Reading the sea: swell, period and a storm far away
1The lesson in plain words
New swell can arrive before its source storm. Log its direction, period and height; swell alone gives no storm distance or arrival time.
Swell runs ahead
Swell is waves that have left the wind that made them. Long swell travels a long way with little loss. So a new swell, from a direction unlike the local wind, can be the first sign of a distant storm. It comes from roughly where the storm was when it made those waves. Growing swell is a reason to check forecasts; it does not show that the storm is heading towards you.
Period
The period is the time between crests passing a point stationary relative to the water. If safe, stop making way through the water and drift clear of hazards. Time 11 crests at the same point on the hull: 10 intervals. Divide by 10. Boat motion through the waves changes this interval: it becomes an encounter period, not the period used below. Ten intervals in 140 seconds gives 14 seconds. Local wind waves often have periods under 8 seconds; distant swell often has periods of 12 seconds or more.
How fast it travels
In deep water, swell energy travels at a group speed of about 1.5 knots per second of period. A 14-second swell has a group speed of about 21 knots, roughly 500 nautical miles a day. Long periods arrive first. Over the next hours the period of a distant storm’s swell usually gets shorter, because shorter waves are slower and arrive later.
Worked example
A 14-second swell arrives from the south-west. Suppose its generating area was 800 nautical miles away when the waves formed. Its group speed is about 21 knots, so travel time is roughly 800 ÷ 21 ≈ 38 hours. That dates the waves, not the storm’s present position. Check the latest chart to see where the system is now.
Other sea signs
A cross sea, two wave trains from different directions, often follows a front or a sharp wind shift. Long swell breaking heavily on a coast, with little local wind, often arrives ahead of the weather. Watch more than the height: note its direction, period and height in your log every watch.
2Key data
- Period
- Seconds for 10 intervals ÷ 10 (time 11 crests)
- Deep-water swell group speed
- About 1.5 × period (s) knots
- 14-second swell
- Group speed about 21 knots, about 500 NM a day
- Distant-storm swell
- Often 12 s or more; local wind waves often under 8 s
- Spanish terms
- Mar de fondo (swell); mar de viento; periodo
3Key points
- 1
A new swell from an unusual direction can be the first sign of a distant storm.
- 2
With no way through the water, time 11 crests (10 intervals), then divide the elapsed seconds by 10.
- 3
Deep-water swell group speed is about 1.5 knots per second of period.
- 4
Long periods arrive first; the period of a distant storm's swell then shortens.
- 5
Log swell direction, period and height every watch.
4Frequent mistakes
Wrong: Measure the period by timing two crests once.
Right: With no way through the water, time 11 crests, giving 10 intervals, and divide by 10. One interval is too short to be accurate.
Wrong: Swell always comes from the same direction as the local wind.
Right: Swell comes from where it was made. It can arrive from a quite different direction, often ahead of the storm.
Wrong: The swell arriving now shows where the storm is now.
Right: Its direction points towards where the waves formed. Use the latest chart for the storm’s present position.
5Practice case & exam keys
Drifting without making way in the Atlantic, you notice a long swell from the north-west that was not there yesterday. You time 10 intervals between crests in 150 seconds. The local wind is light from the south-east. What do you work out and do?
Show the answer
The period is 150 ÷ 10 = 15 seconds. Its deep-water group speed is about 1.5 × 15 ≈ 23 knots. Its period alone gives no distance or storm track. Get the latest high seas forecast or chart to check possible sources to the north-west. Log direction, period and height each watch; use the latest forecast and observations to decide how to prepare.
Exam keys
- Swell can arrive before the storm, from the direction of its source.
- Period: while stopped or drifting, time 11 crests (10 intervals), divide by 10.
- Deep-water swell group speed ≈ 1.5 knots per second of period.
- Long periods arrive first; swell direction points towards where the waves formed.
Finish this unit
When you've read it, match the terms, then answer three questions. Get them all right to finish the unit; opening the page alone doesn't count.
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