What Do the USGS Volcanic Threat Levels Mean?

The United States Volcanic Hazards Program uses threat levels on a national scale to ensure that there is a consistent notification of the dangers of volcanic activity for the general public as well as other government agencies.

Alerts for the Volcanic Hazard program are either ground based, or aviation based. Volcanic ash can pose an extreme threat to respiratory systems on humans, cause crop damage, as well as damage to ground based ventilation system. Airborne ash poses a threat to aircraft. Ash can cause engines to shut down and become inoperable within seconds if an air craft were to fly through the cloud.

Aviation Volcanic Alerts
Volcanic alerts issued by the USGS for aviation use a color coded scheme. in order of increasing danger the colors can be green, yellow, orange, and red.

Green - "Volcano is...in non-eruptive state...volcanic activity has ceased and volcano has returned to non eruptive background state.

Yellow - "Volcano is exhibiting signs of elevated unrest...or after a change from a higher level, volcanic activity has decreased significantly..."

Orange - "Volcano is exhibiting...increased potential of eruption... OR eruption is underway with no or minor volcanic-ash emissions...

Red - Volcanic "eruption is imminent with significant emission of volcanic ash into the atmosphere... or eruption is underway or suspected with significant emission of volcanic ash...

Ground Based Volcano Alerts
Ground based volcano alerts issued by the United States Geological Survey follow a scheme as the aviation alerts. However, ground based alerts for the general public use descriptive wording place of the color codes.

The level of alerts are as follows: Normal, Advisory, Watch, Warning.

Volcano alerts are issued by the United States Geological Society for the following locations:

The Alaska Volcano Observatory
The Cascades Volcano Observatory
The Hawaiian Volcano Observatory
The Long Valley Observatory
The Yellowstone Volcano Observatory

Each one of these observatories maintain a website where it is possible to view a lot of the data that they gather. There are also studies, and plenty of information available about the volcanoes that each of these observatories monitor.

Source: Various United States Geological Websites