Øvelse 2: Påvisning af hotspots og røgsøjler - fortsat   MODIS Bands 31 and 32 are used to detect a volcanic plume and distinguish it from clouds. The bands range from 10.780 - 11.280 μm (31) and 11.770 - 12.270 μm (32), and correspond to the thermal IR.   Band subtraction (Band 31 minus band 32) for ash detection of the MODIS image of 24 July 2001. The difference between the brightness temperatures usually results in negative values for volcanic plumes containing ash (dark), and positive values for atmospheric clouds (light).   Band subtraction (Band 31 - band 32) for ash detection of the MODIS image of 23 July 2001. The difference between the brightness temperatures usually shows negative values for volcanic plumes containing ash (dark) and positive values for atmospheric clouds (light).