Sea ice and icebergs: terms, dangers and handling
1The lesson in plain words
Sea ice forms from frozen seawater; icebergs break off glaciers. Learn the international terms and why small pieces are the most dangerous to a yacht.
Two kinds of ice
Sea ice is frozen seawater. Icebergs are pieces of glacier ice, of land origin, that have broken off, or calved, into the sea. Arctic sea ice usually reaches its maximum in March and minimum in September; Antarctic sea ice usually peaks in September and reaches its minimum in February. Most of an iceberg is under water, typically about nine-tenths of its mass, and it can extend far sideways below the surface.
Sea ice terms
The WMO sea-ice nomenclature is used in English in ice reports worldwide. Fast ice is sea ice attached to the shore. Pack ice is any sea ice other than fast ice, drifting with wind and current. An ice shelf is a floating sheet of glacier ice attached to the coast, ending in a high ice cliff. Bare ice has no snow on it. Ice concentration is given in tenths of the sea surface covered.
Iceberg sizes
An iceberg shows more than 5 m above the sea. A bergy bit shows about 1–5 m, with an area of about 100–300 m². A growler shows less than about 1 m, with an area of about 20 m²: often greenish or almost transparent, and hard to see.
Ice detection limits
Even large icebergs may give weak radar echoes. Small pieces are especially easy to miss in sea clutter, the echoes from waves. They float low, so you may see them only when close. A rising sea temperature or a falling one is no reliable warning. Keep a sharp lookout, slow down in ice areas at night or in fog, and give icebergs a wide berth.
Passing an iceberg
Pass to windward, the side facing the wind: wind and sea drive the bergy bits and growlers that break off it downwind. Keep well clear: underwater spurs reach out from the berg, and a melting berg can capsize or calve without warning. A yacht should not enter pack ice.
2Key data
- Iceberg
- More than 5 m above the sea
- Bergy bit
- About 1–5 m above the sea; about 100–300 m²
- Growler
- Less than about 1 m above the sea; about 20 m²
- Fast ice / pack ice
- Attached to the shore / any other sea ice, drifting
- Arctic sea ice
- Most in March, least in September
- Spanish terms
- Iceberg, témpano, banquisa (pack ice), hielo fijo (fast ice)
3Key points
- 1
Sea ice is frozen seawater; icebergs are glacier ice that has calved into the sea.
- 2
Iceberg over 5 m high; bergy bit about 1–5 m; growler under about 1 m.
- 3
Growlers and bergy bits are hard to see and can hide in radar sea clutter.
- 4
Pass icebergs to windward and give them a wide berth.
- 5
Sea temperature is no reliable warning of nearby ice.
4Frequent mistakes
Wrong: Radar will always show an iceberg in time.
Right: Even a large berg may give a weak echo. Small pieces can hide in sea clutter. Keep a lookout and reduce speed.
Wrong: Pass an iceberg to leeward, on its downwind side.
Right: Pass to windward. Broken-off growlers and bergy bits drift downwind of the berg.
Wrong: Fast ice is sea ice moving fast with the current.
Right: Fast ice is fastened to the shore. Drifting sea ice is pack ice.
5Practice case & exam keys
Crossing south of Newfoundland in June, at night, in a moderate sea with patchy fog, the radar shows a large iceberg 4 miles ahead, fine on the bow. The wind is from the south-west. How do you pass it?
Show the answer
Alter early to pass it on its windward, south-west side, with a wide berth. Reduce speed: growlers and bergy bits from the berg may lie downwind of it, and they can hide in the radar clutter. Post an extra lookout forward, use the searchlight if needed and report the berg's position to the International Ice Patrol.
Exam keys
- Iceberg more than 5 m high; bergy bit about 1–5 m; growler less than about 1 m.
- Fast ice is attached to the shore; pack ice is any other sea ice; an ice shelf is floating glacier ice on the coast.
- Pass icebergs to windward with a wide berth; growlers may not show on radar.
- About nine-tenths of an iceberg's mass is under water.
Finish this unit
When you've read it, match the terms, then answer three questions. Get them all right to finish the unit; opening the page alone doesn't count.
My notes for this unit
No notes