PM2.5 smog caused by yellow dust in Oi, Fukui (map) in February 2014. Photo courtesy Shohei Guccio.

The southwestern Japanese city of Fukuoka is experiencing its first “yellow dust” day of the season, three months earlier than in past years. This is also the first time in five years South Korea has issued a yellow dust alert so early in the spring.

Yellow dust, or kosa (黄砂), is a traditional harbinger of spring in east Asia. Powerful spring westerly winds pick up sand, dust and sediment from arid regions of the Asian continent, including the Gobi Desert in Mongolia and the Loess Plateau in China.

The dust is transported throughout northeast Asia, including to the Korean Peninsula, Japan, and even the Russian Far East.

ANN broadcast news reports on the early arrival of the dust in Fukuoka, and how the dust is obscuring visibility in the normally clear skies of the season:

In Japan, yellow dust traditionally announces its arrival by leaving cars, windows, and clothes hung out to dry coated with a noticeable layer of grime. The wind-blow dust also creates a blue or yellowish haze that can obstruct the sun.

Dust obscures the Yellow Sea and Sea of Japan in March 2001. Image provided by the SeaWiFS Project, NASA/Goddard Space Flight Center, and ORBIMAGE

It's possible that the yellow dust has already struck some cities in Japan earlier this month. This Twitter user reports a coating of the stuff on his car (although the dusting may be from cryptomeria pollen, another bane of early spring in Japan):

黄砂だ！ pm 2.5と花粉対策 広島地方 pic.twitter.com/2bs4t2Xdgr

— hajime (@tattachan1484) February 9, 2015

It's the yellow dust! Time for PM 2.5 and pollen countermeasures! (somewhere in Hiroshima)

Even further east, a Hankyu train rider somewhere near Osaka has also noticed the arrival of the continental dust:

うわ この黄色いのん 黄砂？ pic.twitter.com/0Fus7oXvPi

— 阪急京都線民 (@hankyu100) February 23, 2015

Agh! This must be yellow dust!

In recent years, yellow dust has also been responsible for a spike in reported respiratory ailments in Japan in spring.

The storms, thought to have once be a natural phenomenon, are now considered to be worsening in frequency and intensity as a result of overgrazing, deforestation, soil degradation and desertification in Central Asia and parts of China.

The fine particles of sediment that make up the yellow sand, as it is also known, act as an aerosol that collects industrial pollutants in China's heavily industrialized northeast. 

The result is poor air quality and clouds of “PM 2.5 smog” all over northeast Asia. PM 2.5 fine particles are too small to be seen and are generally generated by industrial pollution. PM 2.5 particulate pollution is generally thought to be harmful to human health because these particles can be easily inhaled to then accumulate in the respiratory system. 

Here is what PM 2.5 concentrations looked like northeast Asia on February 23, 2015:

Source: Air Pollution in Asia – Real-time Air Quality Index Visual Map (February 23, 2015)

The pollution indices and color codes in the map above follow the EPA graduation, as defined by AirNow. 

Numbers greater than 100 indicate air quality that is “unhealthy for sensitive groups.” Numbers greater than 300 indicate air quality that is “hazardous for all.”

These satellite images shared on Twitter demonstrate the sudden scale of the yellow dust onslaught:

今日（23日）西日本で黄砂が観測されています．http://t.co/sINRq5OXC1 22日の衛星可視では，黄海あたりに黄砂っぽいのが見えます．19日と比べるとかなり茶色い．http://t.co/MARPwXwBJD pic.twitter.com/D7N9da4yKs

— 荒木健太郎 (@arakencloud) February 23, 2015

Today (February 23) Yellow Dust is clearly visible… On February Feb. 22 the Yellow Dust is clearly visible over the Yellow Sea [adjacent to Fukuoka]. Compared to February 22, it's really (noticeable).