,
the sand temperature is extremely high: it can easily reach 80 °C or
176 °F or more. A sand temperature of 83.5 °C (182.3 °F) has been recorded in Port Sudan. Ground temperatures of 72 °C or 161.6 °F have been recorded in the Adrar of Mauritania and a value of 75 °C (167 °F) has been measured in Borkou, northern Chad.
Due to lack of cloud cover and very low humidity, the desert
usually has high diurnal temperature variations between days and nights.
However, it is a myth that the nights are cold after extremely hot days
in the Sahara. The average diurnal temperature range is typically
between 13 and 20 °C or 23.4 and 36.0 °F. The lowest values are found
along the coastal regions due to high humidity and are often even lower
than 10 °C or 18 °F, while the highest values are found in inland desert
areas where the humidity is the lowest, mainly in the southern Sahara.
Still, it is true that winter nights can be cold as it can drop to the
freezing point and even below, especially in high-elevation areas. The
frequency of subfreezing winter nights in the Sahara is strongly
influenced by the North Atlantic Oscillation (NAO), with warmer winter temperatures during negative NAO events and cooler winters with more frosts when the NAO is positive.
This is because the weaker clockwise flow around the eastern side of
the subtropical anticyclone during negative NAO winters, although too
dry to produce more than negligible precipitation, does reduce the flow
of dry, cold air from higher latitudes of Eurasia into the Sahara
significantly.
Precipitation
The average annual rainfall ranges from very low in the northern and
southern fringes of the desert to nearly non-existent over the central
and the eastern part. The thin northern fringe of the desert receives
more winter cloudiness and rainfall due to the arrival of low pressure systems
over the Mediterranean Sea along the polar front, although very
attenuated by the rain shadow effects of the mountains and the annual
average rainfall ranges from 100 millimetres (4 in) to 250 millimetres
(10 in). For example, Biskra, Algeria, and Ouarzazate,
Morocco, are found in this zone. The southern fringe of the desert
along the border with the Sahel receives summer cloudiness and rainfall
due to the arrival of the Intertropical Convergence Zone from the south
and the annual average rainfall ranges from 100 millimetres (4 in) to
250 millimetres (10 in). For example, Timbuktu, Mali and Agadez,
Niger are found in this zone. The vast central hyper-arid core of the
desert is virtually never affected by northerly or southerly atmospheric
disturbances and permanently remains under the influence of the
strongest anticyclonic weather regime, and the annual average rainfall
can drop to less than 1 millimetre (0.04 in). In fact, most of the
Sahara receives less than 20 millimetres (0.8 in). Of the 9,000,000
square kilometres (3,500,000 sq mi) of desert land in the Sahara, an
area of about 2,800,000 square kilometres (1,100,000 sq mi) (about 31%
of the total area) receives an annual average rainfall amount of 10
millimetres (0.4 in) or less, while some 1,500,000 square kilometres
(580,000 sq mi) (about 17% of the total area) receives an average of 5
millimetres (0.2 in) or less.
The annual average rainfall is virtually zero over a wide area of some
1,000,000 square kilometres (390,000 sq mi) in the eastern Sahara
comprising deserts of: Libya, Egypt and Sudan (Tazirbu, Kufra, Dakhla, Kharga, Farafra, Siwa, Asyut, Sohag, Luxor, Aswan, Abu Simbel, Wadi Halfa) where the long-term mean approximates 0.5 millimetres (0.02 in) per year.
Rainfall is very unreliable and erratic in the Sahara as it may vary
considerably year by year. In full contrast to the negligible annual
rainfall amounts, the annual rates of potential evaporation are
extraordinarily high, roughly ranging from 2,500 millimetres (100 in)
per year to more than 6,000 millimetres (240 in) per year in the whole
desert.
Nowhere else on Earth has air been found as dry and evaporative as in
the Sahara region. However, at least two instances of snowfall have been
recorded in Sahara, in February 1979 and December 2016, both in the
town of Ain Sefra.
Desertification and prehistoric climate
One theory for the formation of the Sahara is that the monsoon in Northern Africa was weakened because of glaciation during the Quaternary period, starting two or three million years ago. Another theory is that the monsoon was weakened when the ancient Tethys Sea dried up during the Tortonian period around 7 million years.
The climate of the Sahara has undergone enormous variations between wet and dry over the last few hundred thousand years, believed to be caused by long-term changes in the North African climate cycle that alters the path of the North African Monsoon – usually southward. The cycle is caused by a 41000-year cycle in which the tilt of the earth changes between 22° and 24.5°.
At present (2000 AD), we are in a dry period, but it is expected that
the Sahara will become green again in 15000 years (17000 AD). When the
North African monsoon is at its strongest annual precipitation and
subsequent vegetation in the Sahara region increase, resulting in
conditions commonly referred to as the "green Sahara".
For a relatively weak North African monsoon, the opposite is true, with
decreased annual precipitation and less vegetation resulting in a phase
of the Sahara climate cycle known as the "desert Sahara".
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.
Sahel region of Mali
During the last glacial period, the Sahara was much larger than it is today, extending south beyond its current boundaries. The end of the glacial period brought more rain to the Sahara, from about 8000 BCE to 6000 BCE, perhaps because of low pressure areas over the collapsing ice sheets to the north.
Once the ice sheets were gone, the northern Sahara dried out. In the
southern Sahara, the drying trend was initially counteracted by the monsoon,
which brought rain further north than it does today. By around 4200
BCE, however, the monsoon retreated south to approximately where it is
today, leading to the gradual desertification of the Sahara. The Sahara is now as dry as it was about 13,000 years ago.
The Sahara pump theory describes this cycle. During periods of a wet or "Green Sahara", the Sahara becomes a savanna
grassland and various flora and fauna become more common. Following
inter-pluvial arid periods, the Sahara area then reverts to desert
conditions and the flora and fauna are forced to retreat northwards to
the Atlas Mountains, southwards into West Africa, or eastwards into the Nile Valley. This separates populations of some of the species in areas with different climates, forcing them to adapt, possibly giving rise to allopatric speciation.
It is also proposed that humans accelerated the drying out period
from 6,000–2,500 BCE by pastoralists overgrazing available grassland.
Evidence for cycles
The growth of speleothems
(which requires rainwater) was detected in Hol-Zakh, Ashalim, Even-Sid,
Ma'ale-ha-Meyshar, Ktora Cracks, Nagev Tzavoa Cave, and elsewhere, and
has allowed tracking of prehistoric rainfall. The Red Sea coastal route
was extremely arid before 140 and after 115 kya. Slightly wetter
conditions appear at 90–87 kya, but it still was just one tenth the
rainfall around 125 kya. In the southern Negev Desert speleothems did not grow between 185–140 kya (MIS 6), 110–90 (MIS 5.4–5.2), nor after 85 kya nor during most of the interglacial period (MIS 5.1), the glacial period and Holocene. This suggests that the southern Negev was arid to hyper-arid in these periods.
During the Last Glacial Maximum (LGM) the Sahara desert was more extensive than it is now with the extent of the tropical forests being greatly reduced, and the lower temperatures reduced the strength of the Hadley Cell. This is a climate cell which causes rising tropical air of the Inter-Tropical Convergence Zone (ITCZ) to bring 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.
It is associated with high rates of wind-blown mineral dust, and these
dust levels are found as expected in marine cores from the north
tropical Atlantic. But around 12,500 BCE the amount of dust in the cores
in the Bølling/Allerød phase suddenly plummets and shows a period of much wetter conditions in the Sahara, indicating a Dansgaard-Oeschger
(DO) event (a sudden warming followed by a slower cooling of the
climate). The moister Saharan conditions had begun about 12,500 BCE,
with the extension of the ITCZ northward in the northern hemisphere
summer, bringing moist wet conditions and a savanna climate to the
Sahara, which (apart from a short dry spell associated with the Younger Dryas) peaked during the Holocene thermal maximum
climatic phase at 4000 BCE when mid-latitude temperatures seem to have
been between 2 and 3 degrees warmer than in the recent past. Analysis
of Nile River deposited sediments in the delta also shows this period had a higher proportion of sediments coming from the Blue Nile, suggesting higher rainfall also in the Ethiopian Highlands. This was caused principally by a stronger monsoonal circulation throughout the sub-tropical regions, affecting India, Arabia and the Sahara. Lake Victoria only recently became the source of the White Nile and dried out almost completely around 15 kya.
The sudden subsequent movement of the ITCZ southwards with a Heinrich event (a sudden cooling followed by a slower warming), linked to changes with the El Niño-Southern Oscillation
cycle, led to a rapid drying out of the Saharan and Arabian regions,
which quickly became desert. This is linked to a marked decline in the
scale of the Nile floods between 2700 and 2100 BCE.
