Hwang Sa

Hwang sa = yellow sand.
This is the wikipedia definition.
Asian Dust (also yellow dust, or yellow sand) is a seasonal meteorological phenomenon which affects much of East Asia sporadically during the springtime months. The dust originates in the deserts of Mongolia and northern China where high-speed surface winds and intense dust storms kick up dense clouds of fine, dry soil particles. These clouds are then carried eastward by prevailing winds and pass over China, North and South Korea, and Japan. Sometimes, the airborne particulates are carried much further, in significant concentrations which affect air quality as far east as the United States. Areas affected by the dust experience decreased visibility and the dust is known to cause health problems, such as sore throat and respiratory difficulties, in residents. The dust has been shown to increase the daily mortality rate in one affected region by 1.7%. The effects of the dust are not, however, strictly negative, as it is thought to enrich the soil of the regions where it is eventually deposited by contributing important trace minerals.
The dust is caused by desertification in northern China near the Gobi Desert. Although such storms have occurred from time to time throughout history, they became a pronounced problem in the latter half of the 20th century, as agricultural pressures on the Gobi region intensified.
In recent years, South Korea and the People's Republic of China have participated in reforestation efforts in the source region. However, this has not affected the problem in any significant way. In April 2006, South Korean meteorologists reported the worst yellow dust storm in four years.

http://yellow.metri.re.kr/images/adust_warning_sys.jpg

that link shows the difference between an advisory and a warning-like with tornadoes in the midwest. and, of course, there are cartoons to help you know the difference.

Supposedly this year is going to be pretty bad b/c China didn't get much snow. Just what I heard.