Saturday, February 4, 2017

Vog in the Air

During the past month there have been repeated "vog" episodes impacting all of the main Hawaiian Islands. The photo above shows the reduction in visibility above Diamond Head on Oahu.

Below are two photos taken from my house that contrast a voggy day versus a clear day. Notice how the ocean is almost completely obscured in the first image taken today (2/4/17). In this post I would like to explain the weather pattern that contributes to vog episodes in Hawaii. In a future post I will describe how my group models and forecasts vog trajectories and dispersion to mitigate the hazards.

What is vog?

Sulfur-dioxide gas emitted by Kilauea volcano on the Island of Hawaii coverts to acid sulfate aerosol particles in the clear air and in clouds, forming volcanic smog known as vog.

Where does vog come from?

Kilauea volcano is one of the most active volcanoes on earth. The current east rift eruption has been ongoing since 1983, while a new summit eruption began in 2008. The photo below shows the summit emission plume. The most significant effect of this new eruptive vent has been a large increase in the amount of sulfur dioxide (SO2) gas that is emitted into Hawaii’s atmosphere. At this time, the current summit eruption shows no signs of abating. Higher gas fluxes from Kilauea appear to be the new norm.    

What hazards are associated with vog?

Vog poses a threat to the health of some of Hawaii’s people as well as being harmful to agriculture and infrastructure on the Island of Hawai‘i’.  Reductions in visibility can be a hazard to general aviation (see first photo above). Vog can irritate the upper respiratory tract, and  exacerbate pre-existing lung conditions, such as asthma.

Vog Episodes

Vog episodes occur nearly every day, at some location, on the island of Hawai‘i, however, when the winds turn from a northeast trade-wind direction, to southeasterly, the rest of the state can be impacted as well. This happens when the prevailing high pressure center (red "H" below) to the north of Hawaii is pushed eastward by an approaching cold front (winter storm) or kona low. The shading in the map below shows forecast precipitation valid at 10 AM tomorrow (2/4/17) northwest of Hawaii associated with an approaching cold front. The solid yellow contours are the sea-level pressure lines (isobars) and show that the ridge axis (red dashed line) has shifted south of the Islands.   

Shifting of the ridge axis southward has three important consequences that create conditions conducive to bad vog episodes, i) the winds turn southeast bringing vog from Kilauea across the Island chain (see wind barbs below), ii) the wind speeds decrease increasing the emission load from Kilauea into the air mass, and

iii) the sinking motion (subsidence) associated with the ridge reduces vertical mixing in the air and traps the vog in a shallow layer (~3-6,000' deep) further enhancing the vog concentrations. The radiosonde sounding from Hilo at 2 PM today shows a sharp increase in temperature with height (temperature inversion) starting at about 900 mb (~3000'). Vog is essentially trapped below this temperature inversion by the warmer air above (cold air sinks).

As the cold front continues to approach, the winds will turn southerly and then westerly, blowing the vog plume across the Island chain and then off to the northeast. Eventually, when the northeast trades return, a remnant of the vog plume will be advected back over the Hawaiian Islands and on to the southwest. Below is the forecast by the UHM Vog Model for 11 PM on Saturday 2/4/17 and at 9 AM on Sunday 2/5/17.

I will discuss the workings of the Vog Model in a future post. In the mean time you can see the vog model output daily here.  Southeast winds and vog episodes are more common in winter than in summer owing to the greater strength of the Hawaiian high and greater number of trade wind days in summer than in winter (see graph below).  Therefore, expect more vog episodes this winter as cold fronts and low pressure troughs approach Hawaii.


  1. I have lived on Oahu almost 20 years now and nave never experienced such heavy vog as this year. In February I got a lung infection from the thick vog which turned into pneumonia. Even though we have NE winds, they sometimes are not strong enough to carry the vog away.
    After searching for information on the internet, I still do not know why this is happening so greatly now more than ever.

    Also, I am perplexed as to why the weather reports rarely address the vog "ratings" for the day. There is a website that addresses the vog ratings for the big island, but not for the other islands. Look at this air quality website:
    AIRNow - City Not Found

    While it rates Oahu as "good" air quality, when I look out at Manoa valley, I see thick vog.

    I have never been this affected by the vog until now. It has caused me to stay in my bedroom for days because I have a special air filter that removes the sulfur particles. Using a charcoal mask otherwise is not much help. It concerns me to not have enough information to deal with such a serious issue (at least to me). Most people I know are now affected by the vog that have not been before. We are all confused because we do not have enough information/knowledge. Most do not know about the air filter I found or to even use a mask.

    Can you help us? Are there agencies that can give more information? Can a website or app be built with a daily vog rating for each of the islands as well as basic vog information and how to combat it?
    Anything would be much appreciated!

    Ja-ne de Abreu

  2. I agree completely with everything said above!!! It is good to know I am not the only one who feels this way.

  3. I live along the coast in Honomu. just north of Hilo. When it is clear or partly cloudy, but the sky is not deep blue, but seems hazy, can I assume this is vog?