Marine weather forecast – Wind speed, direction and Sea state

by OldSailor on October 2, 2007

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The aim of this post is to make you aware how wind speed at sea decides the sea state. Generally, higher the wind speed, higher the sea state and rougher the sea.

Weather forecasts are extremely important for ships to ensure comfort and safety. Operators can route ships to avoid severe weather, reducing the risk of injury and damage.

The shipping forecast contains a number of unusual terms, used to convey specific meanings that are given below:
Gales

Gale warning Beaufort Wind speed (Knots)
Gale 8 34 – 40
Severe gale 9 41 – 47
Storm 10 48 – 55
Violent storm 11 56 – 63
Hurricane 12 64 – 71

Time periods

Imminent expected within 6 hours of time of issue
Soon expected within 6 to 12 hours of time of issue
Later expected more than 12 hours from time of issue

Speed of movement – pressure systems

Description Speed (Knots)
Slowly Moving at less than 15
Steadily Moving at 15 to 25
Quickly Moving at 25 to 35
Rapidly Moving at 35 to 45
Very Rapidly Moving at more than 45

Wind descriptions

Description Meaning
Direction Direction from which wind is blowing
Wind becoming cyclonic Rapid change in direction, usually associated with frontal system
Veering Wind direction changing in a clockwise direction – S to SW to W etc.
Backing Wind direction changing in an anti-clockwise direction – E to NE to N etc.

Wind direction

Abbreviation Wind direction Degrees
N North
NNE NorthNorthEast 22.5°
NE NorthEast 45°
ENE EastNorthEast 67.5°
E East 90°
ESE EastSouthEast 112.5°
SE SouthEast 135°
SSE SouthSouthEast 157.5°
S South 180°
SSW SouthSouthwest 202.5°
SW Southwest 225°
WSW WestSouthwest 247.5°
W West 270°
WNW WestNorthwest 292.5°
NW Northwest 315°
NNW NorthNorthwest 337.5°

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Visibility

Description Meaning
Fog Less than 1000m (approx 0.6 nautical miles)
Poor Between 1000 and 2 nautical miles
Moderate Between 2 and 5 nautical miles
Good More than 5 nautical miles

The Beaufort Sea State Scale (wind speed)

  • Sea State 0 (Under 1 Knot)
  • Sea State 1 (1 – 3 Knots)
  • Sea State 2 (4 – 6 Knots)
  • Sea State 3 (7 – 10 Knots)
  • Sea State 4 (11 – 16 Knots)
  • Sea State 5 (17 – 21 Knots)
  • Sea State 6 (22 – 27 Knots)
  • Sea State 7 (28 – 33 Knots)
  • Sea State 8 (34 – 40 Knots)
  • Sea State 9 (41 – 47 Knots)
  • Sea State 10 (48 – 55 Knots)
  • Sea State 11 (56 – 63 Knots)
  • Sea State 12 (64+ Knots)

Sea State 0:

Description: Calm

Conditions: Sea like a mirror.

Wind Speed: Under 1 knot

Wave Height: 0 feet

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Sea State 1:

Description: Light Air

Conditions: Ripples but without foam crests.

Wind Speed: 1 – 3 knots

Wave Height: 0.25 feet

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Sea State 2:

Description: Light Breeze

Conditions: Small wavelets. Crests do not break.

Wind Speed: 4 – 6 knots

Wave Height: 0.5 feet

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Sea State 3:

Description: Gentle Breeze

Conditions: Large wavelets. Perhaps scattered white caps.

Wind Speed: 7 – 10 knots

Wave Height: 2 feet

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Sea State 4:

Description: Moderate Breeze

Conditions: Small waves. Fairly frequent white caps.

Wind Speed: 11 – 16 knots

Wave Height: 4 feet

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Sea State 5:

Description: Fresh Breeze

Conditions: Moderate waves, many white caps.

Wind Speed: 17 – 21 knots

Wave Height: 6 feet

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Sea State 6:

Description: Strong Breeze

Conditions: Large waves begin to form; white foam crests, probably spray.

Wind Speed: 22 – 27 knots

Wave Height: 10 feet

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Sea State 7:

Description: Near Gale

Conditions: Sea heaps up and white foam blown in streaks along the direction of the wind. Wind Speed: 28 – 33 knots

Wave Height: 14 feet

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Sea State 8:

Description: Gale

Conditions: Moderately high waves, crests begin to break into spindrift.

Wind Speed: 34 – 40 knots

Wave Height: 18 feet

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Sea State 9:

Description: Strong Gale

Conditions: High waves. Dense foam along the direction of the wind. Crests of waves begin to roll over. Spray may affect visibility.

Wind Speed: 41 – 47 knots

Wave Height: 23 feet

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Sea State 10:

Description: Storm

Conditions: Very high waves with long overhanging crests. The surface of the sea takes a white appearance. The tumbling of the sea becomes heavy and shock like. Visibility affected.

Wind Speed: 48 – 55 knots

Wave Height: 29 feet

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Sea State 11:

Description: Violent Storm

Conditions: Exceptionally high waves. The sea is completely covered with long white patches of foam lying in the direction of the wind. Visibility affected.

Wind Speed: 56 – 63 knots

Wave Height: 37 feet

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Sea State 12:

Description: Hurricane

Conditions: The air is filled with foam and spray. Sea completely white with driving spray. Visibility very seriously affected.

Wind Speed: 64+ knots

Wave Height: 45 feet

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* Photographs from National Weather Service Observing Handbook No. 1, US National Weather Service.

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