Wave height statistics

[baadbee]

So I just read a thread about "will the waves be big for my cruise to XXX" for the umpteenth time. The answers, as always, were that waves are unpredictable and you should be ready for anything. It left me thinking... are there statistical models for wave height per month (per something...) that are useful for evaluating sea conditions for cruising (or any shipping)? I'm hoping one of the expert retired mariners on the forum could enlighten me. Google searching reveals only very specific studies that don't seem useful for prediction by a traveler.

 

It's easy enough to find detailed weather info for anywhere in the world. I can't know for sure if it will be warm somewhere next Feb but at least I can know whether it's likely to be warm (and how warm). Are sea conditions so highly variable that no meaningful statistics can be provided? Are there any interactive maps that can be used to compare typical sea conditions between areas at specific times of year?

 

Just wondering...

 

Chris

[navybankerteacher]

Sea conditions are less predictable than tempertures or even general weather patterns. There are certain areas where, given the right conditions, heavy seas might be expected: the Bay of Biscay, for example, due to its shape and shoaling seabed will likely give you the highest seas of any part of the Atlantic —— IF there has been strong steady winds from the northwest. In a period of variable winds from the south or east, it is likely to be as flat as anywhere. There is a patch of the Mediterranean south of Marseilles which can be very rough with a wind out of the north funneling down the Rhône valley - other times it will be as calm as the Bay of Naples following a windless week.

 

You get what you get - there are seasons: that half of the year when hurricanes MIGHT impact the Caribbean, the Gulf and East Coasts - but some years are active and others not. Around the Spring equinox Hatteras can get very nasty - but mostly not.

 

Sea conditions are so variable that the sort of meaningful statistics you seem to want are not really available.

[CaptnG]

Sea conditions are less predictable than tempertures or even general weather patterns. There are certain areas where, given the right conditions, heavy seas might be expected: the Bay of Biscay, for example, due to its shape and shoaling seabed will likely give you the highest seas of any part of the Atlantic —— IF there has been strong steady winds from the northwest. In a period of variable winds from the south or east, it is likely to be as flat as anywhere. There is a patch of the Mediterranean south of Marseilles which can be very rough with a wind out of the north funneling down the Rhône valley - other times it will be as calm as the Bay of Naples following a windless week.

 

 

 

You get what you get - there are seasons: that half of the year when hurricanes MIGHT impact the Caribbean, the Gulf and East Coasts - but some years are active and others not. Around the Spring equinox Hatteras can get very nasty - but mostly not.

 

 

 

Sea conditions are so variable that the sort of meaningful statistics you seem to want are not really available.

 

 

 

I might be just putting my foot in my mouth here, cause you seem to have a lot of knowledge on the subject.. but as I was reading I was thinking how if you combined the information you shared, like what wind speeds and direction are most likely to cause rough seas with, for example..the historical weather forecast for NW winds near the bay of biscay, or when north winds off the Rhône valley are more frequent, perhaps with enough data you might be able to create something like what the OP was asking for. Like any distant weather forecast it would only provide averages, but come on we’re on CC, clearly we’ll take all the information we can get. [emoji12]Sounds like a really interesting idea to me.

 

 

 

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[Kamloops50]

So I just read a thread about "will the waves be big for my cruise to XXX" for the umpteenth time. The answers, as always, were that waves are unpredictable and you should be ready for anything. It left me thinking... are there statistical models for wave height per month (per something...) that are useful for evaluating sea conditions for cruising (or any shipping)? I'm hoping one of the expert retired mariners on the forum could enlighten me. Google searching reveals only very specific studies that don't seem useful for prediction by a traveler.

 

It's easy enough to find detailed weather info for anywhere in the world. I can't know for sure if it will be warm somewhere next Feb but at least I can know whether it's likely to be warm (and how warm). Are sea conditions so highly variable that no meaningful statistics can be provided? Are there any interactive maps that can be used to compare typical sea conditions between areas at specific times of year?

 

 

Just wondering...

 

Chris[/quote

 

In the ocean there are two sets of waves. The overall wave heights depend on how these waves mix at any given moment,

[Heidi13]

Back in my working days we had access to weather routing services and meteorology forecasts & predictions. Part of these services included sea height predictions.

 

At my home terminal south of Vancouver, we also used a surf boarding website, which had predictions frequently more accurate than Environment Canada.

 

In retirement, I am not concerned with wave heights when cruising, as this is no longer my responsibility and am very happy leaving it to others. Therefore, I have never actually searched for this info in the public domain. So apologies, but I can't help with this one other than checking forecasts for where you plan to visit, or try a variety of Google Searches.

[Korimako]

You might find some info on this website

 

 

 

https://www.windy.com/

[chengkp75]

There are weather buoys in places around the world, that collect weather and sea state data, NOAA is probably the largest user of this data, and their website will have some sea condition predictions on it. The weather buoys are reinforced by the voluntary observing ship (VOS) program, where the ships report local weather and sea state conditions several times a day.

[MicCanberra]

Here in Australia we have this but it really only does a week in advance.

http://www.bom.gov.au/australia/charts/viewer/index.shtml?domain=combinedW&type=sigWaveHgt

 

I am sure other countries would likely have something similar

[MicCanberra]

BTW, a metre is around 3 feet.

[Suzanne123]

I use this one for wave heights

http://www.oceanweather.com/data/

[MicCanberra]

I suppose the forecasts are there it is getting the archived wave heights as an average over history for use as a forecast..

[baadbee]

I use this one for wave heights

http://www.oceanweather.com/data/

 

Thanks for that link, fun to click around on that and see where things are heavy (seems like things are quiet in much of the oceans this week).

[baadbee]

Thanks chengkp75, heidi13, navybankerteacher and all others. It's always made me curious that the sea condition questions aren't answered with hard data where general weather questions are (can be). I guess the problem is that you can't meaningfully summarize highly chaotic data...

[Heidi13]

I only had time for a quick response this morning, so this is a fairly simplified explanation why it is difficult to provide average wave height that provide reasonable accuracy.

 

You will hear many people discussing waves & swell and many believe they are actually synonymous. This could not be further from the actual truth, however current wave height are a combination of both.

 

Wind waves are generated at the vessel's current location and the direction is consistent with the wind direction. The simplified explanation of wind wave height depends on 3 primary factors - wind speed (more wind = higher waves), wind duration (length of time wind direction is constant) and the fetch (uninterrupted distance the wind blows over the ocean). This is the reason in coastal waters and rivers you find wind waves are much smaller compared to ocean crossings.

 

Wind waves are closer together than swells.

 

Swells are created by distant storm systems that can be many hundreds, even thousand miles away. The wind waves created at the storm centre gain significant energy and travel vast distances over open oceans. Swells tend to be longer and lower.

 

All assumes open ocean transits, as shallow water significantly affects wave/swell heights.

 

Hopefully, this fairly simple explanation provides an idea how accurate wave height predictions are really only possible days or a few weeks in advance.

[MicCanberra]

Beware the perfect storm. :eek:

[CaptnG]

I only had time for a quick response this morning, so this is a fairly simplified explanation why it is difficult to provide average wave height that provide reasonable accuracy.

 

 

 

You will hear many people discussing waves & swell and many believe they are actually synonymous. This could not be further from the actual truth, however current wave height are a combination of both.

 

 

 

Wind waves are generated at the vessel's current location and the direction is consistent with the wind direction. The simplified explanation of wind wave height depends on 3 primary factors - wind speed (more wind = higher waves), wind duration (length of time wind direction is constant) and the fetch (uninterrupted distance the wind blows over the ocean). This is the reason in coastal waters and rivers you find wind waves are much smaller compared to ocean crossings.

 

 

 

Wind waves are closer together than swells.

 

 

 

Swells are created by distant storm systems that can be many hundreds, even thousand miles away. The wind waves created at the storm centre gain significant energy and travel vast distances over open oceans. Swells tend to be longer and lower.

 

 

 

All assumes open ocean transits, as shallow water significantly affects wave/swell heights.

 

 

 

Hopefully, this fairly simple explanation provides an idea how accurate wave height predictions are really only possible days or a few weeks in advance.

 

 

 

Great explanation! Thanks for providing so much insight.

 

 

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[GUT2407]

Sorry ohijack the thread, but it seems there are some here who know what they are talking about.

 

So

 

Tides

 

When they say high tide is at 10:20 and 1 metre (3 foot roughly)

 

What are they measuring and where?

 

Why is there never a negative tide?

[MicCanberra]

High tides and low tides are the effects of the moon's gravitational pull on our oceans. As the moon orbits the earth, it pulls the ocean with its gravity. Giving the result of high tide being closest to the moon and low tide to be on the opposite side of the Earth.

 

Spring tide: A tide with the Greatest different between high and low tide that occurs when the sun and the moon are aligned with Earth at the new moon and full moon.

 

Neap tide: A tide with the least different between low and high tide that occurs when the sun and moon pull at right angles to each other at the first and third quarters of the moon.

 

Why do we have tides

First, recall Kepler's Third Law of Orbital Motion, or else work out the orbital speed of an object in a circular orbit by setting centripetal force equal to gravitational force and solving for velocity. Either way, you discover the that velocity of a small object in orbit has to be faster in close to the object it is orbiting, and slower further away. So now think about a large object (i.e. one with an appreciable diameter) in a circular orbit. The whole thing moves at orbital speed for the distance that its centre of mass is at. But some parts of it are appreciably closer to the central object, and some are appreciably further away. That means that the bits that are closer are moving slower than circular-orbit speed for the distance they are at. Gravity is stronger than what would keep them in the orbit they are in and tends to curve their path more, so they tend to fall towards the centre as though they were at the apapsis (high point) of a slower, elliptical orbit. On the other side you find bits of the large body that are further away from the centre than the centre of mass, moving slightly faster than circular-orbit speed at their actual distance. For them, gravity is slightly weaker than what would give them a circular path at their current distance and speed and tends to curve their path less. They tend to rise away from the centre as though at the periapsis (low point) of a faster, elliptical orbit. The result is that one bulge of material rises towards the body that this body is orbiting, on the face towards it, and another rises on the opposite face, away from the body that the body is orbiting. These are the two tidal bulges. The rise to the point where either elastic forces in the orbiting body or the gravitational disequilibrium of its distorted surface provide an equal countervailing force. It doesn't matter if the orbit is not circular: any extended object moving through a gravitational field feels gravity more strongly on the side nearer to the gravitating body than at its centre of mass, and less strongly at the side further from the gravitating body, so its near parts tend to curve more towards the gravitating body, and its further parts less, than its trajectory producing tidal strain . Now, Earth can be considered to be orbiting in the Sun's gravity, and also in the Moon's gravity (never mind the the centre of that orbit is not at the Moon, it doesn't matter). So Earth tends to get a tidal bulge pointing towards the Moon and one pointing away from the Moon, and a smaller one (1/3 the height) pointing towards the Sun and another one pointing away from the Sun. These get displaced from their positions by Earth's rotation, but that's a bit of a tangent. The important thing is that both the ocean and the solid material of the Earth feel the tidal strain, but the oceans respond to it much faster. If Earth were non-rotating the material of the mantle would conform to the tidal bulges, but at Earth's current rotation rate it doesn't get time to. So the tides in the ground are smaller than the tides in the ocean, and not quite in phase with them. Any particular point on the Earth rotates into and back out of the tidal bulges. and the ocean raises and falls, but the land doesn't get time to rise or fall as much. Thus the oceans tend to rise and fall with respect to the land. The atmosphere does too, as you can see if you look at a graph of frequent measurements of the atmospheric pressure. Now, it turns out that the range of the lunar tide on Earth is only about four feet, and the solar tide is a third of that. What gives us the much larger tidal ranges that we actually see in a lot of places is that the solar and lunar tides act as a periodical driver, pushing and pulling water on a regular schedule. In places where the period of these tides corresponds to the natural resonant frequency for waves in a "basin" constrained by the landform, a sort of tidal slosh builds up by resonance. And that gives you the high tides and low tides.

[GUT2407]

Strangely I understand all that (well most of it) what I don’t get is what is one metre, or more acurately where is zero. And why doesn’t it ever go below that.

[chengkp75]

Strangely I understand all that (well most of it) what I don’t get is what is one metre, or more acurately where is zero. And why doesn’t it ever go below that.

 

Tide reference tables use the "mean low water" as the zero point. "Mean low water" is the "mean", or simplistically the average, of the lowest tidal levels ever recorded over the "tidal epoch" (the time that tide data has been recorded). Since this "zero point" is a mean figure, there are low water levels on some days above this number, and some below it, but generally the variations are small, especially when a tidal station has a very long recording era. Negative tides do occur, they are considered to be "lowest astronomical tide", and are the portion of low tides that fall below the mean or average low water.

 

So, the tide chart shows the difference between actual tide level and "mean low water".

[GUT2407]

Tide reference tables use the "mean low water" as the zero point. "Mean low water" is the "mean", or simplistically the average, of the lowest tidal levels ever recorded over the "tidal epoch" (the time that tide data has been recorded). Since this "zero point" is a mean figure, there are low water levels on some days above this number, and some below it, but generally the variations are small, especially when a tidal station has a very long recording era. Negative tides do occur, they are considered to be "lowest astronomical tide", and are the portion of low tides that fall below the mean or average low water.

 

So, the tide chart shows the difference between actual tide level and "mean low water".

Thank you that is more or less what I thought just never heard a low tide of minus x. So wasn’t 109% sure.

[MicCanberra]

Wave Anatomy:

1. Still-Water Line - The level of the sea surface if it were perfectly calm and flat.
   
2. Crest - The highest point on the wave above the still-water line.
   
3. Trough - The lowest point on the wave below the still-water line.
   
4. Wave Height - The vertical distance between crest and trough.
   

[MicCanberra]

http://www.bom.gov.au/marine/about/combinedseaswell.shtml

Combined sea and swell height = [(Wind Wave Height)2 + (Swell Wave Height)2+ (Secondary Swell Wave Height)2]1/2

[Heidi13]

Tides and chart datums is a subject that we spent way too many hours studying when I was a lowly cadet.

 

The Cole's notes explanation is that the Earth's oceans are affected by the gravitational pull resulting from the distances and positions of the Sun and Moon in relation to the Earth. With respect to the Sun & Moon, the Moon has a greater affect on tides. An additional factor to consider in the equation is inertia, since the earth is spinning.

 

Combining the gravitational pull from the Moon and inertia causes two tidal bulges on Earth - 1 towards the Moon and a second one at the furthest point from the Moon. While inertia tries to keep the water in place, the Moon's gravitation pull exceeds inertia resulting in a tidal bulge towards the Moon. On the opposite side of Earth the Moon's gravitational pull is minimal, so inertia exceeds gravitational pull, resulting in another tidal bulge away from the Moon.

 

Spring Tides - these occur at New & Full Moon, when the Sun, Moon & Sun are aligned. These have higher tidal ranges

 

Neap Tides - also occur twice per month, when Sun & Moon are perpendicular. These have lower tidal ranges.

 

Why are the next day's tides a little later? The Moon rotates around the Earth in the same direction as the Earth rotates and takes a little longer than 24 hrs to return to same position - from memory this was just less than 25 hrs, about 24hrs 50 mins. Therefore, the time difference between subsequent high tides is 1/2 a lunar day, or just under 12.5 hrs.

 

Tidal Height - as mariners we require our charts to have a known standard for posted depths, which is known as Chart Datum. Unfortunately, this standard is not consistent throughout the World.

 

- The UKHO, which produces charts world wide uses LAT (Lowest Astronomical Tide) - which is the lowest tidal prediction under astronomical conditions that are considered normal. This system provides a benefit in that all tidal calculations are added to chart datum.

 

- The USHO (NOAA) uses MLLW (Mean Lower Low Water) - which is an average of the lowest tides. Being an average, or mean provides opportunities for negative tides.

 

- The Canadian HO uses LNT, which is the Lowest Normal Tide, which is closer to the UKHO standard. However, I have seen negative tides on rare occassions.

 

Provided the tide tables are consistent with the chart supplier, no challenges are experienced.