The westerlies and high-latitude ocean weather
1The lesson in plain words
Poleward of the subtropical highs, depressions march east one after another. In the south, with almost no land in the way, they make the Roaring Forties.
The westerly belt
Between about 35° and 60° the prevailing wind is westerly, but rarely steady. Along the polar front, where cold polar air meets warm tropical air, depressions form and travel east, usually every few days. The wind changes direction and strength as each low and its fronts pass; the Pressure systems unit shows the sequence. Gales are far more frequent here than in the trades, especially in winter.
Northern hemisphere
In the North Atlantic most lows track north-east between Newfoundland and Iceland, past the British Isles and Norway. In winter they are deeper and further south; in summer they weaken and move north, while the Azores High expands.
Southern hemisphere
South of about 40° S there is almost no land to slow the wind or block the sea. The westerlies blow round the globe: the Roaring Forties, then the Furious Fifties and the Screaming Sixties. Lows follow each other quickly, with gales common in every season and a long, heavy swell from the west.
Cold-air hazards
At high latitude, cold air brings extra risks. Cold water can quickly incapacitate you, even when the air feels mild. Spray can freeze on deck and rigging: superstructure icing, which adds weight high up. Fog forms where warm air crosses cold water, as over the Grand Banks. Icebergs are covered later in this unit.
2Key data
- Westerly belt
- About 35°–60°; travelling depressions, gales common
- North Atlantic storm track
- North-east, Newfoundland – Iceland – Norway; further south in winter
- Roaring Forties
- 40°–50° S; Furious Fifties 50°–60° S
- Cold water
- Cold water can rapidly incapacitate; air temperature is no guide.
- Spanish terms
- Vientos del oeste, frente polar, cuarenta rugientes
3Key points
- 1
In the westerlies, depressions travel east along the polar front every few days.
- 2
Northern storm tracks move south and deepen in winter, north in summer.
- 3
South of 40° S nearly unbroken ocean gives the Roaring Forties and Furious Fifties.
- 4
High latitude adds cold water, icing, fog and ice to the weather risk.
4Frequent mistakes
Wrong: The westerlies blow as steadily as the trades.
Right: Travelling lows and fronts make winds shift and strengthen within the prevailing westerly circulation.
Wrong: The Roaring Forties blow only in the southern winter.
Right: Gales are common in every season there, with heavy swell from the west.
5Practice case & exam keys
You compare March and late May in the North Atlantic. How do the typical storm tracks, gale risk and Azores High differ?
Show the answer
In March, lows are generally deeper and track further south, bringing more frequent gales and heavy seas. By late May, the storm track is generally further north and the Azores High has expanded. Gales remain possible: these are seasonal tendencies, not a forecast.
Exam keys
- Westerlies: about 35°–60°; travelling depressions along the polar front bring variable winds.
- Northern storm tracks: deeper and further south in winter, weaker and further north in summer.
- Southern Ocean: Roaring Forties (40°–50° S), Furious Fifties (50°–60° S).
- High-latitude risks: cold water, superstructure icing, fog and ice.
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.
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