They are the communicational component of an early warning system, accounting for warnings both prior to and during a hazard event1. Observation of eruption phenomena and integration of monitoring of precursory signals such as geodetic, seismic, and gas flux as well as detection and interpretation of magma migration are important in order to issue timely warnings to local people and governments and thus mitigate risks from volcanic phenomena2. Ground and aviation hazards are decoupled, as they might be independently in some cases, e.g. a large fast-moving volcanic debris flow with minimal ash production.
In the USA separate standardized Volcanic Alert Level Systems are used for ground hazards (textual) and for aviation hazards (color code). To improve hazard communication with nonscientists as well as meteorological and aviation authorities on volcanic activity potentially affecting the aviation space, the color-coded volcanic alert-level system for aviation was established by the U.S. Geological Survey (USGS). It was first instituted by the Alaska Volcano Observatory (AVO) during the eruption of Redoubt Volcano in 19903. It is a simple alert scheme which describes changing hazards and conditions at the volcano. Besides the USA it has been adopted for use in Russia, New Zealand, Iceland, and partially in the Philippines, Papua New Guinea, and Indonesia3. The color code alert system is adapted to the different situations and volcanos. Today, a worldwide system of nine Volcanic Ash Advisory Centers (VAACs) detects ash clouds and issues advisories describing their current and expected locations on satellite data, volcano observatory notifications, pilot reports, and atmospheric dispersion modeling. Ongoing activity can be viewed here.
New Zealand also uses two standardized Volcanic Alert Level Systems: one designed for hazards expected at frequently active volcanoes and one for restless and reawakening volcanoes. They are numerically based ranging from 0 to 5 (GNS 2010).
In Japan, volcanic warnings and forecasts are since 2007 issued by the Japan Meteorological Agency (JMA) with a “Volcanic Alert Level” for active volcanoes in order to inform the public and mitigate risks and impacts from volcanic phenomena4. This is done individually for every volcano, and the regions around the volcano where people need to take specific action are designated2. Volcanic Warnings are communicated to residents through the media, prefectural offices and local municipalities. In contrast to the Volcanic Alert Level Systems of the USA and New Zealand which are both based upon the current activity of a volcano, and neither advocate action nor provides advice to users involved in crisis management and mitigation1, the Japanese Volcanic Alert Level System addresses the measures to be taken by specifying areas of danger, indicating extent of evacuation, and outlining the expected volcanic activity: When issuing the volcanic warnings, local governments are advised to take critical mitigation measures (Japan Meteorological Agency 2010). Specific target areas are defined in distances from the volcanic centers or craters. Based on the disaster mitigation measures required in the specific areas, and associated with the protective actions against the phenomena, five Volcanic Alert Levels are defined. The assigned level is determined considering the presumed area where pyroclastic flows, ballistic bombs and lahars may reach. Phenomena such as tephra fall and shock waves are harmful, but can be avoided by sheltering in place.
Also in developing countries Volcanic Alert Level Systems integrate advice on mitigation action or evacuations to civil authorities or emergency managers