Probably unique to Hawaii, at least unique within the United States, is vog. Similar to smog, vog is formed by a combination of sulphur dioxide from Kilauea, oxygen, and water vapour. With the increase in gas output vog has made its presence felt more often recently. Living in Volcano, we notice vog all the time. Most of the time it affects communities such as Glenwood or Mountain View more than us, as the vog blows either over us or to the north of us, then descends onto these communities. Driving through it is like driving through a thick fog, but you get the bonus of sulphuric gas to make your throat hurt.

A good demonstration of how vog affects Hawaii comes from the MODIS instrument on two satellites in orbit around the earth. Their December 7, 2008 image of the day shows the bulk of Hawaii covered by clouds and vog. Clouds are obvious. Vog is the thick haze spreading to the north-west from the southern-most Big Island.

Clicking through to the high-res image shows two sources of heat on the Big Island, denoted by red boxes. The southern-most heat source is the Kilauea ocean entry near Kalapana. The northern heat source is a brushfire — you can even see the smoke plume drifting off to the west.

You can also clearly see evidence for the tropical inversion layer that makes Mauna Kea an excellent site for astronomy. The inversion layer keeps clouds and moisture below roughly 10,000 feet, and the summits of Mauna Kea, Mauna Loa, and Haleakala on Maui are well above this elevation. You can clearly see all three poking above the clouds.

[Thanks to Andrew for the original link to the MODIS page]