The arrival of dust from the distant Sahara to our geographical area is one of the phenomena that typically occurs in June, July, and early August, although it can sometimes occur as early as spring or last until autumn in the northern hemisphere, especially in the easternmost part of the Caribbean Sea. The Sahara Desert (or Great Desert, as the Arabs call it) is the largest warm desert in the world, but it would rank third when compared to the cold deserts of Antarctica and the Arctic. The Sahara covers almost all of North Africa and has an extension of 9,400,000 km2, including ten countries: Algeria, Chad, Egypt, Libya, Morocco, Mauritania, Mali, Niger, the Sahrawi Arab Democratic Republic, Sudan and Tunisia. It has an extension as large as China or the United States. The highest point is the Emi Koussi Mountain, at 3,455 meters above mean sea level, and the lowest point is the Qattara Depression, at 133 meters below sea level. The climate there has had many changes over time. Hundreds of thousands of years ago it was wet, and it became dry in response to a cycle repeated every 41,000 years or so. This cycle responds to regular...