5. DUST 



African Dust and its Transport into the Western Hemisphere 



Meteorologists have recently dis- 

 covered that enormous quantities of 

 dust are raised over arid and semi- 

 arid regions of North Africa and in- 

 jected into the trade winds over the 

 North Atlantic. Outbreaks of dust 

 from the Sahara take about one week 

 to reach the Caribbean. The amounts 

 of dust are highly variable in space 

 and time, both from day to day and 

 season to season, but the period of 

 maximum dust transport across the 

 Atlantic (June to early September) 

 coincides with the Atlantic hurricane 

 season. Dust outbreaks from Africa 

 often appear on meteorological satel- 

 lite photographs as a semi-transpar- 

 ent or transparent whiteness that re- 

 sembles thin cirrus clouds. (See 

 Figure VI-13) In such outbreaks, 

 surface visibility can be moderately 

 reduced as far west as the Caribbean. 



African dust outbreaks and the 

 hurricanes that also have their origin 

 over Africa may be interrelated in 



some ways. While it is highly un- 

 likely that African dust can cause 

 wind disturbances to form into hur- 

 ricanes or hurricanes to dissipate, 

 there is enough observational and 

 theoretical evidence to suggest that 

 the two phenomena might affect each 

 other indirectly or directly in a sec- 

 ondary role. The dust's ability to 

 directly influence hurricanes lies in its 

 ability to affect the thermodynamics 

 of cloud growth through its role as 

 an ice or condensation nucleator. 

 More indirectly, the dust can affect 

 the energy balance of the tropics by 

 its ability to block incoming radiation 

 from the sun or outgoing infrared 

 radiation from the earth's surface. 



Dust can also serve as a tracer of 

 atmospheric air motion. There is 

 some evidence that an enhanced dust 

 transport accompanies the movement 

 of wind disturbances off the west 

 coast of Africa. The dust content of 

 the air can be modified in the disturb- 



Figure VI-13 — DUST OVER THE TROPICAL ATLANTIC 



This satellite photograph was taken by the ATS-3 satellite on the afternoon of August 

 11, 1970. It shows a great cloud of African dust between 30° and 60° W. longitude 

 just north of the Tropic of Cancer. 



ance either by being washed out in 

 rain or by being evacuated to very 

 high altitudes in the updrafts that 

 accompany giant cumulus clouds. 

 When it is transported to levels well 

 above the 3- to 4-kilometer depth 

 over which it is normally found, the 

 dust can more readily affect the en- 

 ergy balance and particulate concen- 

 trations in other parts of the globe. 



Characteristics of Dust Transport 



Since 1965, quantitative measure- 

 ments of windborne dust transport 

 have been made on a year-round 

 basis at a tower on the island of Bar- 

 bados, in the lower Antilles. (Re- 

 cently, two more such stations have 

 been set up to measure dust in Ber- 

 muda and Miami.) These measure- 

 ments, made by scientists from the 

 University of Miami, show that the 

 airborne dust loading is highly vari- 

 able from day to day, season to sea- 

 son, and even year to year. Like 

 hurricanes, the primary activity is in 

 summer when the dust transport 

 averages 10 to 50 times more than in 

 winter, with the daily amounts vary- 

 ing from about 1 to 40 micrograms 

 per cubic meter. 



Variability — Air-trajectory analy- 

 sis shows that the summer dust orig- 

 inates over arid to semi-arid regions 

 in the northwestern corner of the 

 African continent, and is swept south- 

 ward and toward the Caribbean by 

 the strong northeasterly winds that 

 exist in that sector during summer. 

 The width of the dust-carrying air- 

 stream is only 300 to 500 miles wide 

 as it leaves the coast of Africa, and 

 the depth of the dust layer is about 

 12,000 feet as determined by the 

 depth of mixing over Africa. Al- 

 though this flow of dust is more or 



191 



