What's it like during ashfall?

The main thing to expect after an ashfall is a time-consuming, costly, and "dirty" effort by everyone in the affected area to remove and dispose of the ash and to keep ash from entering homes, businesses, and wastewater systems. People need to coordinate their clean-up activities so that ash is only moved once and does contaminate a previously cleaned site. Because water will be in high demand, a rationing strategy may be necessary so that a community's fire-fighting ability is not reduced.

Following are photographic examples of ashfall based on accumulated ashfall thickness. Thresholds for each term are based on severity of various impacts, drawing heavily on the experience and documentation for recent eruptions of Mt. Ruapehu in New Zealand (Johnston, 1997).

Table of ashfall severities based on accumulated thickness.
Term Accumulation (inches) Accumulation (decimal inches & SI unit) Key Impact Thresholds (cumulative)
Trace or dusting <1/32 in 0.031 in (< 0.8 mm) Eye and respiratory irritant, very low level impacts for most people.
Minor 1/32 – 1/4 in 0.031 – 0.25 in (0.8 – 6.4 mm) Possible crop, animal equipment, and infrastructure problems; widespread clean-up likely.
Moderate 1/4 – 1 in 0.25 – 1.0 in (6.4 –25.4 mm) Ash removal efforts significant.
Heavy 1 – 4 in 1.0 – 4.0 in (25.4 –100 mm) Weaker roofs can fail at ~ 4-5 inches of compacted, wet ash accumulation (~40 lb/ft2)
Very Heavy 4 in – 12 in 4.0 – 12.0 in (100 – 300 mm) Danger of roof collapse increases, damage to trees, essential services interrupted.
Severe > 12 in >12 in (>300 mm) Roads impassable, severe infrastructure damage, heavy plant and animal loss.

Trace or Dusting (<0.8 mm or 1/32 in)

Minor (0.8–6.4 mm or 1/32–1/4 in)

Minor ashfall may result in some of these conditions, but the main challenge will likely be the persistent ashy conditions (due to remobilization by wind and other disturbance). Impact is mostly limited to the effort needed to clean and remove the ash.

Moderate (6.4–25 mm or 1/4–1 in)

Heavy (2.5–10 cm or 1–4 in)

During a heavy ashfall and for several days after, normal community and business services are typically severely limited or completely unavailable. Transportation systems are likely to be shut down or restricted—roads may be impassable or purposefully blocked, airports temporarily closed. People will be stranded away from home. Electrical power may be intermittently unavailable when conditions favor arcing on transformer insulators or if power generators have to be shut down for clean-up operations.

After a heavy ashfall, surviving roofs may collapse if the ash becomes saturated before it is removed or they may have suffered structural damage.

Severe (>30 cm or 12 in)