(i) They occurred because of the persistent droughts in that area.
(ii) The rains failed year after year and temperatures soared. The wind blew
with ferocious speed.
(iii) But ordinary duststorms became black blizzards only because the entire
landscape had been ploughed over and the grass was ruined which
could hold soil.
(iv) When wheat cultivation had expanded dramatically in the early 19th
century, zealous farmers had recklessly uprooted all vegetation, and
tractors had turned the soil over and broken the sod into dust. The
whole region had become a Dust Bowl.