On Saturday June 4th, a volcano in the Puyehue-Cordon Caulle chain of Chile erupted after lying dormant for more than 50 years, ejecting huge quantities of ash high into the atmosphere. The ash plume reached a height of more than 10km above the earth's surface, where it was captured by strong upper level winds, which are referred to in meteorology as 'the jetstream'. The jetstream can be imagined as a fast-flowing river that encircles the southern hemisphere. Once the ash plume gets high enough in the atmosphere, it can be caught up in the jetstream and quickly carried around the southern hemisphere from west to east. In this case, it only took a week for the ash to be transported from South America, past South Africa and then over southern Australia. The thickest part of the plume crossed New Zealand during Sunday, so there does look like being an improvement in conditions for air travel over Australia in coming days. Ash can pose a serious danger to jet engines, as was the case when all four seized up on British Airways Flight 9 over Indonesia in 1982. Examples such as this highlight why caution is used by the authorities.