English II
Yellow dust occurs when ne sand dust blown up by the wind from the northern part of China and dry/red clay zones of Mongolia, spreads in the air, covers the sky, and then descends gradual- ly to certain regions. Yellow dust generally occurs from March to May when cyclones are active. Sometimes, they reach North America via strong upper-level west winds that pass over Korea, Ja- pan, and the Paci c Ocean. When yellow dust oc- curs, sunlight is blocked by airborne particulates, so that the sky is seen as yellowish brown, and yellow-brown dust covers the land. The dust causes a variety of problems in human health, agriculture, industry, transportation, and the oceans. Although Korea and Japan are far from the main dust sources, they are normally impacted by them. The intensity and number of yellow dust days have been rapidly increasing since the 1990s. The number of yellow dust days in Seoul has increased as follows: 28 days (1971 – 1980), 39 days (1981 – 1990), 77 days (1991 – 2000), and 122 days (2001 – 2010). The number of events in the 2000s is 4 times greater than that of the 1970s. The yellow dust special weather reports issued by the Korea Meteorological Administration are classi ed as advisories and warnings. An advisory is issued when an hourly average particulate mat- ter (PM-10) density of 400 μg/m3 is expected to last for more than 2 hours. A yellow dust warning is issued when an hourly average particulate mat- 3ter (PM-10) density of 800 μg/m is expected to last for more than 2 hours.
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