Douglas scale: what it is, what it is for and table of sea states

The Douglas scale is a system used to measure the strength of the wind visually, based on the height and state of the waves at sea. It was created by British Vice Admiral Henry Percy Douglas in the 1920s, with the aim of unifying the criteria that sailors used to describe the state of the sea and wind conditions.

Unlike other scales, such as the Beaufort scale (focused on the intensity of the wind), the Douglas scale focuses on how the sea reacts to the wind. That is, it does not directly measure the speed of the wind, but rather the appearance of the sea and the height of the waves it generates.

This scale is made up of two complementary parts:

• Sea state scale: evaluates the local waves produced by the wind blowing at that time.
• Swell Scale: Describes waves formed by distant winds that reach an area where the wind may no longer blow.

The Douglas Scale serves to describe and classify the state of the sea in a standardized and understandable way for anyone who works or sails in the ocean. Its main function is to facilitate communication between sailors, meteorologists, fishermen and sailors, avoiding confusion or subjective interpretations about how the sea is.

In practice, this scale allows:

• Evaluate sailing conditions: Knowing whether the sea is calm, rough or dangerous waves helps you decide whether it is safe to set sail or not.
• Interpret weather reports: maritime weather reports usually include the state of the sea according to the Douglas scale, which makes it possible to predict the behavior of waves in a given area.
• Plan maritime operations: from fishing activities to rescues or transportation of goods, knowing the state of the sea is essential to guarantee safety.
• Unify international criteria: thanks to its widespread use, anyone in the maritime field can understand sea conditions regardless of language or country.

The Douglas scale works as a numerical system that classifies the state of the sea according to the height and type of waves observed. It is divided into ten degrees, ranging from calm sea to completely wooded sea, that is, with huge waves and widespread foam.

Each degree of the scale corresponds to an approximate range of wave heights and a visual description of the sea. In this way, a person can estimate the state of the sea simply by observing its surface, without the need for sophisticated instruments. In the next section, we are going to analyze all the degrees one by one with a table.

Furthermore, the Douglas scale distinguishes between two types of sea:

• Sea of ​​wind: generated by the local wind blowing at that time.
• Swell: formed by waves that originated in another area and travel long distances.

Both aspects are evaluated separately, allowing a more complete description of the maritime conditions.

As we have mentioned before, the Douglas scale is divided into ten levels or degrees, which describe the state of the sea according to the average height of the waves and their general appearance. Each number represents a category ranging from completely calm seas to extreme conditions, with gigantic waves that are dangerous, even for the largest boats.

Let’s see what each level means:

• 0 – Calm: the sea is completely smooth, without waves or foam. It is what is known as the “mirror sea”, ideal for navigation.
• 1 – Rippled sea: small ripples appear on the surface of the water, less than 10 centimeters. No defined waves are forming yet.
• 2 – Swell: the wind begins to influence more strongly, generating small waves that reach up to half a meter. You can see ridges that are just beginning to break.
• 3 – Swell: the waves are more noticeable, between 0.5 and 1.25 meters high. There are already breakers and the movement of the sea is felt more.
• 4 – Strong swell: the waves reach 2.5 meters and white foam is observed on the crests. Navigation becomes uncomfortable for small boats.
• 5 – Thick sea: waves reach between 2.5 and 4 meters. The sea is beginning to be dangerous for medium-sized boats and there is strong agitation.
• 6 – Very thick sea: with heights of 4 to 6 meters, the sea has an imposing appearance. Sailing in these conditions requires experience and caution.
• 7 – Wooded: the waves reach 6–9 meters, with large amounts of foam and a continuous swell. It is a difficult sea even for large boats.
• 8 – Mountainous sea: waves between 9 and 14 meters that rise like mountains. The sea becomes extremely dangerous.
• 9 – Huge sea: more than 14 meters high. It is the maximum level of the scale and describes exceptional conditions, typical of severe storms or hurricanes.

Although the Douglas scale and the Beaufort scale are related, they do not measure exactly the same thing. Both are used in meteorology and navigation, but they evaluate different aspects of the same phenomenon: wind and sea.

• Beaufort Scale: It was created in the 19th century by the British admiral Francis Beaufort and is used to measure the strength of the wind based on its visible effects both at sea and on land. It is made up of 13 levels (from 0 to 12) and describes everything from a complete calm to a hurricane.
• Douglas Scale: focuses on the state of the sea, that is, how the water reacts to the wind. It does not measure the speed of the wind, but the height and appearance of the waves, classifying them into ten degrees (from 0 to 9).

If you want to read more articles similar to Douglas Scale: what it is, what it is for and table of sea states, we recommend that you enter our Meteorological Phenomena category.