PART VI — PRECIPITATION AND REGIONAL WEATHER PHENOMENA 



less continuous, the variations in dust 

 content of the air are often quite 

 abrupt. 



Locally, the variation in dustiness 

 can be due simply to a shift in the 

 dust-laden airstream. In many in- 

 stances, large increases in dust load- 

 ing in the Caribbean can be tied to 

 specific outbreaks of dust-storm ac- 

 tivity over parts of North Africa. At 

 other times, the increases in dust load- 

 ing of the trade winds are attribut- 

 able to the venting of the normally 

 dusty air over the African continent 

 by a favorable wind regime which 

 brings air from deep in the interior of 

 the Sahara into the Atlantic trade 

 winds. In many cases, however, it is 

 impossible to assign a cause to the 

 dust outbreak or even to detect the 

 variation of dust loading downwind 

 from Africa without direct measure- 

 ments. 



Visibility — The presence of Afri- 

 can dust in the Caribbean can be seen 

 as thick haze, with visibility reduced 

 from 20 to 30 miles, in the case of no 

 haze, to only between 6 and 15 

 miles. In exceptionally hazy areas of 

 the Caribbean, the horizon resembles 

 that on a dry day of the American 

 Midwest or on a muggy day in a large 

 city of the Northeast. Indeed, the 

 dust loadings over the Caribbean are 

 probably comparable to or greater 

 than those that would be found over 

 much of continental United States. 



Source — There is an abrupt 

 change in the general source region 

 of the dust between winter and sum- 

 mer. After October and until May 

 (with some rare deviations), the dust 

 is ash-gray to black and is thought to 

 originate over the sub-Sahara from 

 the Cameroons through central Ni- 

 geria and the Ivory Coast. In the 

 summer, however, the flow of dust 

 is primed by the strong northeasterly 

 winds associated with the intense 

 pressure gradient that exists between 

 the low pressure of the central 

 Sahara and the relatively high pres- 

 sure along the western coast. Then, 



the dust is a reddish-brown color 

 with a tinge of yellow. 



Particle Size — A surprising as- 

 pect of the size spectra of the dust 

 reaching the Caribbean is the rela- 

 tively large fraction of the dust (5 to 

 20 percent) with particle sizes in ex- 

 cess of 10 microns. In general, the 

 higher the dust loading the higher is 

 the fraction of dust in the larger size 

 ranges. According to Stokes, settling- 

 velocity particles in excess of 10 

 microns would settle out of the air 

 before reaching the Caribbean unless 

 they were raised to heights well in 

 excess of 20,000 feet. Since the visible 

 dust top is rather distinct at about 

 10,000-15,000 feet over the Carib- 

 bean, and is directly related to the 

 top of the turbulent mixing layer over 

 the Sahara, which is at about the 

 same altitude, one can assume that 

 virtually all the dust falls from below 

 10,000-15,000 feet. Although a sub- 

 stantial fraction of the dust un- 

 doubtedly settles out before reaching 

 the Caribbean, a certain fraction of 

 all size ranges is prevented from 

 being lost by the recycling of air 

 (turbulent mixing) in the dust layer 

 over Africa and in the trade winds. 



Vertical Distribution — Recent ob- 

 servations of the vertical distribution 

 of the dust show that the dust con- 

 centration in the air downwind from 

 the Sahara is greatest in the layer 

 between the dust top and the top of 

 the cumulus layer (say, 4,000 to 8,000 

 feet). In the lower layers, the trade- 

 wind air may be air of non-Saharan 

 (or partially Saharan) origin that 

 flows southward to undercut the 

 original dust airstream, being thereby 

 enriched by mixing and by fallout 

 from above. 



Possible Relation of African Dust 

 to Tropical Disturbances 



A great deal of indirect theoretical 

 and observational evidence exists to 

 suggest that African dust may play 

 some secondary role in the growth 

 or suppression of tropical disturb- 



ances and the entire energetics of 

 the tropical atmosphere. Conversely, 

 some observations indicate that Afri- 

 can disturbances have some effect on 

 the movement of dust into the Carib- 

 bean and that the behavior of the 

 dust is at least superficially affected 

 by the presence of these wave per- 

 turbations. 



Dust as a Nucleator — It is well 

 known that the size spectra and num- 

 bers of condensation nuclei have a 

 profound effect on the population of 

 water droplets in clouds and the 

 ability of the cloud to precipitate. 

 These condensation nuclei are derived 

 from various types of atmospheric 

 aerosols — salt particles, dust, pol- 

 lution, and the like. Much research 

 has been done both in the laboratory 

 and in the field, to determine the 

 nucleating properties of various sub- 

 stances and their relative importance 

 in cloud growth. 



Similarly, the formation of ice crys- 

 tals from supercooled water in clouds 

 depends on the presence of foreign 

 freezing nuclei and on the distribu- 

 tion of existing ice crystals. Almost 

 any substance will nucleate ice at 

 some temperature, but only a rela- 

 tively few types of substances are 

 efficient in this capacity — i.e., are 

 able to promote freezing at tempera- 

 tures warmer than about —20° cen- 

 tigrade. The best-known and most 

 efficient type of nuclei air crystals is 

 silver iodide, which has been used in 

 cloud-seeding experiments. But silver 

 iodide is not found naturally in the 

 air in significant quantities. The most 

 efficient natural ice nuclei are the 

 clay minerals — notably kaolinite, il- 

 lite, and montmorillite. These three 

 minerals are abundant in the soils 

 of North Africa and have been found 

 to be a prominent constituent in the 

 African dust. Since the haze top is 

 near the freezing level, the dust could 

 only be effective in freezing if it 

 were entrained into large cumulus 

 which protrude to heights well above 

 the haze top. 



Until very recently the Atlantic 

 trade winds were thought of in terms 



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