Douglas sea scale

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The Douglas sea scale is a scale which measures the height of the waves and also measures the swell of the sea. The scale is very simple to follow and is expressed in one of 10 degrees.

Contents

The scale

The Douglas sea scale, also called the "international sea and swell scale", was devised in 1921 [1] by Captain H. P. Douglas, who later became vice admiral Sir Percy Douglas and hydrographer of the Royal Navy. Its purpose is to estimate the roughness of the sea for navigation. The scale has two codes: one code is for estimating the sea state, the other code is for describing the swell of the sea. [2]

State of the sea (wind sea)

DegreeHeight (m)Height (ft)Description [3]
0no waveCalm (Glassy)
10–0.100.00–0.33Calm (rippled)
20.10–0.500.33–1.64Smooth
30.50–1.251.6–4.1Slight
41.25–2.504.1–8.2Moderate
52.50–4.008.2–13.1Rough
64.00–6.0013.1–19.7Very rough
76.00–9.0019.7–29.5High
89.00–14.0029.5–45.9Very high
9>14.00>45.9Phenomenal

The Degree (D) value has an almost linear dependence on the square root of the average wave Height (H) above, i.e., . Using linear regression on the table above, the coefficients can be calculated for the low Height values () and for the high Height values (). Then the Degree can be approximated as the average between the low and high estimations, i.e.:where [.] is the optional rounding to the closest integer value. Without the rounding to integer, the root mean square error of this approximation is: .

Swell

DegreesDescription
0No swell
1Very Low (short or average and low wave)
2Low (long and low wave)
3Light (short and moderate wave)
4Moderate (average and moderate wave)
5Moderate rough (long and moderate wave)
6Rough (short and high wave)
7High (average and high wave)
8Very high (long and high wave)
9Confused (wavelength and height indefinable)

Wave length and height classification

Wavelength

Wave height

See also

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References

  1. Owens, Edward H. (September 21, 1984). Schwartz, M. (ed.). Beaches and Coastal Geology. Encyclopedia of Earth Sciences Series. Springer US. p. 722. doi:10.1007/0-387-30843-1_397. ISBN   978-0-87933-213-6 via Springer Link.
  2. Met Office, UK Fact Sheet 6, accessed 10 September 2020.
  3. "EuroWEATHER - Douglas scale". www.eurometeo.com.