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Definitions

Let’s start this lesson with the following definitions:

Sea
: created by the wind at that moment.
Swell: created by the wind that was blowing earlier.
Waves = sea + swell.
Below is a table with the translation in English, French and German.

NLZeegangDeining
ENSeaSwell
DUWindseeDunung
FRMerHoule

Douglas sea scale

 Height (m)Description
0smooth seaCalm (Glassy)
10-0.10Calm (rippled)
20.10-0.50Smooth
30.50-1.25Slight
41.25-2.50Moderate
52.50-4.00Rough
64.00-6.00Very rough
76.00-9.00High
89.00-14.00Very high
914.00+Phenomenal

Wave height predictions

At windy.com, for example, you can check wave forecasts.

Waves have the following characteristics:
– Wavelength
– Period
– Wave velocity
– Steadiness
– Wave height

Significant wave height is average of the highest 1/3rd.
Maximum wave height = H max = 1.8 x H signi.

Breakers, ground sea, surf

Long waves are not necessarily dangerous if they are long. Breakers, on the other hand, are more dangerous because they can capsize the ship. The occurrence of breakers can be explained by the following formula: Wavelength = speed x period. A shallow depth e.g. beach, sandbank inhibits waves and the wave speed thus decreases. The wave period will remain the same, so wavelength becomes shorter. Mass remains the same so wave gets higher and steeper, breakers occur. This happens at when the water becomes shallow (ground sea) and in front of the beach (surf). When there are ground seas in front of the harbour, the waves also contain a lot of sand. The waves are then also sandy.

Current against wind

Tidal current against or in a river greatly affects waves. Current against wind shortens wavelength and increases wave height. If current and wind come from the same direction, we have longer wavelength and lower wave height.

Cross Seas

When a depression passes over quickly, we get different wind directions in quick succession and therefore different wave patterns. Those wave patterns then get mixed up and cross seas form. Sailing is very difficult and unpleasant. A cross sea can also be caused by waves rebounding against a steep deep shore.

Calculate wave height

Wave height is determined by the following factors:
– Duration certain wind
– Wind speed
– Depth of water
– Wind path/fetch

The height depends on wind speed, the duration in which the wind blows, the wind path (fetch). From the graph below, you can get the wave height. The y-axis shows the wind speed in knots and the x-axis the wind path (Fetch). The curved lines show the wave height in feet.

wave height

Source: NOAA

More comprehensive graphs also include wave period and duration.

The chart above shows:

  • On the left axis, the wave height in metres.
  • On the right axis, the wind speed in metres per second.
  • On the horizontal axis the duration of the wind in hours.
  • The transverse lines show the wind path in kilometres.

Questions & Answers

Question 1: Where do cross seas often form?

a: For the warm front
b: In the warm front
c: Behind the cold front

Question 2: When do waves get longer?

a: In case of sudden shoal
b: When current and wind are coming from the same direction
c: In case of current against wind

Question 3: What decreases when a wave goes towards the beach?

a: The height
b: The speed
c: The period

Question 4: When are there higher waves on the Wadden Sea?

a: With south wind
b: At low tide
c: At high tide

Question 5: What is meant by swell in English?

a: Waves that started a day ago.
b: Breakers
c: Surf

Question 6: What is the wave height with a westerly wind force 7bft in IJmuiden?

a: 11 feet
b: 11 meters
c: 6 feet

Question 7: What is the wave height if the wind blows for 3 hours at a speed of 10 meters per second and a wind trajectory of 4 kilometres?

a: 0.5 meters
b: 1 meter
c: 5 feet

Question 8: What kind of waves can be expected after a cold front has just passed?

a: Swell from the north
b: Sea from the southeast
c: Swell from the southwest with sea from the northwest

Question 9: What is the maximum wave height at 20 knots of wind on the ocean?

a: 1.8 meters
b: 2.6 meters
c: 0.6 meters

Question 10: What is the maximum wave height at 5 meters per second and a wind trajectory of 15 kilometres?

a: 4 feet
b: 0.4 meters
c: 4 meters