Category Archives: Weather

Sea and land conditions, environmental factors etc

Understanding Rough Water: Force Three

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When writing the article on how to swim in rough water, I acknowledged that it’s an area I need to do a bit more about. And the popularity of the post caught me a bit by surprise, as it always does when such happens. But I wasn’t sure what I could do next. After all while I love writing about cold swimming, and it continues to provide me new avenues of investigation and expression, rough water is just rough water.

On last Saturday’s visit to the Guillamene, a swim I didn’t want to do, because two days previously I’d had a bad asthma attack while pool training, (for the first time in two years), and I didn’t feel I was recovered. Also, though the start of the fourth week of May, the water temperatures were still down, only nine degrees the previous weekend, worse the air temperature was low, only ten degrees, with a north-easterly wind. When you live at fifty-four degrees latitude, northerly winds are always cold. It felt just like winter and when I measured the temperature at the Guillamenes, it was on a par with the air temperature, ten degrees, yes a rise over the previous week, but with the chilly wind and the lowering dull grey sky and no-one around, it was less than inviting.

And as I said, the wind was north-easterly. As this point it’s useful to show you (again) how the Guillamene is situated.

Tramore Bay

The bay faces south-west (directly up is North in this image), the Guillamenes is on the west wide, so looking out from the platform you are facing south-east. It’s sheltered from the prevailing south-westerlies, but exposed to south-easterlies, easterlies and even north-easterlies.

The bay is just under five kilometres wide from Newtown Head to Powerstown head, a not-insignificant distance. Therefore a Force Three north-easterly wind will have about four kilometers of water to blow over before it reaches the Guillamenes. You may recall we discussed this in Understanding Waves for Swimmers a long time ago. The distance of water over which water blows is called the fetch. The greater the distance over water which wind blows, the greater the waves and chop that can be pushed up.

Apart from the fetch, the strength of the wind is important. I have long thought the ability to observationally measure wind using the Beaufort Scale should be an essential skill for serious open water swimmers.

Force Three on the Beaufort Scale is also known as a “gentle breeze”, a more pleasant title. Force Three is however a critical measurement for open water swimming because at Force Three scattered whitecaps start to appear, as crests start to crumble from the wind. This means an increase in rough water to the swimmer. (If you see choppy-ish water with no whitecaps, it’s Force Two). Below is a very boring picture of all these facts! A Force Three wind, blowing onshore, across a four kilometre fetch. It’s full size if you click on it and want a better look at what Force Three looks like, but it’s pretty unexciting. If you look around you’ll see those occasional whitecaps.

You can of course swim in Force Three, but you’ll be slowed down. It took me almost twenty-five minutes to pass the pier, where it would normally take twenty. I call Force Three critical because it’s the transitional point into rough water.

And because video is better for this, here’s a short clip. (With the wind blowing across the microphone, you might want to turn your volume down first).

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This second video is taken from the water. However, this was taken underneath Doneraile Head, and the fetch that wind was blowing across was less, so though the wind was the same, and blowing the same direction, the waves were less.

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Just one Irish winter’s day weather forecast

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Extremely windy or stormy in places today especially early this morning with gale force southwest to west winds and severe gusts of 100 to 140km/hr at times—– strongest in exposed parts of the north and northwest early today. Heavy rain in eastern and southern areas will clear before dawn and the day will be bright with sunny spells, occasional heavy rain and hail showers especially in western and northern areas with a risk of thunder and some hill snow possible in the north. Cold by afternoon with afternoon temperatures of 3 to 7 degrees.

So that’s windy and calm, wet and dry, cloudy and sunny. Rain, sun, hail and maybe even snow. All in one day, in a small country, January the 3rd, 2012. Wind gusts actually hit 169 kph at Malin Head.

Introducing a precise open water swimming temperature scale

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Next year’s Cork Distance Week will have a record number of attendees, many from outside Ireland. Some will be coming nervous or terrified about the potential temperatures especially if they heard any of 2011′s details.

They need a scale of reference for that fear and we need a common terminology!

Steve Munatones on Daily News of Open Water Swimming had a great post recently on the temperatures at which people consider water cold.

I hope he won’t mind me showing the poll results here:

I remember Finbarr once saying to me that; “10ºC is the point at which you can start to do some proper distance”.

{Fin, I need either a blog or picture from you for the constant references. Either one of you in your UCC Pirate Polo Speedos or one of you swimming directly over some poor unsuspecting swimmer going round a buoy would be the most appropriate.}

I hope Jack Bright might have some input into this also. :-)

I think it would be fair to say that many, if not most (but not all), of the (serious) Irish and British swimmers would fall into the 7% category, it’s getting cold under 10° C.

So here’s my purely personal swimmer’s temperature scale:

Over 18°C (65°F): This temperature is entirely theoretical and only happens on TV and in the movies. The only conclusion I can come to about the 32% who said this is cold are that they are someone’s imaginary friends. Or foetuses.

16°C to 18°C (61 to 64°F): This is paradise. This is the temperature range at which Irish and British swimmers bring soap into the sea. The most common exclamation heard at this stage is “it’s a bath”!!! Sunburn is common. Swimmers float on their backs and laugh and play gaily like children. They wear shorts and t-shirts after finally emerging. They actually feel a bit guilty about swimming in such warm water. Exposures times are above 40 hours.

14°C to 16°C (57° to 61°F): Aaahhh, summer. All is well with the world, the sea and the swimmers. Exposure times are at least 20 to 40 hours. Sandycove Swimmers will swim 6 hour to 16 hour qualification swims, some just for the hell of it and because others are doing it. Lisa Cummins will see no need to get out of the water at all and will just sleep while floating, to get a head start on the next day’s training.

13°C (55° to 56°F): Grand. You can do a 6 hour swim, and have a bit of fun. Daily long distance training is fine. Barbecues in Sandycove. The first Irish teenagers start to appear.

12°C (53/54°F): Well manageable. You can still do a 6 hour swim, it’ll hurt but it’s possible. Otherwise it’s fine for regular 2 to 4 hour swims. This the temperature of the North Channel.

11°C (51/52°F): Ah well (with a shrug). Distance training is well underway. Ned, Rob, Ciarán, Danny C., Imelda, Eddie, Jen & myself, at least, have all recorded 6 hour qualification swims at this temperature. Lisa did 9 hours at this temperature. Swimmers chuckle and murmur quietly amongst themselves when they hear tourists running screaming in agony from the water, throwing children out of the way… 

10°C (50°F): Usually known as “It’s Still Ok”. The key temperature. This is the one hour point, where one hour swims become a regular event. We start wearing hats after swims.

9°C (48/49°F): “A Bit Nippy”. No point trying to do more than an hour, it can be done, but you won’t gain much from it unless you are contemplating the Mouth of Hell swim. Christmas Day swim range. Someone might remember to bring a flask of tea. No milk for me, thanks.

8°C (46/48°F): The precise technical term is ”Chilly”. Sub one-hour swims. Weather plays a huge role. Gloves after swims. Sandycove Swimmers scoff at the notion they might be hypothermic.

7°C (44/45°F): ”Cold”. Yes, it exists. It’s here. The front door to Cold-Town is 7.9°C.

6°C (42/43°F): “Damn, that hurts”. You baby.

5°C (40/41°F): “Holy F*ck!” That’s a technical term. Swimmers like to remind people this is the same temperature as the inside of a cold domestic fridge. Don’t worry if you can’t remember actually swimming, getting out of the water or trying to talk. Memory loss is a fun game for all the family.

Under 5°C (Under 40 °F). This is only for bragging rights.There are no adequate words for this. In fact speech is impossible.  It’s completely acceptable to measure exposure times in multiples of half minutes and temperatures in one-tenths of a degree. This is hard-core.  When you’ve done this, you can tell others to “Bite me, (’cause I won’t feel it)”. (4.8°C is mine). Carl Reynolds starts to get a bit nervous. Lisa tries to remember her suntan lotion.

Ned Denison during the winter

2.5°C  to 5°C. South London Swimming Club and British Cold Water Swimming Championships live here. If you are enjoying this, please seek immediate psychological help. Lisa might zip up her hoodie.

1.5°C to 2.5°C: Lynn Coxian temperatures. You are officially a loon.

0°C to 1.5°C: Aka ”Lewis Pughian” temperatures. Long duration nerve damage, probably death for the rest of us. Lisa** considers putting on shoes instead of sandals. But probably she won’t.

*Grand is a purely Irish use that ranges from; “don’t mind me, I’ll be over here slowly bleeding to death, don’t put yourself out … Son“, to “ok” and “the best“, indicated entirely by context and tone.

** Lisa Cummins, for the win.

Looking forward to your opinions.

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Is the water too cold to swim?

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This article is, once again, a variation of the most popular question here: “What temperature of water is too cold to swim in”?, which I’ve written about before.

Thermometer

Image by Ben+Sam via Flickr

The temperature at the Guillamene last Sunday week (October 16th, 2011) was about 13° Celsius (55° F). That’s far warmer than what most people will imagine, not far off the highest normal summer water temperature (about 15° to 16°, excluding unusual warmer pockets or days) for Ireland’s South Coast. And by the end of last week it was down to about 11.5° Celsius.

The weather is changing though, autumn and early winter storms have shown up and the water is rough most days. There’s been fog that has lasted for days,and the days of grey skies and continuous rain. Days and nights are cooler (though given the crap summer, again, in Ireland, that’s not much of a real change, only about 4° to 6° Celsius change for now.) Surely, many people will say, the water is cold!

Annual mean sea surface temperature from the W...

Image via Wikipedia

Occasional swimmers have changed to wetsuits weeks back. But experienced swimmers are still, should they desire, putting in two or three hours without wetsuits, (if they haven’t gone back to pool training or like me, have slackened off for the end of season).

So this is a critical time for those considering a big swim for next year, or wanting to improve their open water ability. Time when you should be asking yourself:

How much more do I really want to able to do?

You can stop now, leave the sea, and just do pool training. or you can retain your sea swimming. You can use a wetsuit, and get used to the sea in winter. Or you can stay in skin, and discover that for maybe another three or four weeks, it’s not that cold.

You can approach this as a multi-year project, this winter just keeping swimming regularly in rubber, maybe dumping the neoprene for a few minutes of skin only here and there, and then next year going a bit further before donning it. The only mistake is to expect to be able to handle cold without doing any work.

An important thing to remember now is Rate of Change, rather than deciding what temperature is your cutoff (because without experience you won;t know anyway). The water temperature will drop soon, (I’ll let you know when The Big Drop happens, it could be as soon as three weeks or could be as long as six or seven). The Big Drop is when the water temperature goes below ten degrees Celsius 9 50° Fahrenheit). Yes, yes … don’t tell you can even get that low, I can hear you from here.

Last year the coldest day was late November, after the coldest spell Ireland had in something like 60 years. And it recovered afterwards. By Christmas the temperature was back to normal for that time of year, at about nine degrees (48° F.).

So now is the time and chance to do address two big issues:

1: Your perception of the world around you, especially the sea.

2: Your perception of yourself, and your limits and capabilities.

I know what some of you are thinking: but this guys is already experienced at cold, and I couldn’t do it. Nonsense. Anyone can, as I keep repeating, you just have to decide whether you want to or not.

There’s already lots of writing about cold on this site, see the top menu bar up there? ^^^

Go beyond your limits. Go on. Do it. I’ll meet you at the Guillamene.

P.S. As I was wondering what images to add to this, I really wished I had one of a swimmer with a meat thermometer stuck in them. But, apart from the pictures of Gábor, this is a Safe For Work site.

Fog (aka Indian summer my arse)

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Foggy famine ruins in Kerry

From a quick check on the M5 buoy, I knew what the conditions were. But when I neared the coast for a new swim, the mist and fog closed in. I knew it probably wouldn’t be better right on the coast, and such was the case. Visibility was about 500 metres. Not too heavy but still not great. If I was at the Guillamene it wouldn’t have bothered me since I could follow the cliffs. But since I had hoped to tackle a new swim, I decided against it.

Indian Summer-like conditions were forecast for Ireland for this week. For the entire south coast yesterday and today however, the prevailing conditions are fog and low-lying cloud and mist.

Fog is very dangerous for swimmers. A swimmer is less visible to boats, and it’s easier to lose track of progress or location. (One way of differentiating fog and mist is that mist has a visibility of greater than one kilometre, while fog is visibility of less than one kilometre).

Fog is a prevalent feature of coasts, and some places such as London or San Francisco are notorious for fog.

Fog is the condensation on your kettle or your bathroom mirror writ large. It happens when warm moist (saturated) air meets cool air and the moisture condenses into small drops as the air rapidly cools and can no longer hold the moisture.

So the times and places fog is most likely encountered are when the possibilities of air masses with different temperatures are greater. That is what has happened this week with suddenly warmer air meeting colder air on the south coast, (rather than coming in from the more usual south-west direction). Coasts are conducive to fog. Cold air can roll over warm water and cause coast fog, or roll down off mountains and do the same or can form ahead of warm weather fronts as happened yesterday.

Less than 50 metre fog at Vista point on the Ring of Kerry - high summer

 

In Ireland, if my experience is anything to go by, Kerry and the Iveragh Peninsula are particularly prone to very dense summer fogs, which can last up a couple of days.

Pressure (synoptic) weather charts explored

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Following on from the discussion on hectopascals and atmospheric pressure:

Let’s use the same chart again.

This time you know what the numbers mean, various high and low pressure areas.

But there are different kinds of lines to be seen also. In this chart we are concerned with five types

  • light black line with numbers
  • short curved bold black line 
  • line with hemispheres
  • lines with triangles
  • lines with paired hemispheres and triangles 

Some of the lines have triangles for a short length, then hemispheres for another length. These can be treated as different lines and are different from the lines where the hemispheres and triangles are beside each other in pairs.

  • In the previous related post, the plain lines (above) with inset numbers running in curves all over the chart were explained as isobars, the different pressure contours.
  • Next is the plain bold black line, which is a Trough, a longer area of lower pressure, associated with increased risk of rain and cloud
  • Next are the lines with a series of hemispheres. This a Warm Front, usually at the front end of an advancing warm air mass. It can often bring cloud and increased humidity. In coloured charts it easier to remember as these are red as seen in the example above.
  • Next is an advancing Cold Front, the line with triangles, which in coloured charts is blue.
  • Next is the Occluded front. This means the front of a cold mass catches up with a warm front and pushes it upwards. Usually brings rain.
These aren’t all the lines and markings used on pressure charts, but are the ones most relevant for the understanding and forecasting we require.

“What does 965hPA mean?”

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Courtesy of another search that brings people here, and following Hurricane Katya last week and the synoptic chart that I linked for it.

Hurricane Katya synoptic (pressure) chart

So we’ve started with that same synoptic chart again.

In this chart we have a low pressure area, Hurricane Katya, north west of Ireland, in sea area Rockall, right in the centre at L 960. This means a Low Pressure area, 960 hPa, hectopascals. Hectopascals are a measure of  atmospheric pressure, which is a measure of the force applied by the weight of air above the area.

The mean sea-level Atmospheric Pressure is 1013, (not 1000), at a standard temperature of 20° Celsius. Hectopascals used to be known as millibars, and mean pressure sea-level air pressure was 1 Bar. So air pressure 1013 or under is Low and 1014 is obviously High, both of which can be seen in the chart.

(As altitude increase, pressure decrease. Also, it is the weight of air that dictates the boiling point of water. On top of Everest etc, the weight of air is so low that water boils at a much lower temperature, boiling is just the ability of the water to move from one state to another, liquid to steam, vapour pressure).

As you know, Low Pressure is typically associated with deteriorating, poor or bad weather.

In the chart the centre is surrounded by circular lines, called Isobars. Like lines on a map indicate height, isobar lines indicate differences in pressure, which rises as you move away from the centre.

960 hPa is a particularly low pressure system, common to bigger storms. Pressure can drop even lower. The recent Hurricane Irene which so threatened the eastern US, dropped to 942 hPa and we’ve experienced 945 hPa on this side of the Atlantic in the big blow of 1988. The lowest ever recorded was 882 hPa in 2005.

The tightness of the isobars, i.e. their closeness, indicates high winds. A Low Pressure area is called a Cyclone in meteorological terms, and a High Pressure area is called an Anti-Cyclone. In the northern hemisphere, cyclones rotate anti-clockwise. See my three part series on weather and climate, and prevailing winds across the globe.

The winds in the Hurricane/weather system above are air flowing from surrounding high pressure areas into the low pressure area, literally “filling it in” like water flowing down a hill.

A synoptic or pressure chart can tell you a lot about the weather over a large area. I’ll return to this subject again and explain the rest of those triangles and circles and lines on the chart.

Thanks Google, for fixing my storm videos

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Google added Video Stabilization to YouTube, a boon for crap video takers, mobile phones and video shot without benefit of a tripod (all apply to me), (and a frightening indication of just how much CPU processing power they have available). I stabilized both the Shakespeare Beach Storm Force 10 videos and they are much improved. They were shot in howling wind, so there’s still shaking but they’re much better (I could see them side by side and see the difference).

30 sec one:

1 minute one:

Here’s a fun 8 second video from Malin Head on Monday of the full force of Hurricane Katya, beats my leaning into the wind at Varne. I wish I’d been out west to take some good footage for you. I’ve been out there in Force 10 before and it’s a sight to behold.

Donal on Varne cliffs in Force 10

Studying weather online during a Force 10/11 blow

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When you are in Dover weather is the beginning and end of everything. We felt we had a better handle on the coming storm to hit Ireland as we were trying to get a good of idea of what coming toward the Strait.

Everyone in Ireland today knows it’s windy (but really unusually it’s dry, even sunny). The remnants of Hurricane Katya are over Sea Area Rockall. But what you do for a better understanding of it? I’ve written before about how to understand weather forecasts.

And also about using the Marine Buoys to “know before you go”.

Well first I’ll check the MET Eireann sea area forecast. (MET Eireeann also have a nice explanation of terms).

Gale warning: in operation. 
Small craft warning: see gale warning. 
Meteorological situation at 0300: A storm depression 160 nautical miles northwest of Erris Head will track northeastwards. 
Forecast for coasts from Valentia to Rossan Point to Fair Head 
Wind: Southwest gale force 8 to storm force 10. Occasionally reaching violent storm force 11 this morning between Erris Head and Malin Head. Later today, winds will decrease force 7 to strong gale force 9. 
Weather: Scattered showers, some heavy in the north. 
Visibility: Moderate to good. 
Forecast for coasts from Fair Head to Carnsore Point to Valentia and the Irish Sea 
Wind: Southwest force 7 to gale force 8, occasionally reaching strong gale force 9 today. 
Weather: Patchy rain or drizzle, clearing to isolated showers. 
Visibility: Moderate to good, occasionally poor at first. Becoming mainly good today. 
Warning of heavy swell: nil. 
Outlook for a further 24 hours until 0600 Wednesday 14 September 2011: Gale force westerly winds will persist on Tuesday for many sea areas, with strong gales at first in northern sea areas. On Tuesday night, winds will decrease fresh to strong. Further showers, mainly in the north. 

Irish Sea Area Map

Remember you need to know where these areas are for Ireland. For Ireland particularly, as you can see from the Sea Area above, we use Headlands instead of the larger Areas that are used by the UK Met Office Shipping News.

(Last week in Dover for example , we were watching Shannon, Fastnet, Sole, Biscay, Dover, Fitzroy because we were trying to grab any hope/prospects from the big picture).

Oh, by the way, the video of Shakespeare Beach from last week was hugely popular. I had about a thousand views on YouTube. Big Time!

Today I look at the satellite radar and Infrared imagery, something I might only do once a year.

Katya Northwest of Ireland

And of course, M5, the buoy off the South East coast. Today I’ll even look at the hourly data from it, just to see the wind trend and gusts.

M5 hourly data for Hurricane Katya

Note especially the hourly Speed of maximum Wind Gusts, which are up to 49 knots. This means Force 10, Violent Storm.

Another by-the-way: one knot is about 1.15 miles per hour or roughly two kilometres per hour for easy calculation, so those Gust speeds mean approximately 100 kilometres per hour currently.

I keep reminding you of the Beaufort Scale, which I believe and also written about previously with explanation is an ESSENTIAL observational skill for Open Water Swimmers. Knowing at least the lower numbers is a tool that allows you to gauge what you may be able to do and allows you to take other’s observational bias into account. It takes practice to able to read the water. MET Eireann’s page linked above is a nice simple page on the Beaufort Scale. By the way, the Beaufort Scale was the invention of an Irishman over 200 years ago.

All of this is what I do for Ireland. It’s entirely relevant for everywhere else, just learn what your locals buoys are, where to get the Marine forecast. And learn it. Then understand it. Gauge your experience by it.

Older people will remember when TV weather forecasts included the Synoptic (Pressure) Charts. These are still the basis of understanding weather.

Last night’s synoptic chart. I’m going to do a separate post on this.

Another stupid thing I’ve done. No. 5124 in an ongoing series, (i.e. my life)

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Coming home from Cork yesterday, the conditions driving across by the Comeraghs were terrible, in fact from Dungarvan to Carrick there was often heavy snow for miles on end. By last night everything had frozen to lethality.   So when I looked out this morning and saw this: I of course immediately thought, “I must go for a sea swim”.

Doggits and myself into the car.   And then a long drive to Tramore. Normally a 30 minute drive, I was unable to cross the bridge at Piltown due to the conditions. Through Waterford, I actually took the toll bridge, the only other time I’ve done this was the Christmas swim in similar conditions last year. I hoped as I approached the coast that the snow and ice would disappear as happened last year. Instead, conditions that I’ve never experienced myself at the coast prevailed and the temperature dropped further as I entered Tramore Town. Unwilling to risk the steep hill through the town, I went up onto the bypass and approached the coast and the Guillamenes from the west side through the trees. I saw this view when I arrived outside the car park, having taken twice as long as long as normal to get there. The grant for redoing the platform at the Guillamenes came through a few weeks ago and so the car park is closed and access down the cliff steps is blocked. So, quick walk for the doggits.

Looking toward the town I could see snow right on the beach.

The steps down were ice sheeted and the platform and rocks were covered in snow and ice. The air temperature was -3.5C! The coldest air temperature I’ve previously ever swum in was about +2 or 3C.

There was also a light Force 2 Northeasterly breeze blowing, just to add to the fun. The water was calm.

I’ve many times swam here on a Saturday or Sunday morning when there are none of the regulars around, but in those circumstances the water conditions were always such they wouldn’t risk it. Today however the water looked lovely and calm. But the risk just getting to the Guillamenes & Newtown Cove was obviously too much.

The cold air temperature meant steam was rising off the sea.

The freezing air and wind meant by the time I got togged out with cap on, my hands were completely numb before I got in the water. I had to wear my coat right to the edge and hang it on the top railing, something else I’ve never done, but it was bitterly cold.

The second pour on the new steps was completed this week, and while the rails remain to be added the new steps are now wide enough to allow a few people to exit and enter the water simultaneously, though it does look like the railings are going to be put to the side instead of the center, which would make more sense and allow entry and exit at the same time, but I long ago ceased to expect foresight from Local Government, retaining now only a perpetual sense of disappointment. But maybe this time I’ll be wrong.

The water itself was of course fine. I loaned my infrared thermometer but I’d say the water was probably 9.5C.

I only swam about 20 minutes. The steam rising off the water mean a reduction in horizontal visibility while the sky was perfectly clear overhead.

My shoulders and back were cold due to air and wind exposure. (Otherwise just normal November water temperature). Because of the cold air, I cut the swim short at about 20 minutes.

It was one of the utterly memorable swims. Blue overhead & white and pink streaks towards the south, with that peculiar washed-out almost urine-like colour of the southern sky toward the horizon mixed with cold grey rain/snow clouds.

While out there I had the usual sense of being the only person alive in the world. But today, unusually, I actually also thought about Liz, Craig, Rob, Gábor and Lisa who were also swimming this morning in Sandycove.  I’m not far from passing my Sandycove C target, but it looks like I won’t reach it until the new year.

Because of the wind and air, getting dressed was the same as if I’d been swimming in 5 or 6 C water and dropped my temperature more significantly.

I had left my rubber changing mat on the concrete and it had already frozen and was as cold to stand on as the concrete! I was unable to towel dry, just pulled the clothes straight on. (One reason why I wear Merino wool when going swimming, it retains body heat while damp.)

As I left I could see snow out in the bay, and the steam still rising.

The trip home was worse. Getting off the Guillamenes road was difficult It took half an hour just to get out of Tramore. For a short local swim, I spent half the day.

Apparently the coldest November since 1973.

But it was fun, as always.

Why Weather Prediciton will NEVER be perfect…

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“The problem with weather forecasting is it’s right too often to ignore it, and wrong too often to rely on it” – Patrick Young

So given our understanding of the principal influencing factors, why isn’t prediction better than it is?

It’s best explained by the much-abused and overused statement about the butterfly flapping it’s wings in China, causing a hurricane in the Caribbean.

What this saying is attempting to explain is that weather is a non-linear system. Weather is a Dynamic (changing) system whose outcome is entirely dependent on it’s initial starting position. It doesn’t change predictably with simple changes in those starting positions. Even tiny variations in the starting position result in wildly different outcomes, outcomes that can’t be predicted.

It’s what’s called a Chaotic System*. Chaotic Systems generate Emergent Properties, which means that out of those unpredictable starting parameters, some patterns will emerge. A Pattern is simply something which displays Order.

(We treat the Earth & Sun as a single system because other factors are nowhere near as significant.

Life itself is Order, an Emergent Property of the Earth/Sun System. And so is weather.

For weather prediction, the starting parameters is the current weather.

So given all that, I still haven’t answered the question about the problem of Weather Predicting.

Think of the area of the Atlantic. Imagine it divided into 100 mile square boxes. In each of those boxes place a very accurate weather sensor, capturing all the data needed. Put all that data into a very good computer simulation. The output is the weather prediction (usually with some experienced human intervention also).

BUT, between each sensor there’s 100 miles of unmeasured area. Within that area maybe there’s an area of ice, or land or cloud or something else. Something that can make a small variation. That change can become bigger, eventually affecting the surrounding weather, and getting picked up by the sensors. But the change came from outside what the sensors originally measured.

So what’s the solution. Well, make you measurements more accurate. Place a sensor every 50 miles or 10 or 1 kilometre. It doesn’t matter. The system (weather) is sensitive and the changes will still accumulate. Eventually you’d have to cover the whole surface of the world in sensors to try to achieve the accuracy you desire.

And of course the reality is we currently have nowhere near even 1 sensor every 100 square miles of the Atlantic, let alone the whole world. You can’t just use shipboard sensors as they’ve all have to be calibrated to the same standard.

The computer GIS systems (GIS; geographical information systems) will get better, but they will Always be limited by the initial measurements. Better data measurement, inputting into better expert systems, whose outputs are then tested against real world results, will make (and have made) predictions better.

So it’s not the meteorologists but the interaction of measurement and computer systems. In the real world we can never measure the starting parameters with 100% accuracy.

*System is a term that means something that has a boundary by which energy neither enters or leaves. People who generally misunderstand science or Thermodynamics will often say that the Second Law of Thermodynamics states that Entropy (Chaos) will always increase and that order can never emerge from Chaos, so whence comes Life? The Earth however isn’t a System, since energy is input from the Sun. At the true Universal scale, the entire Universe that is, then Entropy does always increase.

Weather and Climate, Part 3

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Part 2.

Almost done.

Obviously meteorology is a far more complex subject than the thumbnail I’m giving again. I’m just covering some of the major influencing factors for Ireland and the North East Atlantic.

So last time we saw some of the prevailing winds ran opposite those across Ireland.

To explain these difference we need a few further scientific names.

The first is Circulation Cells.

These were originally used to describe the circulating patterns of air. Hadley Cell is the movement in the equatorial region, Ferrell Cell is the mid-latitudes and Polar Cell is Latitude above 60 deg. North.

In the Hadley Cell, warm air rises at the equator, flows north (and south) and DROPS when colder and denser at about the sub-tropical (30 deg. Latitude) point to flow south again.

In the Ferrel Cell the air flows along the surface and RISES when warmer at around the 60 deg latitude point.

In the Polar Cell cold air flows along the surface south (or north in the souther hemisphere), until about the 60 deg latitude line, where, warmer, it rises again and flow back toward the pole.

So the upshot of all this for Ireland is:

  • South of us is a semi-permanent High Pressure region.
  • North of us is a semi-permanent low pressure region.

So low pressure systems will be blocked on the south and flow north eastwards generally. This , you will recognise as the prevailing pattern in Ireland. Bad weather (cyclones) come in from the west, and move up toward Scandanavia.

Of course other factors are required also for this to happen. For one thing, the theoretical model in the attached picture is affected by different land and water geography.

Given our position on the edge of the Atlantic we are subject to something most the rest of Europe isn’t, namely the edge of the Marine Polar air mass.

The world is also covered in Air Masses. The North Atlantic is covered by the mP, i.e. marine Polar, i.e. cold, Air Mass. Whereas continental Europe isn’t covered by a stable air mass.

Around the Azores is the differentiation to the mT, marine Temperate, zone. This acts as a block to low pressures more northerly than the 30 deg N, Ferrel Cell boundary.

And there you have some broad outlines.

An offshoot of this for Ireland is you can understand a low pressure system better. Due to the Coriolis Effect, low pressure systems (anti-cyclones) in the northern hemisphere circulate anti-clockwise. Think about an anti cyclone off the west or north west coast. The air will flow southward and swing around through westerly until it is again flowing northwards (a southerly wind). These are our prevailing local wind conditions, onshore southerly winds on the south coast which are offshore on the north coast.

IANAM (I am not a meteorologist)

Part Three.

Weather and Climate, Part 2

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I finished Part 1 with the Coriolis effect.

I said last time that the warmer air at the equator flows toward the poles. The Coriolis Effect impacts this to cause the major weather patterns.

Since the world is a sphere, the surface of the earth at the equator is moving much faster than the surface at the poles. Like a wheel.
For every revolution the outside of a wheel has to cover a much greater distance in the same time as the centre/hub. So the outside of the wheel is moving at a faster speed. For the Earth this means the air at the Equator is moving faster than the air further north and south.

Given the “westerly” spin of the earth, air moving away from the equator carries this extra speed and generally curves eastwards. So our weather fronts, as you know, come from the Atlantic, not just because it’s a large body of water, but because of the Coriolis Effect. These are our prevailing (or dominant) winds.

Along with this is the different densities of the air dependent on temperature. Warmer air is lighter and rises, while colder denser air slips under the warm air.

By now you’ve realised though that not all weather comes from the west, despite what I’ve said. Depending on your global location the pattern is different.

See the first picture for the global prevailing winds pattern. Of most importance to Ireland are two facts. At 30 deg North Latitude there is a “sub-tropical high” and at 60 Deg. North latitude there is a subpolar low.

When you watch the weather forecast you’ll typical see low pressure systems arrive from the west and flow north-easterly, catching us

And you see some of the winds, particularly equatorial and tropical, go the opposite direction.

Part 3.

Weather and Climate, Part 1

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“Climate is what we expect, weather is what we get” – Mark Twain

So I’ve written about the Shipping News and the databuoys and waves and wind etc. I thought I might do some more on the weather subject and move up to a bigger scale, particularly as it affects Ireland and the North Atlantic.

Last winter in Ireland especially, saw constant discussion about the weather. In the media the discussion always went the same way:
“If climate change is real, why are we having the coldest winter (or wettest summer, etc) in years?”.

To which the meteorologists response is that you can’t mistake weather for climate.

Weather is a localized effect, whereas climate is a pattern. You can have climate at various scales from localised micro-climates (e.g. because of some effect like mountains near sea) up to global climate.

We all know what Irish climate is like. Moderate winters, cool and wet summers. But why?

Almost anyone you talk to in Ireland will recall one bit of science from school: Ireland doesn’t freeze in the winter because of the Gulf Current.

The Gulf Stream Current is part is a larger North Atlantic system called the North Atlantic Gyre which includes the Equatorial Current and is one of the five main oceanic gyres. (See pic).

A Gyre is a circulating current. It is driven by the Earth’s Coriolis Effect, which means the rotating earth has a friction effect on the atmosphere.

The Equatorial Current feeds across from Africa to the Caribbean, driving the Gulf Stream. The Gulf Stream in turn flows North East. part turns into the North Atlantic Drift and runs further North east toward the Arctic, while part splits and flows back southward, passing Portugal and cooling, and feeding in the turn the Equatorial Current again, thus setting up a circulating current.

The gyres themselves operate independently, but they are a surface effect usually going no deeper than about one kilometer.

At an even bigger scale however they fit in a larger pattern often called the global ocean conveyor belt (Thermohaline circulation). This moves heat around the globe with for example the warmer water from the Gulf Stream getting colder as it goes further north, and becoming more dense and sinking, while southward flowing water warms and becomes less dense and rises. (Cold water also carries more nutrients and drives the global food ecosystem). The global conveyor belt connects all the oceans with a current that flows around the world rising and falling, taking about 1500 years.

The Oceanic Gyres are only part of the story though.

The most basic fact about weather: It’s all driven by the Sun. The Sun inputs the energy that drives the whole system. That energy varies slightly, (for example with Sunpots) and the amount of energy hitting the Earth varies according to local effects like cloud cover, ice, water or land. This is all aggregated over the whole world/system. However even more important is that the amount of energy hitting the Earth at the equator is significantly greater than that at the Poles. This discrepancy is the generator for the changing weather. The mechanism by which the weather propagates is the Coriolis Effect.

 

Part Two.

 

 

 

 

Know before you go

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So let’s say like most of us, you don’t live close enough to the sea to able to look out your living room window before deciding whether to swim. But you want to know what the conditions at the coast are.

The easy solution is the use the Irish Marine Buoys. The map shows their locations.

All Marine databuoys around the coast belong to an international Station ID database. M5, my “local” buoy is actually Station ID 62094. The Sandettie Lightship in the English Channel is 62304. But I digress.

Let’s have a look at today’s readings.

Buoy M1 M2 M3 M4 M5 M6
Date 14 Feb 11:00 21 Oct 09:00 21 Oct 09:00 21 Oct 09:00 21 Oct 09:00 21 Oct 09:00
Meteorological
Atm. Pressure (mb) n/a 1021.4 1025 1020.2 1024.2 1024.2
Wind Speed (kn) n/a 15 5 18 13 n/a
Max Gust (kn) n/a 20 10 25 23 n/a
Wind Direction (°) n/a 250 290 270 280 240
Air Temp. (°C) n/a 9.8 12.6 11.9 10.9 13.4
Relative Humidity (%) n/a 84 58 73 78 75
Oceanographic
Sig. Wave Height (m) n/a 1.1 1.2 n/a 1.1 1.6
Sig. Wave Period (s) n/a 4 6 n/a 4 7
Max Wave Height(m)
Max Wave Period(s)
Mean Direction (°)
Sea Temp. (°C) n/a n/a 14.6 13.2 13.7 14.1
Salinity

bo
The important stuff here for us are quite simple. All the buoys are Offshore but give an idea of the conditions within a region. M5 is about 50 miles offshore. M3 is closest buoy for Cork (because it gives a better idea what is coming than M5).

The Atmospheric Pressure. (All around 1024 in this case). And as I look out the window the sky is blue with little wind. It’s a high pressure,which given it’s October makes the night cold and the day dry.

Wind speed you are now familiar with (it’s in Knots here), but keep an eye also on the Max Gusts.

The wind direction is based on a 360 degree circle to indicate the direct from which it’s coming. 0 degrees is Due Nothe, 180 degrees is dues south. So readings here of 250 or 270 say the wind is coming roughly from the South West.

Sig. Wave Height is the height of the swell from trough to crest. Since it’s deep water, the swell isn’t breaking. Experience will help determine that this s once it hits the shore, but under 1.5 Metres is quite ok.

Sig. Wave Period is the time between peaks. I’ve mentioned this previously. Lower number (under 7 or 8) will indicate the swell is being driven by wind, and will be more messy at the shore.

The last (and for OW swimmers one of the most important) is Sea Temp.

Note: M5 is 50 miles out. Experience has shown that M5 reads higher than the shore temperature almost all the time. Which also makes sense if you think about. Offshore the water is deeper, and therefore is a larger volume, and less likely to be affected by wind than the shallower coastal water.

You can click on anyone of the buoys to get detailed hour reading, and see the change in wind or swell height.

Finally, when you look at the page, keep an eye on the latest news. It doesn’t often change, but some buoys go offline for months at a time, or get renamed as another (as is currently the case with M1 & M6)

How to understand Sea Area Forecasts / Shipping News

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I though I’d continue on from yesterday’s post and talk about how to interpret the radio sea weather forecasts.

In Ireland we get the Sea Area Forecast, also called the Marine Forecast on RTE Radio 1 (which comes from Met Eireann) twice a day. In the UK it’s more famously know as The Shipping News.

RTE 1 Marine Area Forecast is broadcast at 06:02, 12.53 & 23:55. In Ireland these are also broadcast more regularly on Marine VHF radio.

Radio Four Shipping News is broadcast at 00:48, 05:20, 12:01 & 17:54. (Useful for your next trip to Dover. All the pilots will listen to this).

Shipping New Forecasts use the Admiralty Sea Area regions (generally going clockwise). The Shipping News cover pretty much the entire Northern Europe Coastal waters.

So you need to see the map.
(See attached to this post also.)

For the English Channel the most relevant area is obviously Dover. For the South of Ireland the region is Fastnet with Shannon covering the South West and most of the West. Now you realise you can ignore most of the Shipping News stuff and just focus on the relevant area.

In Ireland however the Marine Forecast uses the Primary Headlands and off-shore Buoys (M1 to M6, usually only using M5 off the South-East) and is out to 30 Nautical miles form the coast. (See attached also.)

The Sea Area forecast always follows the same format.

It’s starts with a General Situation, describing the main weather systems affecting Ireland.
Also any Gale Warning in operation (Beaufort Force 8 or greater).
Small Craft Warning are issued if the wind is Beaufort Force 6 or greater.
Swell Warning is given with swell greater than 4 metres is expected.

The words that are used are not arbitrary but have specific meanings.

In time, Imminent means in the next 6 hours, Soon is between 6 and 12 hours, and Later is between 12 and 24 hours. The movement of the systems also have terms from Slowly (up to 15 knots) to Very Rapidly (greater than 45 knots).

Following the general situation, the Coastal Waters forecast is given, covering Wind, Weather and Visibility. Wind direction also includes some other words. Veering (a clockwise change), Backing (an anti-clockwise change).

Where the Forecast is similar, different areas are grouped together.

Wind is given using the Beaufort Scale I wrote about yesterday.
Weather uses a short list of self-explanatory words, Fine, Fair, Cloudy, Mist, Haze, Rain, Hail, Snow.
Fine means dry, mainly sunny and clear after dark. Fair means dry and sunny but may have some cloud.
Visibility in miles. Good means greater than 5 (nautical) miles, Moderate is 2 t0 5 miles, Poor is 1 kilometer to 2 miles. Less is Fog.
Pressure in hectopascals and the pressure tendency. For example “985 hPa, falling rapidly” means a low pressure dropping 3.5 t0 5.9 hPa per 3 hours, indicating bad weather coming quickly.

(For pressure 1000 hPa is the mean. Above is high, below is low, however the mean actually changes over the year, so a centre of 1005 hPa surrounded by centres of 1010 to 1020, is still a relative low pressure. This is a type of situation that can occur in the summer.)

So the forecast can be something like:

Forecast for coasts from Carlingford Lough to Roche’s Point to Erris Head and the Irish Sea -
Wind: North force 2 to 4.
Weather: Mist with patchy drizzle.
Visibility: Moderate to poor in mist but locally good.

Beaufort Wind Scale – An essential observational skill for the OW Swimmer

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One thing I’ve really noticed a lot this year is the inability of many swimmers to accurately gauge wind speed and water conditions.

The way to do this is the Beaufort Scale, which measures wind speed based on empirical observed conditions. (It is, by the way, named after an Irish explorer and British Navy Admiral.)

I’m just pasting the Wikipedia chart in here. I first used a version of this when I was learning to parachute, although that scale used the leaf and tree movement. When I started surfing, I starting using the modern Beaufort scale of sea conditions.

When I was crewing for Gábor, it was how I was able to give current wind conditions during the swim. It’s very easy with just a bit of practice. Sailors will also use the description in the second column. At one point I described that we had moved to Force Three for about 45 minutes. This was based on observation that the waves had just started to break. For my own swim, it had moved to Force Five by the end, disorganised larger waves, lots of whitecaps, and water washing unpredictably across my face.

For sea swimming, the critical noticeable tipping point (for me anyway) is between Force Two and Force Three. At Force Three it becomes more work and at this point the chance of aspirating water increases with the unpredicability of the water surface conditions.

Also remember, that these conditions can occur On Top Of Groundswell, which I’ve previously described. The combination of Force Four to Five wind and groundswell leads to conditions that should only be swum by experienced open water swimmers, and who have a defined water exit point.

For the much mentioned “lumpy” day outside Sandycove, it’s either Force Three or Force Four. By Force Five we rarely go outside the island, though those who can recall the Champion of Champions race in 2008, and who made it to the eight mile, will recall how tough that was, which I estimated at Force Five headwind (South Easterly).

Beaufort number Description Wind speed Wave height Sea conditions Land conditions Sea state photo
0 Calm < 1 km/h 0 m Flat. Calm. Smoke rises vertically.

< 1 mph
< 1 kn 0 ft
< 0.3 m/s
1 Light air 1.1–5.5 km/h 0–0.2 m Ripples without crests. Smoke drift indicates wind direction, still wind vanes.

1–3 mph
1–2 kn 0–1 ft
0.3–1.5 m/s
2 Light breeze 5.6–11 km/h 0.2–0.5 m Small wavelets. Crests of glassy appearance, not breaking Wind felt on exposed skin. Leaves rustle, vanes begin to move.

4–7 mph
3–6 kn 1–2 ft
1.6–3.4 m/s
3 Gentle breeze 12–19 km/h 0.5–1 m Large wavelets. Crests begin to break; scattered whitecaps Leaves and small twigs constantly moving, light flags extended.

8–12 mph
7–10 kn 2–3.5 ft
3.4–5.4 m/s
4 Moderate breeze 20–28 km/h 1–2 m Small waves with breaking crests. Fairly frequent white horses. Dust and loose paper raised. Small branches begin to move.

13–17 mph
11–15 kn 3.5–6 ft
5.5–7.9 m/s
5 Fresh breeze 29–38 km/h 2–3 m Moderate waves of some length. Many white horses. Small amounts of spray. Branches of a moderate size move. Small trees in leaf begin to sway.

18–24 mph
16–20 kn 6–9 ft
8.0–10.7 m/s
6 Strong breeze 39–49 km/h 3–4 m Long waves begin to form. White foam crests are very frequent. Some airborne spray is present. Large branches in motion. Whistling heard in overhead wires. Umbrella use becomes difficult. Empty plastic garbage cans tip over.

25–30 mph
21–26 kn 9–13 ft
10.8–13.8 m/s
7 High wind,
Moderate gale,
Near gale		 50–61 km/h 4–5.5 m Sea heaps up. Some foam from breaking waves is blown into streaks along wind direction. Moderate amounts of airborne spray. Whole trees in motion. Effort needed to walk against the wind.

31–38 mph
27–33 kn 13–19 ft
13.9–17.1 m/s
8 Gale,
Fresh gale		 62–74 km/h 5.5–7.5 m Moderately high waves with breaking crests forming spindrift. Well-marked streaks of foam are blown along wind direction. Considerable airborne spray. Some twigs broken from trees. Cars veer on road. Progress on foot is seriously impeded.

39–46 mph
34–40 kn 18–25 ft
17.2–20.7 m/s
9 Strong gale 75–88 km/h 7–10 m High waves whose crests sometimes roll over. Dense foam is blown along wind direction. Large amounts of airborne spray may begin to reduce visibility. Some branches break off trees, and some small trees blow over. Construction/temporary signs and barricades blow over.

47–54 mph
41–47 kn 23–32 ft
20.8–24.4 m/s
10 Storm,[6]
Whole gale		 89–102 km/h 9–12.5 m Very high waves with overhanging crests. Large patches of foam from wave crests give the sea a white appearance. Considerable tumbling of waves with heavy impact. Large amounts of airborne spray reduce visibility. Trees are broken off or uprooted, saplings bent and deformed. Poorly attached asphalt shingles and shingles in poor condition peel off roofs.

55–63 mph
48–55 kn 29–41 ft
24.5–28.4 m/s
11 Violent storm 103–117 km/h 11.5–16 m Exceptionally high waves. Very large patches of foam, driven before the wind, cover much of the sea surface. Very large amounts of airborne spray severely reduce visibility. Widespread damage to vegetation. Many roofing surfaces are damaged; asphalt tiles that have curled up and/or fractured due to age may break away completely.

64–72 mph
56–63 kn 37–52 ft
28.5–32.6 m/s
12 Hurricane-force[6] ≥ 118 km/h ≥ 14 m Huge waves. Sea is completely white with foam and spray. Air is filled with driving spray, greatly reducing visibility. Very widespread damage to vegetation. Some windows may break; mobile homes and poorly constructed sheds and barns are damaged. Debris may be hurled about.

≥ 73 mph
≥ 64 kn ≥ 46 ft
≥ 32.7 m/s

Tides For Swimmers, Part 2 – Local Effects

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Part 1 – Theory

So, why a different post for what we can call the observed local effects?

Well, depending where you are in the world, this may not be needed. There are some places where the observed local effects match those predicted by theory.

However, why isn’t that the case for everywhere?

Simply, the oceans aren’t uniformly shaped basins. They have obviously irregularly shaped coasts, and differing sizes and depths, all of which interfere with the tidal wave. Gravitational effects (lunar and solar) cause waves. But inertia and drag also interfere with the tides.

This is a map of global tides. We need to look at the Global situation to understand the local situation.

world tidal constituent

Whoa there.

Okay, how about this simpler-to-read one?

global tides

All right, that second one’s a bit easier to read. Forget everything else and have a look at the Atlantic. Notice there is a node, a black point, in the centre of the North Atlantic, south of Greenland, east of Newfoundland, from which lines radiate out?

That’s the Atlantic Amphidromic Point.

This means it’s the point in the Atlantic where the tidal range is zero, i.e. it has no tide, it’s all at the same tide height. The tides radiate out and away from that.

The further you go the greater the tidal range.

Each of those lines reaching indicate different tide heights. The further away from the amphidromic point the greater the tide height. Each of those lines also has a number associated which is the (very) approximate tide range.

Now go back to the coloured chart. See how one you get up to Europe the tidal range (colour) gets higher (a deeper colour)? Compare the difference between California (and the Catalina Channel) and the Atlantic Isles and the English Channel.

Now lets back up again. Imagine the Atlantic is a rectangular basin. You pick it up sloosh the water from on side to another. Assuming you are not spilling it out, and keep it moving, there’s a point somewhere toward the middle where the water is hardly moving up and down, just backwards and forwards. The water at the ends though will pile up against the basin for the “high tide”.

Now also think of the moving water. All the water doesn’t move at the same time and most of the water from either side will never get to the other side. So this affects the locality of the tide even within the basin.

Now imagine swishing the water and trying to add a circular motion. It gets more complicated.

This also then applies on a global scale. The tidal wave doesn’t all go from A to Z and back to Z every 12 hours. A to Z is a long way with lots of intermediate points. So lets the water from A goes to C, the water from B goes to D etc. Or better yet. The water from Kerry makes it to West Cork, the water from West Cork makes it to Cork, the water from Cork makes it to Youghal etc. So that means each area’s tide is at a different time, as you already know.

So that’s the whole Atlantic situation: Here’s an animation of the simple and complex (real) models.

Imagine this applying to Ireland. Think of that tide coming towards Ireland. It’s coming from west. So imagine it arriving as a single wavefront. Some flowing into the west coast, some flowing up around Donegal, across the north coast, some flowing across the south coast, around Carnsore Point and up the east coast.

Ok, the tide wave doesn’t cover all the area. As said previously it’s actually water moving back and away from the originating point. And then what? See it?

Ireland is an island, with water flowing around it. Logically that flow must reach around the coast and each flow start to interfere with other. Which is does. There’s actually an amphidromic point north of Ireland near Skye, and in the Irish Sea just south of the Isle of Man and yet Lancashire and Cumbria, on the NE coast of England, not that far away, have the highest tides at eight metres and Carnsore in Wexford, is just 1.75 metres. According to the Irish Cruising Club, there is an amphidromic point close to the east coast 25 miles north of Carnsore.

But we’re not finished. So you’ve heard of Capes, Cape Horn etc. Ok you channel swimmers, your favourite, Cap de Griz. Well in geographical terms, a Cape is a headland that significantly affects the ocean currents around it.

It does this, and also other headlands to a lesser extent, by essentially forcing the tide to flow through a smaller opening like putting you finger over the end of a tap. (Hello Channel tides!) Or maybe the coast “falls away” froma tide flow, having the opposite effect.

And then there’s the estuarine effect where a tide flowing into a estuary gets higher as it is forced further in. The tide here outside my door is about 7 metres, 20 miles from the sea, where is about 5m normally.

The Irish Sea, because of the narrow North Channel, also acts as estuary, forcing higher tides further north along it. In fact there are really varying effects in the Irish Sea.

Courtown for example sometimes has 5 tides instead of 4, with a tiny variation between them. The tides around Kilmore Quay also can be 8 knots. Some other places have similar very strong tides, while a few miles away there are negligible tides.

So lots of effects. In the case of the south coast of Ireland, the only bit I feel qualified and experienced enough to talk about, all this combines to produce a “lag” effect, where the slack portion of the tide is NOT necessarily at the high and low water marks, as we saw is the theoretical case in the first post. Instead, depending where along the coast you can be, slack water may be the middle of the tide or other times.

The charts to determine this are published by the South West Cruising club and printed every year in Reed’s Almanac, the yearly sailing almanac, so I can’t really post them here. PM me if you want them and i’ll send them on, though they are a bit complicated apparently first time you see them.

All of which serves to demonstrate that adhering, as so many swimmers do, to a theoretical tide idea should be tempered with acknowledging that it’s necessarily the truth.

For good tide knowledge, sailors and kayakers are the best. Like us they are at the mercy of tides, but are further out so their knowledge is often more extensive and their literature is far more developed.

Tides for Swimmers, part 1 – Theory

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One occasional bee I have in my swimming bonnet is the lack of understanding of tides amongst many OW swimmers. It is however quite understandable, if the location where you swim isn’t particularly affected by tides.

But what about expedition swim planning or trips to spots you don’t know? Or from a safety point of view planning a new participation swim? Or if like me you are prone to going solo. It’s good for swimmers to understand these factors.

In Dover, I spent an hour explaining tides generally, and how they affect the Channel, to two Italian relay swimmers, Martino and Michale (The first ever successful Italian relay team). In their case they swim in the Mediterranean, which is not very tidal, as any of you have been there will know. And they had no clue about tides. And it reminded me I was going to do some stuff here on tides months ago.

So we better start with the theory (don’t worry, it’s easy). Because some people who do know something about tides repeat the theory only, to the exclusion of local effects, or just don’t believe there are local effects. I learned this stuff when surfing and it was certainly my case when surfing, never realising that I was incorrect until I started swimming.

Ok. Whether you know it or not, tides are caused by the Moon AND the Sun. (A lot of people think it’s just the Moon), with the Moon being the main influence.

When the Sun, Earth and Moon are in a Direct Line, Spring tides are caused. (Pic 1. Spring Tides Diagram).

(These tides are called sub-lunar and antipodal).

(One of the biggest misunderstandings of non-coastal people is thinking Spring Tides happen in the Spring, instead of actually every two weeks).

In the case where you can’t see the Moon, that’s called a New Moon (another common misunderstanding) and the Moon is between the Sun and the Earth. Both gravitational forces, of the Sun and Moon, combine to pull the sea water away from the earth’s surface, the Spring Tides.
A Full Moon is when the Moon is on the opposite side of the Earth from the Sun, but the gravitational forces are still directly in-line.
Spring tides therefore happens approximately twice a month.

It’s “approximately” because the tide cycle follows the Lunar cycle and is around six and a quarter hours, meaning a Tide Period, one High and one Low tide, every twelve and a half hours. So you don’t have exactly four tides per day (24 hours), which is why you can’t go swimming on the same high tide every Saturday morning (or whenever) as the tides shift time gradually every day. Occasionally you get a second Full Moon within one calendar month, which is called a Blue Moon. (This is where the well known phrase originates.)

At the New Moon, the tides are called sub-lunar, while those on the other side of the world, away from the Moon’s closest influence are called antipodal, as the Moon’s gravitational effect is lessened and the Earth is pulled more than the ocean.

With Spring Tides, since the water is pulled MORE, that means the tides are both HIGHER and LOWER. The difference can be up to almost double the height. This is really important and I’ll come back to it at the end.

So the New Moon is the first Spring Tide. The second then occurs at the Full Moon. Full Moon is actually a Half Moon in lunar phase terms, that is, Second Quarter.

A Full Moon in NOT a New Moon!

So what about Neap tides?

Neaps also obviously then occur every other two weeks from the Springs. They happen when the Sun and Moon and Earth are at Quadrature, that is not directly aligned. This is around the First and Third Quarter periods of the Moon,.

The Sun and moon’s gravitational influence are acting in partial opposition (transverse) on the ocean, so the gravity pulling the water away from the surface is lessened. The Sun is pulling in one line and the Moon is pulling perpendicular to that direction.
(Pic 2 -Neap tides).

Ok. Got that?

Briefly:

Spring tides are higher and lower.
Neap’s and Spring’s cycle around each other with each occurring about twice a month.

Moving on… and back to the issue to tide height.

The single biggest misunderstanding of tides is just assuming that tides only go in and out, and therefore up and down. I think it’s a misunderstanding because again (non-coastal) people don’t pause to ask themselves what happens to all that water.

With tidal movement there is something called “The Rule of Twelfths”.

For this we average a tide into 6 hours.

In the first hour before AND after high (or low) tide, the tide will move one-twelfths of the total distance that it will move over the full 6 hour tide. So that’s 1 twelfth PLUS 1 twelfth equals 2 twelfths moved in two hours.

In the second tide before AND after high (or low) tide, the tide will move two-twelfths of the total distance that it will move over the full 6 hour tide. And that’s 2 twelfths PLUS 2 twelfth equals 4 twelfths moved in these two hours.

So for four of the six hours the die moves 2 twelfths plus 4 twelfths equals 6 twelfths which is only half the distance, in two-thirds of the time.

For the remaining 2 hours, which is the third and fourth hour of the tide, the tide moves three-twelfths in each hour, so it move the other half of the total distance in only one third of the time.

So…this means, that the tide is moving fastest in the central 2 hours of the tide.

Hour Distance (Speed)
1 1/12th
2 2/12ths
3 3/12ths
4 3/12ths
5 2/12ths
6 1/12th

So theory says: the tide is at it slackest, lowest or no movement, around high and low tides and the tide is moving at it’s quickest at mid tide. That speed is then directly related to whether it is Spring or Neap.

A Spring tide will have a greater range from higher to lower, so more water will move, so the speed of the running tide will be greater. A Spring High will be higher, a Spring Low will be lower.

For example a Spring Low will expose kelp at Tramore pier but a Neap Low Tide won’t.

In Ireland we have quite a big Tidal Range. Lowest Spring Tides are only 0.1 metres (above the Mean Datum of 0), while highest Spring Tides can be well over 6 metres. In the English Channel the highest is above 7 metres. The biggest tidal range in the world is 15 metres but the Mediterranean is only a metre or so!

So, almost there…with Spring Tides the two issues which you have to consider are:

  • Tidal Range. (Will it affect your ability to swim at a particular location, for example when it’s low tide, like at Sandycove?
  • Will the speed of the tide affect your ability to swim a particular distance in a particular time? (For example timing a swim across T-Bay or across the Waterford Estuary, or across the English Channel).

BUT…

Recall: this is Theory. It will apply directly in some places, but not in other because of local effects. This is very important. I cannot tell you your local topography and geography.

Part 2.

Understanding Waves, for Swimmers, Part 1

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If you want to swim Open Water more safely at the coast it’s a good idea to develop some understanding of various aspects of your environment. This is  a thumbnail sketch of breaking coastal waves for swimmers.

1: Waves are mostly caused by wind, either close or far-away.

2: There are many days without waves.

3: But new wave-fronts can arrive seemingly suddenly, regardless of local weather conditions.

4: It’s vitally important to have a general understanding of different waves.

When wind blows across water, it causes turbulence. The longer timer period and greater distance the wind blows over, causes greater turbulence, which leads to waves. The distance that the wind blows over is called the Fetch. An easterly wind blowing across Tramore Bay is only blowing over a fetch of five kilometers, yet can still cause very choppy water at the Guillamene, but this is a very short fetch. A long fetch may be hundreds of kilometers long in big storms in the Caribbean or Arctic, or, in the case of the Pacific, up around the Aleutian islands, leading to huge waves thousands of kilometers away in Hawaii.

Waves caused by close winds in the immediate area will be messy and disorganised, and will be close together. (A short period of much less than 10 seconds between them). this makes them more difficult to get past.

Waves caused by winds far away will have longer gaps between them and will be more potentially dangerous.

In Ireland that period can regularly be 13 to 15 seconds (and could in rare circumstances get up to 20/21 secs). These are called groundswell waves or just groundswell. Without a good understanding, groundswell looks more regular, but can be much more dangerous and unpredictable. Groundswell will generally be bigger and more powerful. As one wave catches up with a sightly different slower speed wave, their height will combine to result in a bigger, faster, more powerful wave.

A five meter wave catching up with a four metre wave will result in a nine metre wave. Groundswell gets more powerful the further it travels (the “fetch”) and the greater distance the wind travels to cause it. Waves hitting Ireland in the spring or autumn from a storm in the Caribbean will have a “fetch” of two and half thousand miles and can be very large and powerful. (I’ve seen an 11 metres swell on the West Coast back in 2002/3. It looked like mountains moving out at sea.) Note: -

If the wind is blowing to land it’s called “on-shore”. This will make the waves messy regardless of their origin. They will “close-out”, that is they will break all along the wave at the same time, and have lots of foam and whitewater in from of the wave.

Note: -

Wind blowing from land to sea is “off-shore”, the conditions sought by surfers. The south of Ireland is unfortunately prevailing on-shore southerly winds, while the north of Ireland is prevailing off-shore. Off-shore winds will cause any wave to “clean-up”. The waves will break gradually from left to right or from right to left, depending on the sea-floor under them. The face of the wave will be unbroken green or grey (for Ireland) water until it breaks. If the off-shore in on a groundswell, these will combine to give the biggest and most powerful waves.

Probably my own favourite wave location ever

Part two.