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Lascar volcano, in northern Chile, erupted on July 20, 2000, sending ash
40,000 feet into the air. These images from the NOAA Geostationary
Operational Environmental Satellite (GOES) show the ash cloud as it blew
The top images use the GOES visible light band. The dark ash cloud
does not stand out well from the Earths surface. To enhance the
appearance of the ash cloud, we compared the two GOES thermal infrared
channels in the bottom images (called the split window technique).
Silcate particles in the ash cloud absorb
more 11 µm radiation than 12 µm radiation, so the ash is
emphasized when one channel is subtracted from the other. This technique
is especially useful in overcast conditions, because water droplets and
ice crystals absorb less in the 11 µm channel than the 12
Image by Marit Jentoft-Nilsen, NASA GSFC Visualization Analysis Lab,
based on data provided by NOAA.