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Volcanic alert-levels characterizes conditions at U.S. volcanoes

The U.S. Geological Survey (USGS) employs a nationwide volcano alert-level system for characterizing conditions (quiet, unrest, eruption) at U.S. volcanoes. Notifications about the status of activity at U.S. volcanoes are issued through the five regional U.S. Volcano Observatories. Additional information on the alert-notification system can be found in the Fact Sheet U.S. Geological Survey's Alert-Notification System for Volcanic Activity.

The USGS alert-level system for volcanic activity has two parts – 1) ranked terms to inform people on the ground about a volcano's status and 2) ranked colors to inform the aviation sector about airborne ash hazards.

ALERT-LEVEL TERMS.
When the volcano alert-level is changed, a Volcano Activity Notice (VAN) is issued.
NORMAL Volcano is in typical background, noneruptive state
or, after a change from a higher level,
volcanic activity has ceased and volcano has returned to noneruptive background state.
ADVISORY Volcano is exhibiting signs of elevated unrest above known background level
or, after a change from a higher level,
volcanic activity has decreased significantly but continues to be closely monitored for possible renewed increase.
WATCH Volcano is exhibiting heightened or escalating unrest with increased potential of eruption, timeframe uncertain,
OR
eruption is underway but poses limited hazards.
WARNING Hazardous eruption is imminent, underway, or suspected.

AVIATION COLOR CODES
When the volcano color code changes, a Volcano Observatory Notification for Aviation (VONA) is issued.
GREEN Volcano is in typical background, non-eruptive state
or, after a change from a higher level,
volcanic activity has ceased and volcano has returned to noneruptive background state.
YELLOW Volcano is exhibiting signs of elevated unrest above known background level
or, after a change from a higher level,
volcanic activity has decreased significantly but continues to be closely monitored for possible renewed increase.
ORANGE Volcano is exhibiting heightened or escalating unrest with increased potential of eruption, timeframe uncertain,
OR
eruption is underway with no or minor volcanic-ash emissions [ash-plume height specified, if possible].
RED Eruption is imminent with significant emission of volcanic ash into the atmosphere likely
OR
eruption is underway or suspected with significant emission of volcanic ash into the atmosphere [ash-plume height specified, if possible].