A three in Indonesia might be a four in New Zealand, which may be the color orange if you’re a pilot, and potentially a triangle with an eyeball if you’re American. These are all volcano alert levels and provide ways of talking about volcanic activity; however, most of them don’t translate over borders or seas.
There is no global standardized alert or measurement system for categorizing ongoing volcanic activity. While both hurricanes and cyclones have scales or categories that provide an international understanding of developing storm strength, and there is a unified network creating a global early warning system for tsunamis, there is no equivalent for volcano status.
So, when Indonesian officials increased its alert level for the volcano Anak Krakatau from two to three, understanding the degree of change meant a search for a description of that country’s particular system.
Volcanic eruptions are so unique that it’s almost impossible to create a wide-ranging system to warn of current hazards and possible outcomes. The lack of international cohesion is due to the great variation both in the behavior of volcanoes themselves and the monitoring capabilities of their home nations, says the World Organization of Volcano Observatories. This, then, leaves the world with a jumbled mess of scales and levels, which can more or less be grouped into three types.
Pilots actually have a standardized way of assessing volcanic activity, though it’s not particularly useful for those on the ground. The International Civil Aviation Organization’s (ICAO) color-coded method places an emphasis on one of the greatest volcanic hazards to aircraft: ash. Ash can cause jet engines to fail, and winds can carry it far from its point of origin.
Aviation alerts are divided into four colors: green, yellow, orange, and red. Red indicates an eruption in progress, or imminent, and indicates to pilots there is likely to be significant ash in the area. Many countries have included some form of the colors in their own alert systems, including the US.
In 2006, the United States Geological Survey (USGS) standardized the alert notification systems (pdf) across its volcanic observatories. It is composed of icons meant to correspond to the aviation colors; however, because hazards in the air and on the ground are different, at times the levels will change independent of one another. This system consists of four levels—normal, advisory, watch, and warning— that range from dormant to eruption.
The majority of countries have some form of numeric system. But be aware: Even if numbers correspond across scales, the severity of what they indicate will differ significantly. Indonesia is an example of countries using a numeric system; its categories range from one to four. New Zealand also uses numbers, though its scale goes to five and includes zero.