The United States Geological Survey (USGS) issues a volcano alert notice (VAN) to describe the risk associated with a volcano. This is converted to a color code to notify planes about the hazard.
Differences from USGS volcano alert levels:
(1) use of a color rather than term
(2) inclusion of ash plume height in the report
Alert levels:
(1) green (normal)
(2) yellow (advisory)
(3) orange (watch)
(4) red (warning)
Volcano Status
Term
typical background, noneruptive
Normal
any signs of elevated unrest above background level
Advisory
increasing potential for eruption
Watch
nonhazardous eruption
Watch
hazardous eruption imminent, underway or suspected
warning
decrease in volcanic activity so no longer hazardous
Watch
decrease in volcanic activity with potential to have renewed activity
Watch
significant decrease in volcanic activity with continued monitoring to detect renewed increase
advisory
cessation of volcanic activity with return to typical background
normal
The height of the plane relative to the ash-plume would determine the hazard to the plane. If the plane flies through the ash then there is risk of catastrophic engine failure. If the plane flies much above the plume then the risk should be low.
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