Chile’s Puyehue Volcano began to erupt on June 4 of this year, and despite being 10,000 km away from the eastern states, wreaked havoc on Australian and New Zealand air travel. How did the cloud of ash manage to travel so far? The answer is trade winds.

Trade winds are named for their use by trade ships in crossing the globe, and in the southern hemisphere the ones that were most commonly used were the ones blowing toward the west. To understand how trade winds work, we need to understand how air moves around the earth. At the equator air warms up and rises, creating a low pressure system. This air moves toward the poles, where it is cooled and sinks back down. Air moves from high to low pressure, and so cooled air from the poles is pulled back up toward the equator, creating a continuous cycle of air, known as a convection current. We discuss these sorts of currents in our Flight and Weather workshop.

In addition to this, the rotation of the Earth causes other air currents in a phenomenon known as the Coreolis Effect. (The Coriolis Effect is the same effect that is commonly blamed for the direction that water swirls in a sink or a toilet – however, it isn’t observed on such a tiny scale, rather operating only on really large scales.) This means that there is movement of air from west to east above 30 degrees latitude and east to west below it (as a point of reference Sydney is at 33 degrees). This is a general trend and does vary due to local differences on the surface such as temperature and land masses.

Tracking clearly showed the ash clouds being blown west to east around the south pole from June 5th to June 12th. So, the winds didn’t blow the ash cloud direct from South America, but rather it traveled tens of thousands of kilometres around the globe in the other direction before hitting Australia! There is no real limit to how far the particulate matter can travel before it disperses.

As of the 22nd of June, the Bureau of Meterology’s Volcanic Ash Advisory Centre was reporting that the ash cloud was beginning a second trip around the world, and in 1991 the ash cloud from another Chilean eruption circumnavigated the world twice. For a cloud like this to disperse, it needs to be interrupted by other winds, and we can see this happen to a small part of the as it hits the east coast of Australia, blowing the ash cloud back over Sydney.

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