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Helpful Weather Information

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The Beaufort Wind Scale

Force

Description

Wind Speed kmh

Wind Speed knot

Wind Speed mph

Land Signs

Sea State

0

Calm

0 – 1.4

0 – 0.8

0 – 0.9

Smoke rises vertically Mirror Smooth

1

Light Air

1.5 – 5

0.9 – 2

1 – 3

Smoke Drifts Scaly Ripple

2

Light Breeze

5 – 11

2 – 5

4 – 7

Leaves rustle, wind vane moves Small wavelets, crests do not break

3

Gentle Breeze

12 – 20

6 – 9

8 – 12

Light flag will wave, small twigs and leaves move constantly Large wavelets, crests may break

4

Moderate Breeze

21 – 29

10 – 14

13 – 18

All flags extended, small branches move, dust and paper blow about Small waves some white horses

5

Fresh Breeze

30 – 39

15 – 20

19 – 24

Small trees begin to sway Moderate waves, many white horses

6

Strong Breeze

40 – 50

21 – 26

25 – 31

Large branches move Large waves with foam crests

7

Moderate Gale

51 – 61

27 – 32

32 – 38

Walking into wind difficult, whole trees move White foam from breaking waves blown in streaks

8

Fresh Gale

62 – 74

33 – 40

39 – 46

Twigs break from trees, walking difficult High long waves Spindrift

9

Strong Gale

75 – 87

41 – 47

47 – 54

Root and fence damage High waves, dense streaks of foam

10

Fierce Gale

88 – 101

48 – 54

55 – 63

Trees uprooted, structural damage Sea white, violent waves

11

Storm

102 – 121

55 – 65

64 – 75

May cause widspread damage, rare inland  

12

Hurricane

122+

66+

76+

Causes devastation.  

 

National Weather Service Terms

The weather service issues marine weather warnings of the following form, based on observed or forecast winds:

Small Craft Advisory

 

Small Craft Warning

18 – 34 (knots)

… and/or seas greater than 7 feet.

Small Craft Advisories and Warnings may also be issued for hazardous sea conditions or lower wind speeds that may affect small craft operations.

The Warning is more serious than the Advisory. Issued up to 12 hours ahead of conditions.

Note: there is no legal definition of the term “small craft” 

Gale Warning

34 – 47

Associated with a non-tropical storm. Issued up to 24 hours ahead of conditions.

Storm Warning

47+

Associated with a non-tropical storm. Issued up to 24 hours ahead of conditions.

Tropical Storm Warning

34 – 63

Associated with a tropical storm. Issued up to 24 hours ahead of conditions.

Hurricane Warning  

64+

Associated with a hurricane or tropical storm. Issued up to 24 hours ahead of conditions.

Special Marine Warning

34+

Associated with a squall or thunderstorm and expected to last for 2 hours or less. Issued up to 24 hours ahead of conditions.

  Comparing the Small Craft Advisory with the Beaufort Scale ( Force Level 5 ) we can expect seas in the 6-8 foot range. Such predictions are far from perfect, however ! To this day the scale is still useful in weather reporting, as it allows an experienced weather observer to report conditions as one of thirteen Force Levels, without the need of instruments or measurements. There is also a Beaufort Scale for use on land, which speaks of smoke, trees, and flags instead of waves, foam, and spray.

 

 

Beaufort Scale of Winds and Sea Conditions

Force Level  

Wind Speed (knots)

Wave Height (feet)

Sea State Description  

0

< 1

0

Sea like a mirror

1

1 – 3

.25

Ripples with appearance of scales, no foam crests

2

4 – 6

.5 – 1

Small wavelets, crests of glassy appearance, not breaking

3

7 – 10

2 – 3

Large wavelets crests begin to break, scattered whitecaps

4

11 – 16

3.5 – 5

Small waves becoming longer, numerous whitecaps.

5

17 – 21

6 – 8

Moderate waves taking longer form, many whitecaps, some spray

6

22 – 27

9.5 – 13

Larger waves forming, whitecaps everywhere, more spray

7

28 – 33

13.5 – 19

Sea heaps up, white foam from breaking waves begins to be blowing streaks along direction of wind

8

34 – 40

18 – 25

Moderately high waves of greater length, edges of crests begin to break into spindrift, foam blown in well-marked streaks

9

41 – 47

23 – 32

High waves, sea begins to roll, dense streaks of foam along wind direction, spray may reduce visibility

10

48 – 55

29 – 41

Very high waves with overarching crests, sea takes white appearance as foam is blown in very dense streaks, rolling is heavy and shock-like, visibility is reduced

11

56 – 63

37 – 52

Exceptionally high waves, sea covered with white foam patches, visibility is still-more reduced

 

The Beaufort Scale describes a fully-developed sea. In the local area, winds from the west to north have a much lesser or often even an opposite effect, such that a west wind of 20 knots might result in a sea state of 3 rather than 6. Apparently, there are no nine-foot waves !
  • 0 to 10 knots: Smooth water with small waves, The boat will be easy to handle under full sail.
  • 11 to 16 knots: Moderate seas with some white caps. If the boat feels overpowered, consider reefing.
  • 17 to 21 knots: Lengthening waves with many white caps and some spray. The boat will be more difficult to manage and you should seriously consider reefing.
  • 22 to 27 knots: Large waves, many white caps and spray. The boat will need at least one reef in the mainsail and a smaller jib. These conditions require considerable sailing experience. Listen to radio weather for small craft advisories.
28 to 47 knots. Gale conditions. High waves with white caps and foam. This is a good time to Stay Ashore !!!