As of 2:00 PM HST Friday, April 28th 2017, a trailing front associated with the cut-off low to the north brings easterly winds to the eastern part of the state, and northerly winds to the western half of the state. All sites are currently reporting good air quality with Ocean View and Kona on the Big Island reporting SO2 ~0.01 ppm (the detectable limit).
Acceptable/good air quality over the Big Island for the weekend. Occasional traces of vog indicator species at Ocean View and Kona sites through this afternoon, shifting to Hilo/Hilo side through the weekend. This will be in the detectable range but falling within the Acceptable category.
Continued good air quality through the weekend with some potential for traces of detectable vog within the Acceptable range over Maui tomorrow (4/29).
Active weather limits vog influence to "detectable but Acceptable" range in the Hilo/Hilo side over the weekend with some potential trace influence over Maui tomorrow (4/29).
The transport of volcanic aerosols, or volcanic smog (Vog), is primarily controlled by two factors - (1) the wind direction and (2) the height of the tradewind temperature inversion. The winds determine which direction the vog will be transported. Typically the winds in Hawaii are out of the northeast or east, as demonstrated by the graph below. This results in the vog being transported around the southern tip of the Big Island of Hawaii. The winds wrap around the westside of Hawaii and are often found over Kailua-Kona during these periods. When the winds are southerly, or are light, the vog is transported north over the northern end of Hawaii and across the northern islands of Maui, Molokai, Lanai, Oahu, Kauai, and Niihau. Again, from the graph below it can be seen that this will most likely occur during the winter months but can also happen in the spring and autumn. It is least likely to occur in summer (June, July, August) whenthe northeast trade winds are most persistent. The tradewind temperature inversion determines how high the vog can mix in the atmosphere. The trade wind inversion is typically between 2000 meters (6500 feet) and 2500 meters (8200 feet). Since the volcanic emissions are quite hot when they exit the vents the gas and aerosols rise quite rapidly, however, they mix with the surrounding air and cool just as fast. This results in large amounts of vog at higher elevations where it is trapped by the tradewind inversion, which it cannot mix across. Regions downwind of the volcano, be it to the south or north, are thus greatly effected near the base of the tradewind inversion and may experience high concentrations of vog periodically.
Photograph by M. Poland, November 13, 2008, USGS HVO.