황사

Known in Korea as 황사 (aka Hwangsa), the Yellow Sand phenomenon is a dust storm season that straddles March and April and blankets the country with end-of-the-world skies and end-of-your-life breathing conditions. Even though it has been translated into many English names like yellow sand, yellow dust, yellow wind, or even Asian Dust the phenomenon itself is when:

...a noxious brew of Gobi desert sand particles and assorted effluent from China's industrial development comes roaring out of the west and dumps down on Japan and Korea.

And with an industrial cloud of heavy metals floating over our heads we can expect to get some yellow rain or even the rare case of 2006's Yellow Snow; Asian dust season can start as early as February and run as late as May but I'm told that here in Mokpo, April is always the worse month.

There's an informational Yellow Sand site with some interesting facts and documentaries. But there's a slightly more useful monitoring site (via Jess) with the closest recording station to Mokpo being Gwangju:

Health Levels (micrograms of dust per cubed meter)
0-99	Acceptable level of Air Pollution.
100-199	Korean Acceptable level of Air Pollution.
200-399	Maybe you should stay home today level of Air Pollution.
400-799	Maybe you should not die today level of Air Pollution.
800+	Gas Masks for Everyone!