Saharan dust cloud heading toward South Florida, US
While summer thunderstorms pound South Florida, a very concentrated area of Saharan dust is coming all the way from Africa. The northern Caribbean islands have already been dealing with Saharan dust with very hazy skies. The dust should arrive sometime on Thursday and last until next Monday. The dust is carried over 10,000 kilometers (6,000 miles) from North Africa, by the same atmospheric waves that bring tropical storms to US. The dust layer typically remains 1,5 km (5,000-6,000 feet) above the ground.
This is one of the largest concentration of dust for years making it that far west across the Atlantic Ocean. Usually dust starts to dissipate as it reaches the western Atlantic. Saharan dust is a limiting factor for tropical development in the Atlantic and also causes nasty hazy skies when it is so densely concentrated.
The Sahara is the greatest deposit of sand and dust particles in the world, stretching about 3.5 million square miles across northern Africa. Rainfall is rare across much of the Sahara because it is limited by persistent high pressure with resultant sinking and drying of air. Persistent northeasterly winds, squeezed between an area of high pressure over the northern Sahara and low pressure over the equator, are often strong enough to stir loose sand and dust in the Sahara. The smaller dust particles can be lofted up to 8 km (5 miles) high into the sky. Strong winds can blow over thousands of square kilometers of the desert and they can scour enormous volumes of dust from the surface. The amount of dust is said to be in the millions of tons.
Sometimes large dust clouds can traverse westward across the Atlantic as they get steered by trade winds. Under favorable settings, dust aloft can reach customary tropical cyclone breeding areas. The clouds of dust are associated with large zones of dry air, which is an inhibiting factor for tropical storms and hurricanes.
The large zone of dust and dry air has been one of the factors in the recent quiet state of the tropics in the Atlantic. Another large cloud of dust has moved in mid-July off the western coast of Africa and appears like it will continue to move westward across the Atlantic (see image bellow).
Beautiful sunsets are observed since the Saharan dust clouds resides so high in the atmosphere. The dust particles act to scatter certain wavelengths of light, allowing red hues to be illuminated.
Sources: CIMSS Satellite Blog, SSEC, MODIS, Miami Herald, Palm Beach Post, AccuWeather