The Saharan Air Layer is an intensely dry, warm and sometimes dust

Dust

Dust consists of particles in the atmosphere that arise from various sources such as soil dust lifted up by wind , volcanic eruptions, and pollution...

-laden layer of the atmosphere

Atmosphere

An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...

 which often overlies the cooler, more-humid surface air of the Atlantic Ocean

Atlantic Ocean

The Atlantic Ocean is the second-largest of the world's oceanic divisions. With a total area of about , it covers approximately 20% of the Earth's surface and about 26% of its water surface area...

. In the Sahara Desert region of North Africa

North Africa

North Africa or Northern Africa is the northernmost region of the African continent, linked by the Sahara to Sub-Saharan Africa. Geopolitically, the United Nations definition of Northern Africa includes eight countries or territories; Algeria, Egypt, Libya, Morocco, South Sudan, Sudan, Tunisia, and...

, 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

Marine layer

A marine layer is an air mass which develops over the surface of a large body of water such as the ocean or large lake in the presence of a temperature inversion. The inversion itself is usually initiated by the cooling effect of the water on the surface layer of an otherwise warm air mass...

. This arrangement is an inversion

Inversion (meteorology)

In meteorology, an inversion is a deviation from the normal change of an atmospheric property with altitude. It almost always refers to a temperature inversion, i.e...

 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

Lapse rate

The lapse rate is defined as the rate of decrease with height for an atmospheric variable. The variable involved is temperature unless specified otherwise. The terminology arises from the word lapse in the sense of a decrease or decline; thus, the lapse rate is the rate of decrease with height and...

 within the SAL itself is steep, that is, the temperature falls rapidly with height.

Disturbances such as large thunderstorm complexes over North Africa periodically result in vast dust and sand storms

Dust storm

A dust / sand storm is a meteorological phenomenon common in arid and semi-arid regions. Dust storms arise when a gust front or other strong wind blows loose sand and dirt from a dry surface. Particles are transported by saltation and suspension, causing soil to move from one place and deposition...

, some of which extend as high as 6,000 meters. These can be driven out to sea within the SAL as far west as North America

North America

North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...

.

In the case of 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 think that the July 2000 measurements made in Puerto Rico

Puerto Rico

Puerto Rico , officially the Commonwealth of Puerto Rico , is an unincorporated territory of the United States, located in the northeastern Caribbean, east of the Dominican Republic and west of both the United States Virgin Islands and the British Virgin Islands.Puerto Rico comprises an...

, nearly 8 million tonnes, equaled about one-fifth of the total year's dust deposits.

This phenomenon can happen at any time of year but is usually associated with the hot air found over the islands during the summer months, ranging in time scale from a few hours up to a week. Calima as it is called, is caused by a duststorm that is stirred up by high winds in the Sahara

Sahara

The Sahara is the world's second largest desert, after Antarctica. At over , it covers most of Northern Africa, making it almost as large as Europe or the United States. The Sahara stretches from the Red Sea, including parts of the Mediterranean coasts, to the outskirts of the Atlantic Ocean...

 and is then driven over the Canary Islands

Canary Islands

The Canary Islands , also known as the Canaries , is a Spanish archipelago located just off the northwest coast of mainland Africa, 100 km west of the border between Morocco and the Western Sahara. The Canaries are a Spanish autonomous community and an outermost region of the European Union...

 by south easterly winds. The fine sand particles cause the air to become thick and visibility becomes rather like that experienced during a thick fog, depending on the severity. During the calima, every surface will be covered in fine reddish brown dust.

These clouds of dust are visible in satellite

Satellite

In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....

 photos as a milky white to gray shade, similar to haze

Haze

Haze is traditionally an atmospheric phenomenon where dust, smoke and other dry particles obscure the clarity of the sky. The World Meteorological Organization manual of codes includes a classification of horizontal obscuration into categories of fog, ice fog, steam fog, mist, haze, smoke, volcanic...

.

Findings to date indicate that the iron-rich dust particles which often occur within the SAL reflect solar radiation, 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

Condensation

Condensation is the change of the physical state of matter from gaseous phase into liquid phase, and is the reverse of vaporization. When the transition happens from the gaseous phase into the solid phase directly, the change is called deposition....

 as they drift into the marine layer below, but not precipitation

Precipitation (meteorology)

In meteorology, precipitation In meteorology, precipitation In meteorology, precipitation (also known as one of the classes of hydrometeors, which are atmospheric water phenomena is any product of the condensation of atmospheric water vapor that falls under gravity. The main forms of precipitation...

 as the drops formed are too small to fall and tend not to readily coelesce. 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

Tropical cyclone

A tropical cyclone is a storm system characterized by a large low-pressure center and numerous thunderstorms that produce strong winds and heavy rain. Tropical cyclones strengthen when water evaporated from the ocean is released as the saturated air rises, resulting in condensation of water vapor...

s, which may be related directly to these factors.

The SAL is a subject of ongoing study and research.