When Thunderstorms Get Down and Dirty

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When we think about volcanology and meteorology, we usually put them in separate pigeonholes. At low emission rates, atmospheric pollutants produced by volcanoes, gases, ash, and heat, are dissipated by winds and don't significantly affect the weather. However, during large eruptions, the line between the two sciences becomes blurred.

When Thunderstorms Get Down and Dirty...

Space Shuttle (Mission STS-43) photograph of the Earth over South America taken on August 8, 1991, showing double layer of Pinatubo aerosol cloud (dark streaks) above high cumulonimbus tops.

(Public domain.)

Surprisingly, the most obvious volcanic pollutant, volcanic ash, doesn't have a long-range or long-term effect on the weather, because ash settles rapidly out of the atmosphere. Rather, the pollutant with the greatest long-term effect is sulfur dioxide, a gas that is all-too-familiar to vog sufferers in Hawai`i. During large explosive eruptions of sulfur-rich magma, sulfur dioxide is injected into the stratosphere, which begins 8-14.5 km (5 to 9 miles) above the earth's surface and extends to 50 km (31 miles). Here it forms sulfuric acid, which, in turn, forms sulfate aerosols. These tiny particles absorb some of the sun's radiation and, thus, heat the stratosphere. They also reflect some of the radiation from the sun back into space and cool the atmosphere below the stratosphere (the troposphere).

The most recent and best-studied example of a weather-changing eruption is the June 15, 1991, eruption of Mount Pinatubo in the Philippines. Within a few hours, the volcano injected about 20 megatons of sulfur dioxide gas into the stratosphere. The resultant stratospheric sulfate load produced global climate anomalies. Surface air temperatures over northern hemisphere continents were up to 2 degrees Celsius (3.6 degrees Fahrenheit) cooler than normal in the summer of 1992, and up to 3 degrees Celsius (5.4 degrees Fahrenheit) warmer than normal in the winters of 1991-91 and 1992-93.

While volcanic ash doesn't have the long-range and long-term climate impact of sulfur dioxide, it does have localized, short-term effects on weather. This was demonstrated by 13 eruptions that occurred at Pinatubo during the 24 hours before the big June 15 eruption.

In each of these precursory eruptions, the ejecta shot out of the vent vertically, rose to great height, then fell around the vent. Imagine a water fountain more than 5 miles high, then substitute a mixture of volcanic ash, pumice, rock fragments, and hot gas for the water. The mixture fell because it was denser than the surrounding air.

As the mixture landed around the vent, it behaved much like a fluid and raced outward, radially away from the vent, as a dense, ground-hugging cloud known as a pyroclastic surge. As it moved away from the vent, the turbulent surge incorporated air, which expanded as it was heated by the hot ash. This lowered the surge density, so the larger fragments began to drop out. These two processes, air ingestion and particle deposition, progressively lowered the density of the surge cloud.

Eventually, many kilometers away from the vent, the surge density dropped below that of the ambient air. At more or less the same time, all around the vent, the surge cloud became buoyant and lifted off the ground, still raining ash from its base.

Like an enormous hot-air balloon, the hot, ash-laden cloud, laced with red-orange lighting, rose near the volcano. Because atmospheric pressure decreases as altitude increases, the cloud expanded and cooled as it rose. The rising cloud contained a lot of water vapor, because the magmatic gas that originally drove the surge-producing eruption was mostly water vapor, and because the air incorporated into the surge was moist. The cooling caused the water to condense on the ash particles.

The water droplets collected more ash until the resultant ash balls, known as accretionary lapilli, became so large that they began to fall out of the cloud. This ashy precipitation produced a downdraft of relatively cool, dense, ash-laden air. When it reached the ground, a few tens of minutes after its parent eruption, the downdraft moved outward radially as a wet, grey, ashy cloud. Because it was denser than the ambient air, it hugged the ground as it moved. Scientists observed one of these clouds 20 kilometers (12 miles) from the volcano--it was about 1 kilometer (0.6 mile) thick.

From the point at which the surge cloud lifts off the ground, the processes described here are quite similar to those that operate within ordinary thunderstorms. In thunderstorms the source of the hot air isn't a volcanic eruption, so the rising air doesn't contain volcanic ash. So rain or hail forms instead of accretionary lapilli, and the downdraft is usually not visible. Because of their similarity to ordinary thunderstorms, the eruption-induced storms have been called "dirty thunderstorms."


Volcano Activity Update

This past week, activity levels at the summit of Kīlauea Volcano have remained at background levels. After stopping in early October 2006, extension of the summit caldera resumed at the end of December 2006. The number of earthquakes located in the summit area is slightly elevated but still considered to be at low levels (usually fewer than 10 per day are large enough to locate).

Eruptive activity at Pu`u `O`o continues. On clear nights, glow is visible from several vents within the crater. Lava is fed through the PKK lava tube from its source on the southwest flank of Pu`u `O`o to the ocean. About 1 km south of Pu`u `O`o, the Campout flow branches off from the PKK tube. The PKK tube feeds a long-lived ocean entry at East Lae`apuki, while the Campout tube is the source for ocean entries East Ka`ili`ili and Kamokuna. The amount of lava entering the ocean at East Lae`apuki and East Ka`ili`ili has decreased significantly over the last several weeks. The entry at Kamokuna, active since late December, is now the dominant entry. All three ocean entries are located inside Hawai`i Volcanoes National Park.

In the last week, intermittent breakouts from the Campout and PKK tubes have been seen on the slope of Pulama pali and on the coastal plain. A breakout from the Campout tube below the pali continues to host minor surface flows inland from the sea cliff at East Lae`apuki and on the East Lae`apuki delta. This is a rather unusual situation, lava from two different tubes, the PKK and Campout tubes, intermingling to form a single ocean entry.

Access to the sea cliff near the ocean entries is closed, due to significant hazards. The surrounding area, however, is open. If you visit the eruption site, check with the rangers for current updates, and remember to carry lots of water when venturing out onto the flow field.

Two earthquakes beneath Hawai`i Island were reported felt within the past week. A magnitude-3.4 earthquake occurred at 11:27 p.m. H.s.t on Friday, March 2, and was located 7 km (5 miles) northwest of Ka`ena Point at a depth of 10 km (6 miles). A magnitude-3.1 earthquake occurred at 12:45 p.m. on Sunday, March 4, and was located 18 km (11 miles) east Mauna Loa summit at a depth of 8 km (5 miles).

Mauna Loa is not erupting. During the past week, earthquake activity was slightly elevated, but still considered at low levels beneath the volcano's summit (six earthquakes were located). Extension of distances between locations spanning the summit, indicating inflation, continues at slow rates.