Dust Bowl

The Dust Bowl, or the Dirty Thirties, was a period of severe dust storms causing major ecological and agricultural damage to American and Canadian prairie lands in the 1930s, particularly in 1934 and 1936. The phenomenon was caused by severe drought combined with farming methods that did not include crop rotation, fallow fields, cover crops, soil terracing and wind-breaking trees to prevent wind erosion. Extensive deep plowing of the virgin topsoil of the Great Plains in the preceding decade had displaced the natural deep-rooted grasses that normally kept the soil in place and trapped moisture even during periods of drought and high winds. Rapid mechanization of farm implements, especially small gasoline tractors and widespread use of the combine harvester were significant in the decisions to convert grassland (much of which received no more than 10 inches (250 mm) of precipitation per year) to cultivated cropland.

• http://en.wikipedia.org/wiki/Dust Bowl