It is estimated that over 500 million people around the globe live in the  proximity of  volcanoes.  This has caused a certain interest in the studying of volcanic activity.  When a volcano erupts, the lava flows are what the local people are most concerned about.  But in the last few decades there has been a shift from studying a volcano locally to focusing on volcanoes as a global concern.  This shift is due mainly in part to the increase of air travel around the world.
     During an eruption, the volcano emits a cloud filled with gases and ash.  These ash clouds can rise high into the atmosphere into the cruise altitudes of aircraft and, as wind carries the ash, can also affect air traffic thousands of miles away from the eruption site.  The ash can ruin airplanes, and cause a loss of thrust or even flameouts.  It can also damage planes on the ground and cover runways in either a thick layer of ash making them unusable, or in a thin layer causing the runways to become slick.
     Volcanic ash is composed of microscopic silicate minerals and shards of glass.  If a plane flies through an ash cloud, the engines, which take in enormous amounts of air and operate at high temperatures, take in the ash.  The ash can melt and solidify, clogging airflow, eroding parts, and possibly leading to engine flameout.  The particles and shards of glass can also wear the leading edges of the aircraft.
     Worldwide, over the past two decades, there have been reports of more than 80 airplanes encountering volcanic ash along flight paths.  These encounters can not be avoided because the onboard radar cannot detect the microscopic ash particles.  The ash problem is compounded with more planes in the air, often traveling at night when the clouds are less visible, and flying more flexible routes.  More planes are also flying above potential volcanic hotspots, such as the Pacific’s “Ring of Fire”.
     Two satellite-based systems can detect volcanic clouds: 1. In the ultraviolet spectrum, the Total Ozone Mapping Spectrometer (TOMS) detects SO2 gas and collects volcanic cloud position data globally about once each day during daylight hours only. 2. Infrared detectors aboard weather satellite platforms (NOAA and GOES) are by far the most useful volcanic cloud detectors because they detect volcanic ash directly, and because these satellites give coverage very frequently        (twice daily for NOAA and every 15-30 minutes for GOES).
AVHRR vs. TOMS data (Mt Spurr, Alaska)
View Volcanic Cloud Tracks
A list of all Active Volcanoes is posted on the Internet by the
Smithsonian Institute
World Reference Map
Volcano Watch
 satellite images showing the world's 10 most active volcanoes as determined by the folks from the MTU Volcanoes Page
 Volcanic Ash Forecast Transport and Dispersion trajectory model from the
NOAA Air Resources Laboratory