ASHFALL FORECASTS

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Overview: Science Issues: Why are forecasts of ashfall important?; How is Ashfall Forecasted?; Future

ASHFALL FORECASTS

Forecasting ashfall is a difficult but very important task that involved the understanding of many disciplines. Ash particles produced during an eruption can remain in the atmosphere for days and have been attributed to many health and machinery problems.

Why are forecasts of ashfall important?

The volcanic ash that is created during an eruption can be hazardous to one's health. Fine ash can remain airborne for long periods of time and upon falling out, are easily resuspended by wind or human activities such as driving. Fine ash that is inhaled can lead to a number of serious health effects including silicosis of the lungs, respitory problems such as asthma and fluorosis.

Many governments have incorporated an upper limit of the concentration of fine particles an individual should inhale before it becomes a health risk. In the United States, the Environmental Protection Agency has incorporated Particulate Matter Standards in an attempt to quantify the amount of particluate matter at specifice size intervals that may be hazardous. The standard for PM10, or particulate matter of 10 microns or less, is not to exceed 50 mug/m3 per year averaged over 3 years and not to exceed 150mug/m3 more than once per year averaged per three year period.(i'm not really sure how they calculate the 24 hr standard...something about 99th%ile). Likewise the PM2.5 for the annual and 24 hour standard is 15 mug/m3 and 65 mug/m3 respectively.

Ash also presents a problem to machinery and structures. The average weight of ash 1 inch deep is 10 lbs per square foot. Significant ashfall can therefore easily collapse structures. Also, if volcanic ash is encountered by an aircraft significant damage can be incurred because the ash can be remelted in engines and shut them down or cause corrosion to numerous parts of the aircraft. In fact, the awareness of the threat of volcanic clouds to aircraft has become large enough that 9 Volcanic Ash Advisory Centers have been set up around the world to inform aviators of the presence of volcanic ash and its projected movement.

How is Ashfall Forecasted?

For the most part, geologists have a strong understanding of how large particles fall from an ash cloud. Typically, these particles fall out in a matter of hours. The behavior of fine ash particles however is poorly understood. These particles can remain in the atmosphere for days but frequently fall out surprisingly quickly due to mechanisms that are poorly understood.

Ashfall can be predicted using information about the eruption, the erupted particles and local wind vectors at a variety of altitudes. The amount of time a particle takes to falls depends on the height at which it was erupted and its terminal velocity in the atmosphere. The particle falls from altitudes above the vent and below the top of the eruption column. The altitude of the vent can be attained from online databases such as the Smithsonian Institution or from a local topographic map of the region. The height of the eruption column can be either visually esitmated or calculated from remotely sensed data. Once the range of starting altitudes has been determined, the rate the particle will fall from a given altitude can be calculated according to properties of fluid mechanics. The forces acting on the particle are dependant upon on the dimensions and composition of the particle and the atmospheric conditions.

The horizontal winds are responsible for carrying the ash particles from the volcano. The distance that the particle can travel is related to the amount of time that the particle falls and the strength and direction of the atmospheric winds. The wind vectors are typically measured using meteorological balloons called radiosondes and generally change at various altitudes.

Several trajectory models predict the location of an ashcloud in time and space. These algorithms require input about the eruption and the meteorological conditions and assume simple particle shapes and generally do not incorporate the possibility of particle aggregation.

Future Needs

In order to better predict the fallout of volcanic ash a better understanding of how ash actually falls out from the atmosphere is needed. Factors that are assumed in the presently used forecasting models do not accurately describe the physical characteristics of ash particles and do not account for particles sticking to each other and then falling faster than predicted.

http://www.geo.mtu.edu/volcanoes/vc_web/overview/o_ashfall_forecasts.html -- Revised: 06 NOVEMBER 2000