Use the Product Listbox to view the two images on this page. In the images shown here, the 11-µm (Ch 4) data are subtracted from the 12-µm (Ch 5) data to produce a brightness temperature difference product image that allows for discrimination between ash, water clouds, and even most tropospheric cirrus cloud types. Note the rapid spread in the ash cloud between 05:12 and 13:38 UTC (21:12 and 05:38 local time).