Thermal disequilibrium at the top of volcanic clouds and its effect on estimates of the column height

Abstract

SATELLITE images of large volcanic explosions reveal that the tops of volcanic eruption columns are much colder than the surrounding atmosphere1. We propose that this effect occurs whenever a mixture of hot volcanic ash and entrained air ascends sufficiently high into a stably stratified atmosphere. Although the mixture is initially very hot, it expands and cools as the ambient pressure decreases. We show that cloud-top undercoolings in excess of 20°C may develop in clouds that penetrate the stratosphere; this is consistent with observations of the 4 April 1982 eruption of El Chichon1 and the 18 May 1980 eruption of Mount St Helens2. Furthermore, from our model results, we predict that for a given cloud-top temperature, variations in the initial temperature of 100–200°C may correspond to variations in the column height of 5–10 km. We deduce that the present practice of converting satellite-based measurements of the temperature at the top of volcanic eruption columns to estimates of the column height will produce rather inaccurate results and should therefore be discontinued.