"a brown, turgid curtain of darkness that spanned the horizon"

Dust storms and sandstorms are common in arid and semi-arid areas, mainly in Asia,

Dust storm - Gold Coast, Australia
Public DomainDust storm - Gold Coast, Australia - Credit: LunarBlitz
Africa and parts of Australia and North America.  The wind can carry huge amounts of dust, giving the impression of a solid wall.  The picture to the right shows a dust storm at the Gold Coast, Australia.