Saturday, November 12, 2005

Racing through the vortex

I've twice sailed the seas around the Canary Islands and both times thoroughly enjoyed it. Its warm without being hot and with good winds - most of the time. The first time I sailed there was a week on one of the Challege 67 boats pottering between the islands, while the second was the start of the Atlantic Rally for Cruisers, or ARC, heading out across an ocean to reach St Lucia 3 weeks later.

But for ocean races it's often just an annoying obstacle. The Transat Jacques Vabre fleet had to pass these islands on its way to Brazil recently and yet again some of them were caught out unawares. For though the wind can blow well it can also disappear, and from sea level it's sometimes hard to predict where and why.

But from space the reason is clear as can be seen in the satellite photo below.

This picture shows clearly what are called von Karman vortices. These naturally form when a fluid flow is disturbed by an object, causing vortices to form alternately on either side down stream. In this case the object is the volcanic peaks of the Canary Islands and the flow is the trade winds from the North East. The result is disturbed air, especially to the South East.

More on this effect can be found here.

From watching fleets from the Volvo to the Global Challange the message is often quite simple: keep clear of islands with high mountains. Or see your position drop down the vortex.

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