ORIGINAL PAPER Tall clouds from small eruptions: the sensitivity of eruption height and fine ash content to tropospheric instability Andrew Tupper Æ Christiane Textor Æ Michael Herzog Æ Hans-F. Graf Æ Michael S. Richards Received: 9 February 2008 / Accepted: 20 July 2009 / Published online: 2 September 2009 Ó Springer Science+Business Media B.V. 2009 Abstract A critical factor in successfully monitoring and forecasting volcanic ash dis- persion for aviation safety is the height reached by eruption clouds, which is affected by environmental factors, such as wind shear and atmospheric instability. Following earlier work using the Active Tracer High Resolution Atmospheric Model for strong Plinian eruptions, this study considered a range of eruption strengths in different atmospheres. The results suggest that relatively weak volcanic eruptions in the moist tropics can trigger deep convection that transports volcanic material to 15–20 km. For the same volcanic strength there can be *9 km difference between eruption heights in moist tropical and dry sub- polar environments (a larger height difference than previously suggested), which appears consistent with observations. These results suggest that eruption intensity should not be estimated from eruption height alone for tropospheric eruptions and also that the average height of volcanic eruptions may increase if the tropical atmospheric belt widens in a changing climate. Ash aggregation is promoted by hydrometeors (particularly liquid A. Tupper (&) Northern Territory Regional Office, Bureau of Meteorology, PO Box 40050, Casuarina, NT 0811, Australia e-mail: A.Tupper@bom.gov.au A. Tupper School of Mathematical Sciences, Monash University, Clayton, Vic, Australia C. Textor Deutsche IPCC-Koordinierungsstelle Projekttra ¨ger im DLR Umwelt, Kultur, Nachhaltigkeit Heinrich- Konen-Str.153227, Bonn, Germany M. Herzog Á H.-F. Graf Centre Atmospheric Sciences, University of Cambridge, Cambridge, UK M. S. Richards Cooperative Institute for Meteorological Satellite Studies, University of Wisconsin, Madison, WI, USA Present Address: M. S. Richards Federal Aviation Administration, Atlantic City, OK, USA 123 Nat Hazards (2009) 51:375–401 DOI 10.1007/s11069-009-9433-9