October 25, 2001

A weekly feature provided by scientists at the Hawaiian Volcano Observatory.

Long-lived eruptions may contribute to local droughts

Over the past several weeks, visitors to Kilauea have been treated to views of spectacular surface lava flows cascading down the pali and a new ocean entry complete with a vigorously growing coastal bench. Behind the scenes are the sighs and whispers of an active volcano that are unnoticed by most observers. For instance, Kilauea is quietly releasing around 1000-2000 tons of noxious sulfur dioxide (SO2) each day. Island residents and visitors are familiar with some of the effects of this gas. It is a major contributor to the volcanic air pollution known as vog, which is responsible for much of the haze seen on the leeward side of the island, particularly during prevailing tradewind conditions. In East Hawai'i, during calm or southerly winds, such as those of the past week, Kilauea produces air pollution that ranges from barely detectable to nearly unbreathable.

There may be a less obvious but more far-reaching effect from Kilauea's daily gas production. Once released, SO2 and other gases from Kilauea react in the atmosphere to form tiny particles. These acidic particles are physically and chemically similar to those of urban pollution from power plants and industrial processes. Researchers studying NOAA satellite data, validated by ground-based measurements, have concluded that substantial urban air pollution can suppress rainfall by inhibiting raindrop formation.

Viewed simply, the rainfall process starts when microscopic dust and other airborne particles are entrained in an aspiring rain cloud. The particles act as a core on which water droplets condense. These tiny droplets measure about 0.01 mm or one-fifth the diameter of a human hair. They grow by colliding and combining with one another until they are about 100 times larger than their initial size. At this point gravity wins out - the droplets are too large and heavy to remain aloft, and fall from the cloud as rain. However, when there are many very tiny particles in the air, such as in highly polluted urban areas, the available water in a cloud is spread over an increased number of extremely small droplets. Because each of these core droplets starts out about 100 times smaller than normal, they take longer to mature to raindrop size. The formation of raindrops in polluted air is therefore less efficient, and an average cloud would yield less rainfall over the course of its lifetime. This same effect can occur downwind of a volcano like Kilauea, which actively degasses copious amounts of SO2.

Now before we point the finger at Kilauea as our sole cause of drought, it is important to note that the most significant contributor to reduced rainfall statewide is widely accepted to be the El Nino Southern Oscillation (ENSO) events that occur irregularly every two to seven years. These anomalous events cause a slackening of tradewinds with winter and spring rainfall generally reduced throughout the state. The driest ten years in Hawaiian weather records were all associated with ENSO events.

As most residents here are aware, however, the southern areas of the Big Island are suffering from a long-term reduction in rainfall. This drought has caused severe impact to agricultural activities and to households relying on water catchment. In some areas, rainfall measured during the mid-1990s decreased as much as 50 percent from levels recorded during the 1930s-1960s. It is reasonable to suspect that the steady stream of volcanic pollution from Kilauea contributes to the continuing drought conditions in some leeward areas of the Big Island. South Kona and Ka`u are particularly impacted by vog particles, especially during the summer months when the tradewinds blow steadily. Researchers have linked the long-term rainfall reduction in this area to a combination of ENSO events, which significantly reduce winter rainfall, and vog particles, which contribute to reduced summer rains.

It is likely that other factors also influence areas of drought around the state as well. However, it is noteworthy that the volcano in our backyard potentially contributes to regional effects beyond the reach of those beautiful lava flows and ocean entries.

Eruption Update

Eruptive activity of Kilauea Volcano continued unabated at the Pu`u `O`o vent during the past week. Lava moves away from the vent toward the ocean in a network of tubes and descends Pulama pali in several separate tubes. Breakouts from the tube system feed two surface flows above and on the pali. Many surface flows, mainly from breakouts of the ocean entry tubes, are also observed in the coastal flats. Lava continues to enter the ocean at Kamoamoa and the area east of Kupapa`u.

The public is reminded that the benches of the two ocean entries are very hazardous, with possible collapses of the unstable new land. The steam clouds are extremely hot, highly acidic, and laced with glass particles. Swimming at the black sand beaches of the benches can be a blistering or even deadly venture.

Two earthquakes were reported felt during the week ending on October 25. Residents of Volcano, Wai`ohinu, Discovery Harbour, Hawaiian Ocean View Estates, Miloli`i, and Captain Cook felt an earthquake at 11:38 p.m. on the night of October 23. The magnitude-3.3 earthquake was located 13 km (7.8 mi) southeast of the summit of Mauna Loa Volcano at a depth of 11.9 km (7.1 mi). A resident of Kona felt an earthquake at 11:24 p.m. on the night of October 24. The magnitude-3.1 earthquake was located 18 km (10.8 mi) ENE of Keahole Point at a depth of 32 km (19.2 mi).