2. The appearance of volcanic ash clouds in selected spectral bands and RGB types
Figure 10: Hunga Tonga-Hunga Ha'apai volcanic eruption January 14, 2022, Source: Wikipedia © Tonga Geological Services, CC BY
The constituents of volcanic plumes such as volcanic ash, water vapor and SO2 can be observed in visible and infrared channels from polar orbiting and geostationary satellites. The movement of the volcanic plumes can be easily followed in animations of satellite images.
Volcanic ash and gases are often mixed in proximity to the source, but they can also be separated at the onset of eruptions, possibly due to differences in injection height. In the latter case, volcanic plumes are seen to follow different routes in cases of vertical wind shear.
Generally, larger ash particles and lapilli are first to deposit after being ejected. Only the smallest ash particles (smaller than 5 microns) remain airborne over a period of days, depending on ejection height. When volcanic ash is released, about 98% of fine ash (1 to 10 microns) is removed by gravitational deposition within 3 to 4 days. However, low ash concentrations are hard to detect for the sensors of satellites whose primary purpose is to monitor water and ice clouds.
SO2 is removed by chemical reaction and conversion to sulphate aerosol, which, as with volcanic ash, is then deposited predominantly by gravitational forces. In many eruptions, the SO2 cloud is seen circling the Earth before it becomes thinned out.
The following sections provide a detailed overview on the spectral channel combinations used to detect volcanic plumes.