What is the Douglas sea scale?

The Douglas sea scale is a World Meteorological Organization standard that grades the state of the sea surface from 0 to 9 based on significant wave height. It was devised by Captain Henry Percy Douglas in 1917 and is widely used in marine weather reporting.

What is significant wave height?

Significant wave height is the average height of the highest one-third of waves in a record. It approximates the wave height a trained observer would report by eye, which is why the Douglas scale is defined in terms of it.

How does the Douglas scale differ from Beaufort?

The Beaufort scale measures wind force, while the Douglas sea scale measures the resulting sea state by wave height. They often correlate, but sea state lags wind and also depends on fetch, duration and swell, so the two numbers are not interchangeable.

What does degree 9 phenomenal mean?

Degree 9, phenomenal, covers significant wave heights above 14 metres. These are extreme storm seas associated with hurricanes and severe ocean storms and are dangerous to all but the largest vessels.

Is there a separate swell scale?

Yes. Douglas defined two scales: one for wind-sea state by wave height, shown here, and a separate swell scale that grades swell by length and height. This tool covers the sea-state scale.

Douglas Sea State Reference

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Reading the sea by wave height

The Douglas sea scale assigns a single digit, 0 through 9, to the state of the sea surface based on significant wave height. It gives mariners and forecasters a compact, standardised way to describe conditions from a glassy calm to a phenomenal storm sea. This reference lists every degree with its wave-height band and a plain description, plus a lookup that classifies any height you enter.

How it works

Each degree corresponds to a band of significant wave height in metres:

0  Calm (glassy)     0 m
1  Calm (rippled)    0 – 0.10 m
2  Smooth            0.10 – 0.50 m
3  Slight            0.50 – 1.25 m
4  Moderate          1.25 – 2.50 m
5  Rough             2.50 – 4 m
6  Very rough        4 – 6 m
7  High              6 – 9 m
8  Very high         9 – 14 m
9  Phenomenal        over 14 m

The lookup takes a height and returns the band that contains it, using upper-inclusive boundaries so that, for example, exactly 2.5 m reads as degree 4 (moderate) rather than degree 5.

Tips and notes

The scale measures the sea, not the wind — pair it with Beaufort for context.
Significant wave height is the mean of the highest third of waves, not the max.
Individual rogue waves can be roughly twice the significant height.
A separate Douglas swell scale grades swell length and height independently.
Sea state lags wind; a rising wind can sit ahead of the matching degree.