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The Dust Bowl, also known as the Dirty Thirties, was a period of severe dust storms that greatly damaged the ecology and agriculture of the US and Canadian prairies during the 1930s; severe drought and a failure to apply dryland farming methods to prevent wind erosion (the Aeolian processes) caused the phenomenon. The drought came in three waves, 1934, 1936, and 1939–40, but some regions of the high plains experienced drought conditions for as many as eight years.〔 〕 With insufficient understanding of the ecology of the plains, farmers had conducted extensive deep plowing of the virgin topsoil of the Great Plains during the previous decade; this had displaced the native, deep-rooted grasses that normally trapped soil and moisture even during periods of drought and high winds. The rapid mechanization of farm equipment, especially small gasoline tractors, and widespread use of the combine harvester contributed to farmers' decisions to convert arid grassland (much of which received no more than 10 inches (250 mm) of precipitation per year) to cultivated cropland.〔(【引用サイトリンク】title=The American Experience: Drought )〕 During the drought of the 1930s, the unanchored soil turned to dust, which the prevailing winds blew away in huge clouds that sometimes blackened the sky. These choking billows of dust – named "black blizzards" or "black rollers" – traveled cross country, reaching as far as the East Coast and striking such cities as New York City and Washington, D.C. On the Plains, they often reduced visibility to 1 metre (3.3 ft) or less. Associated Press reporter Robert E. Geiger happened to be in Boise City, Oklahoma, to witness the "Black Sunday" black blizzards of April 14, 1935; Edward Stanley, Kansas City news editor of the Associated Press coined the term "Dust Bowl" while rewriting Geiger's news story. While the term "the Dust Bowl" was originally a reference to the geographical area affected by the dust, today it is usually used to refer to the event, as in "It was during the Dust Bowl". The meaning of the term "bowl" – a hollow container – in this context is however still not quite clear. The drought and erosion of the Dust Bowl affected 100,000,000 acres (400,000 km2) that centered on the panhandles of Texas and Oklahoma and touched adjacent sections of New Mexico, Colorado, and Kansas. The Dust Bowl forced tens of thousands of families to abandon their farms. Many of these families, who were often known as "Okies" because so many of them came from Oklahoma, migrated to California and other states to find that the Great Depression had rendered economic conditions there little better than those they had left. Author John Steinbeck, wrote ''Of Mice and Men'' (1937) and ''The Grapes of Wrath'' (1939) about migrant workers and farm families displaced by the Dust Bowl. Babb's own novel about the lives of the migrant workers, ''Whose Names Are Unknown'' (2004), was eclipsed and shelved in response to the success of Steinbeck's works,〔(【引用サイトリンク】title=Whose Names Are Unknown: Sanora Babb )〕 and was finally published in 2004. ==Geographic characteristics and early history== The Dust Bowl area lies principally west of the 100th meridian on the High Plains, characterized by plains which vary from rolling in the north to flat in the Llano Estacado. Elevation ranges from in the east to at the base of the Rocky Mountains. The area is semiarid, receiving less than of rain annually; this rainfall supports the shortgrass prairie biome originally present in the area. The region is also prone to extended drought, alternating with unusual wetness of equivalent duration. During wet years, the rich soil provides bountiful agricultural output, but crops fail during dry years. The region is also subject to high winds. During early European and American exploration of the Great Plains, this region was thought unsuitable for European-style agriculture; explorers called it the Great American Desert. The lack of surface water and timber made the region less attractive than other areas for pioneer settlement and agriculture. The federal government encouraged settlement and development of the Plains for agriculture via the Homestead Act of 1862, offering settlers plots. With the end of the Civil War in 1865 and the completion of the First Transcontinental Railroad in 1869, waves of new migrants and immigrants reached the Great Plains, and they greatly increased the acreage under cultivation. An unusually wet period in the Great Plains mistakenly led settlers and the federal government to believe that "rain follows the plow" (a popular phrase among real estate promoters) and that the climate of the region had changed permanently. While initial agricultural endeavors were primarily cattle ranching, the adverse effect of harsh winters on the cattle, beginning in 1886, a short drought in 1890, and general overgrazing, led many landowners to increase the amount of land under cultivation. Recognizing the challenge of cultivating marginal arid land, the United States government expanded on the offered under the Homestead Act—granting to homesteaders in western Nebraska under the Kinkaid Act (1904) and elsewhere in the Great Plains under the Enlarged Homestead Act (1909). Waves of European settlers arrived in the plains at the beginning of the 20th century. A return of unusually wet weather seemingly confirmed a previously held opinion that the "formerly" semiarid area could support large-scale agriculture. At the same time, technological improvements such as mechanized plowing and mechanized harvesting made it possible to operate larger properties without high labor costs. The combined effects of the disruption of the Russian Revolution, which decreased the supply of wheat and other commodity crops, and World War I increased agricultural prices; this demand encouraged farmers to dramatically increase cultivation. For example, in the Llano Estacado of eastern New Mexico and northwestern Texas, the area of farmland was doubled between 1900 and 1920, then tripled again between 1925 and 1930.〔 The agricultural methods favored by farmers during this period created the conditions for large-scale erosion under certain environmental conditions.〔 The widespread conversion of the land by deep plowing and other soil preparation methods to enable agriculture eliminated the native grasses which held the soil in place and helped retain moisture during dry periods. Furthermore, cotton farmers left fields bare during winter months, when winds in the High Plains are highest, and burned the stubble as a means to control weeds prior to planting, thus depriving the soil of organic nutrients and surface vegetation. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「dust bowl」の詳細全文を読む スポンサード リンク
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