In 1934, people in the city of Boston in the north-eastern USA watched in amazement as clouds of choking yellow dust passed over them on their way to the Atlantic Ocean. The dust had come from the Great Plains in the American Midwest. These plains are generally treeless, because the average yearly rainfall is less than 500mm (20 in) and droughts are common.
The plains were formerly grasslands that supported herds of buffalo, but from the late 19th century they were turned into farmland. Some areas were ploughed and others became cattle ranches. Overgrazing and ploughing removed the grasses, whose roots had bound the soil together. Strong winds blew the dry, loose soil about, breaking it down into fine dust which was blown away by westerly winds. Formerly fertile soil ended up on the sealed, and the Great Plains were turned into an infertile dust bowl. In recent years similar dust bowls have been created in the drier parts of Africa.