1.3 The propagation of volcanic ash clouds and gases
The propagation of volcanic ashes and gases is wind driven. Depending on vertical wind shear, ash and gases can be transported in different directions at different heights (Figure 8).
Figure 8: GOES-16 Volcanic Ash RGB of Mount Fuego eruption, 1 February 2018 07:00-23:00 UTC, © NOAA
Because volcanic ash particles are either washed out or removed by gravitational deposition, SO2 is often the last remaining atmospheric component that indicates a major volcanic eruption. SO2 propagation has particular significance when it comes to climate change. It can reach the stratosphere where it interacts with water vapor to form small droplets of sulphuric acid (H2SO4). These droplets reduce incoming solar radiation by absorbing and reflecting light, which results in surface cooling. Figure 9 shows an SO2 plume (bright green band) from the Kasatochi eruption (2008) that circled the entire northern hemisphere over more than 20 days.
Figure 9: Meteosat-8 Cloud Microphysics RGB of Kasatochi eruption, 21 August 2008, 12:00 UTC - 22 August 2008, 06:00 UTC. © EUMETSAT
Quiz
Compare the orientation of the visible part of the volcanic plume with wind direction at various heights.
Which wind levels fits best? Click on an image and go through the gallery to determine the answer. Pay attention to the caption below the image.
Cumbre Vieja eruption (28 September 2021 at 12:00 UTC).