While we're at it, reader Colin Seftor, an atmospheric scientist who helped develop the "TOMS" monitoring system mentioned below, has another improvement to suggest for the USGS ash-fall map that I mentioned here (and ran a previous comment on here):
The map from the USGS is a rather odd one. I suppose it shows the area of direct ash fall from the 1980 eruption, but it doesn't indicate the area affected by ash clouds lofted high into the atmosphere (and that could, therefore, be hazardous to airplanes). [And of timely interest right now.]
Over the last twenty years or so, techniques have been developed to detect (and track) volcanic ash from satellites. One such technique uses measurements from satellite sensors designed to determine the amount of ozone in the atmosphere (such as the Total Ozone Mapping Spectrometer, or TOMS). An example of TOMS data used to detect and track the movement of ash clouds from the 1980 eruption of Mt. St. Helens can be found here: [after the jump]
You can see the ash clouds covering large areas over the western part of the US, then moving east over the mid Atlantic region and into eastern Canada. (On May 21st, one orbit of data is missing, hence the abrupt change in the image. Images via the SO2 (sulfer dioxide) group at the University of Maryland, Baltimore County, UMBC.)
The Mount St. Helens image shows something called the aerosol index, which is a measure of the amount of aerosol in the atmosphere. In this case, the aerosol is volcanic ash. At other times, in other areas, the aerosol index can detect desert and track dust (Sharan dust storms or asian dust storms) or smoke from brush fires; one of the disadvantages of this index is it can't distinguish between the different types of aerosol. For the case of the Mt. St. Helens eruption it's pretty easy to associate the clouds with volcanic ash (although there might be some smoke from Canadian brush fires).
Other techniques use measurements from other satellites to detect such clouds; a network of "Volcanic Ash Advisory Centers" (or VAACs) use satellite measurements (and visual reports, etc) to continuously monitor volcanic eruptions and the ensuing volcanic ash clouds; NOAA maintains the VAAC for the US, Mexico, and parts of South America:
It's interesting to note that a series of proposals have been made (to US funding agencies and others) to develop and fly satellite sensors that are optimized to detect and track volcanic ash (all of the current satellite sensors, which were designed for other purposes, have one problem or another in detecting ash). To date, none have them have been funded. Given the current situation (with the airlines losing up to $200 million a day, and the increasing criticism that government officials are relying on computer modeling and simulation that is too conservative in denoting the danger regions), it might be worth governments reconsidering the costs of developing and flying such sensors versus the cost of not flying them...