Here be icebergs!

17th January 2003 at 00:00
This is Graham Dalton's first time tackling the Southern Ocean and his greatest concern is icebergs. "We've got ice charts, so we kind of know where the concentrations of ice are," he says. "But if you're by yourself and you hit a bit of ice or miss it, it's largely down to luck."

How do icebergs form?

Icebergs are born, or "calve", from the edge of the Antarctic ice-sheet as glaciers and ice-shelves break off into the sea. From there, the icebergs float away and drift with the currents until they are broken up by the ocean swell and melt.

What do they look like?

Millions of icebergs, known as the white wolves of the Southern Ocean, roam in packs or alone. An iceberg called B15 that broke off the Antarctic ice sheet in 2000 was as big as Jamaica - while some are no bigger than a shoebox. Most are somewhere between pure white to deep blue, although a few are bottle green due to algae.

How much of an iceberg is submerged?

Most icebergs lie 80 to 90 per cent beneath the water.

Why do icebergs float?

Because ice is less dense than salt water.

How long can icebergs last?

The largest bergs can last for decades and travel thousands of kilometres before melting.

Is it hard to spot an iceberg?

It depends on the size and the time of day. At night, radar registers only the big bergs. The smaller ones, known as "bergy bits", or the very small but deadly "growlers" (less than a metre high and 3 metres long) that lurk just below the surface are nearly impossible to spot and would cause severe damage to a speeding yacht.

How will Graham Dalton avoid icebergs?

To some extent, he will rely on satellite maps of sea-ice and the latest ice reports. But he will also have to use keen eyesight, looking for signs such as "ice-blink", a yellowish-white glare on the underside of clouds.

What would happen if he hit a berg?

It would be similar to hitting a dock - the bow would crumple and the mast would almost certainly come down. It is unlikely the boat would sink.

Are icebergs blown by the wind? No. With 80 per cent of their mass below the water, they are more likely to follow the ocean currents. Many large icebergs are more than 100 metres thick and are driven by currents beneath the surface of the sea. This means that an iceberg can move in apparently calm water.

Which icebergs are most dangerous to a yacht?

Mega-icebergs pose less of a threat to yachts than small ones. Large icebergs are relatively easy to spot on radar, but small ones can remain unnoticed both to the naked eye and to radar.

An activity on icebergs will be put on the Global Education Challenge website, www.education.hsbc.com, on February 15.

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