(Volcano Watch is a weekly article written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory.)

Alert observers of Kilauea’s ongoing summit eruption often note the changing character of the ever-present plume emerging from Halemaumau. Sometimes it’s energetic, sometimes its color is a visibly ash-rich grey or brown, and at other times the plume appears translucent, even wispy. What causes these variations?

Several factors affect the plume’s appearance. Changes in physical conditions at the vent, meteorology, and even chemical reactions occurring as gas boils out of the melt can conspire or act alone to produce the variably visible plume.

The summit plume is principally composed of water vapor (H2O), carbon dioxide (CO2), and sulfur dioxide (SO2), all of which are clear and colorless gases. The visible part of the plume includes a minor amount of ash, along with a suspended mixture of tiny droplets and particles, called aerosol. Volcanic ash consists of rock dust and glass particles less than 2 mm (0.08 inches) in diameter.

The aerosol in the plume is even smaller, less than 0.0025 mm (0.0001 inch) in diameter — about one-tenth the diameter of a strand of human hair. Aerosol is composed chiefly of dilute sulfuric acid (H2SO4), formed when SO2 reacts in the presence of oxygen and water vapor.

Physical changes occur intermittently at Halemaumau as the unstable rim crumbles and releases rocks and debris into the vent. These rockfalls disturb the top of the magma column and sometimes result in the production of brief ash-rich plumes, ranging from gray to brown, before returning to their prior appearance.

Meteorological variations, including relative humidity, temperature, cloud cover, and wind speed also affect plume appearance. Because the plume is visible, owing partly to the presence of water-laden droplets, air temperature and relative humidity work in concert to changing the plume’s visibility. Higher humidity and lower temperature conditions cause moisture condensation, producing a more visible plume.

The converse temperature/humidity story is true, as well. Increasing wind speed tends to make the plume more compact, and usually more opaque, except at very high wind speeds.

Cloud cover diffuses the sun’s rays; diffuse sunlight is scattered in a way that appears different to our eyes than does direct sunlight. The result of this effect can be that a plume appearing blue and translucent in bright sun light, turns milkier and denser under heavy cloud cover.

Chemical transformations occurring with the plume are of great interest with respect to their effects on plume appearance. These same changes also provide clues about processes occurring at the top of the summit magma column, a place often not directly visible from the surface.

As noted earlier, the visible aerosol is comprised of H2SO4, formed by chemical oxidation of SO2. A primary factor accounting for variability in the amount of H2SO4 produced is obviously the amount of SO2 bubbling out of the magma to start with, but the temperature of the vent has a strong effect, as well. Higher temperatures within the vent tend to oxidize more SO2.

A useful lesson showing the complex effects of temperature and chemistry on the visibility of SO2- and H2SO4-bearing plumes occurred at coal-fired power plants on the mainland in the 1990s. Power companies worked hard to clean up sulfur and nitrogen oxide emissions by installing chemical scrubbers.

The scrubbers reached their goal of effectively decreasing the amount of clear, colorless and noxiously polluting SO2 in their exhaust gases, but, partly because of gas temperature and humidity effects, had the unintended consequence of producing dense blue plumes containing light-scattering H2SO4.

The experience of the power stations underscores the importance of temperature and humidity on plume visibility. It also demonstrates how decreases in SO2 emissions do not always result in a less visible plume. Conversely, a more dense plume doesn’t necessarily mean more SO2, either.

At Kilauea, we confirmed this lesson by using ultraviolet spectrometers to measure SO2 emission rates of dense white plumes and thin, wispy blue ones. We’ve found that plumes appearing thin and wispy often contain as much or more SO2 then dense white ones.

A substantial and valuable part of the science of volcanology is based on simple but careful observations. Combining these direct observations with instrumental ones has helped volcanologists answer numerous questions about how volcanoes work. As our toolbox of high-tech methods bulges, we remind ourselves of the value and place of simple human-based observations.