The collection of polar orbiter and geostationary satellites provide global coverage and their data enable forecasters to track a volcanic ash cloud over long distances as long as it can be distinguished from water-bearing clouds.
In standard visible and infrared satellite imagery volcanic ash clouds can resemble water-bearing clouds. However, the radiative absorption properties of the silicate in the volcanic ash are different to those of water in the infrared wavelength range 10-12 microns. An image showing the brightness temperature difference between channels at 10.8 and 12.0 microns (BT10.8 - BT12.0) can be used to distinguish volcanic ash from water-bearing clouds.
Met Office forecasters operating the London VAAC service currently use BT10.8 - BT12.0 imagery from the Advanced Very High Resolution Radiometer (AVHRR) on board polar orbiting satellites to monitor and track the movement and dispersion of volcanic ash clouds in their area of responsibility. The Met Office is now also producing BT10.8 - BT12.0 images every 15 minutes from the geostationary satellite series, Meteosat Second Generation.
AVHRR BT10.8 - BT12.0 imagery