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
The Atlantic Ocean is the second-largest of the world's five oceans, with an area of about . It covers approximately 20% of Earth's surface and about 29% of its water surface area. It is known to separate the " Old World" of Africa, Europe ...
. It carries upwards of 60 million tonnes of dust annually over the ocean and the Americas.
This annual phenomenon sometimes cools the ocean and suppresses Atlantic
tropical cyclogenesis
Tropical cyclogenesis is the development and strengthening of a tropical cyclone in the atmosphere. The mechanisms through which tropical cyclogenesis occurs are distinctly different from those through which temperate cyclogenesis occurs. Tr ...
.
The SAL is a subject of ongoing study and research. Its existence was first postulated in
1972
Within the context of Coordinated Universal Time (UTC) it was the longest year ever, as two leap seconds were added during this 366-day year, an event which has not since been repeated. (If its start and end are defined using mean solar tim ...
.
Creation
The dust cloud originates in
Saharan Africa
, photo = Sahara real color.jpg
, photo_caption = The Sahara taken by Apollo 17 astronauts, 1972
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and extends from the surface upwards several
kilometers
The kilometre ( SI symbol: km; or ), spelt kilometer in American English, is a unit of length in the International System of Units (SI), equal to one thousand metres (kilo- being the SI prefix for ). It is now the measurement unit used for ex ...
. As the dust drives, or is driven out over the Atlantic ocean, it is lifted above the denser
marine air. This atmospheric arrangement is an
inversion where the temperature actually increases with height, as the boundary between the SAL and the marine layer suppresses or "caps" any
convection
Convection is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the conve ...
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.
Disturbances such as large
thunderstorm
A thunderstorm, also known as an electrical storm or a lightning storm, is a storm characterized by the presence of lightning and its acoustic effect on the Earth's atmosphere, known as thunder. Relatively weak thunderstorms are some ...
complexes over North Africa periodically result in vast
dust and sand storms, some of which extend as high as . The layer is transported westwards cross the Atlantic by a series of broad
anticyclonic eddies that are typically found
above sea level.
[ ] An estimated 60-200 million tons of dust particles are carried from the
Sahara Desert
, photo = Sahara real color.jpg
, photo_caption = The Sahara taken by Apollo 17 astronauts, 1972
, map =
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region of
North Africa
North Africa, or Northern Africa is a region encompassing the northern portion of the African continent. There is no singularly accepted scope for the region, and it is sometimes defined as stretching from the Atlantic shores of Mauritania in ...
, where it originates, and moves westward annually.
Sometimes a depression to the southwest of the
Canary Islands increases the
wind speed
In meteorology, wind speed, or wind flow speed, is a fundamental atmospheric quantity caused by air moving from high to low pressure, usually due to changes in temperature. Wind speed is now commonly measured with an anemometer.
Wind speed ...
and intensity of the SAL, which can lift the dust around into the air and often carries the dust as far as the
Caribbean.
Effects
The SAL passes over the Canary Islands where the phenomenon is named "''calima''" (English: "haze") and manifests as a fog that reduces visibility and deposits a layer of dust over everything. It is especially prevalent in the winter months. Canary islanders suffer
respiratory problem
Respiratory diseases, or lung diseases, are pathological conditions affecting the organs and tissues that make gas exchange difficult in air-breathing animals. They include conditions of the respiratory tract including the trachea, bronchi, br ...
s during this weather event, and sometimes the dust is so bad that public life and transport halt completely. On January 8, 2002, the dust was so heavy over the
Tenerife South Airport
Tenerife South Airport ( es, Aeropuerto de Tenerife Sur) , also known as Tenerife South–Reina SofĂa Airport, is the larger of the two international airports located on the island of Tenerife (the other being Tenerife North Airport) and t ...
, dropping the visibility to , that it was forced to close.
From Northern Africa, winds blow twenty percent of dust from a Saharan storm out over the Atlantic Ocean, and twenty percent of that, or four percent of a single storm's dust, reaches all the way to the western Atlantic. The remainder settles out into the ocean or washes out of the air with rainfall. Scientists believe the July 2000 measurements made in
Puerto Rico
Puerto Rico (; abbreviated PR; tnq, Boriken, ''Borinquen''), officially the Commonwealth of Puerto Rico ( es, link=yes, Estado Libre Asociado de Puerto Rico, lit=Free Associated State of Puerto Rico), is a Caribbean island and unincorporated ...
, nearly 8 million
tonne
The tonne ( or ; symbol: t) is a unit of mass equal to 1000 kilograms. It is a non-SI unit accepted for use with SI. It is also referred to as a metric ton to distinguish it from the non-metric units of the short ton ( United State ...
s, equaled about one-fifth of the year's total dust deposits.
The clouds of dust SAL creates are visible in
satellite
A satellite or artificial satellite is an object intentionally placed into orbit in outer space. Except for passive satellites, most satellites have an electricity generation system for equipment on board, such as solar panels or radioi ...
photos as a milky white to gray shade, similar to
haze
Haze is traditionally an atmospheric phenomenon in which dust, smoke, and other dry particulates suspended in air obscure visibility and the clarity of the sky. The World Meteorological Organization manual of codes includes a classificati ...
.
Findings to date indicate that the
iron
Iron () is a chemical element with Symbol (chemistry), symbol Fe (from la, Wikt:ferrum, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 element, group 8 of the periodic table. It is, Abundanc ...
-rich dust particles that often occur within the SAL reflect
solar radiation
Solar irradiance is the power per unit area (surface power density) received from the Sun in the form of electromagnetic radiation in the wavelength range of the measuring instrument.
Solar irradiance is measured in watts per square metre ( ...
, thus cooling the atmosphere. The particles also reduce the amount of sunlight reaching the ocean, thus reducing the amount of
heating of the ocean.
They also tend to increase
condensation as they drift into the marine layer below, but not
precipitation
In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravitational pull from clouds. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hail. ...
as the drops formed are too small to fall and tend not to readily coalesce. These tiny drops are subsequently more easily evaporated as they move into drier air laterally or dry air mixes down from the SAL aloft.
[ ] Research on
aerosols also shows that the presence of small particles in air tends to suppress winds. The SAL has also been observed to suppress the development and intensifying of
tropical cyclone
A tropical cyclone is a rapidly rotating storm system characterized by a low-pressure center, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain and squalls. Depen ...
s, which may be related directly to these factors.
See also
*
African easterly jet
References
{{Reflist
NOAA FAQ: Saharan Air LayerReal Time SAL data
External links
NASA's 2013 HS3 Hurricane Mission to Delve into Saharan Dust
Meteorological phenomena
Atlantic Ocean
Sahara
Satellite interpretation
Tropical meteorology
Winds
Environment of Western Sahara