Obsessive Weather Watching

A common activity for people awaiting a Channel Swim is obsessive weather watching.  Weather conditions can make or break a swim.  These days, there are many sources of information for the anxious Channel-Swimmer-To-Be to obsess over.

Earlier this year, Helen and I crossed the Strait of Gibraltar (Link) on an excellent day.  The days before the crossing were windy, and from the wrong direction, which would have created a very nasty ‘wind over current’ chop situation.  The day after we got there, the wind was blowing at 43 miles per hour out of the east.  The days after the swim were similarly bad, as the wind once more swung round to come out of the east.

Tarifa Weather the Week of Our Crossing

Tarifa Weather the Week of Our Crossing

But we did get to swim, in the short period of calm as the winds switched from easterly to westerly. We even got a slight push from the north, helping to propel us over to Morocco in an unexpectedly fast time (3 h 16 minutes).

Typically swimmers will try and avoid wind if possible.  Some years in the English Channel there are long periods of settled weather, with light winds, normally associated with an extended period of high pressure.  This year there has been very little of this sort of weather.  Swim ‘windows’ have been short, resulting in an unusually small number of swims, and an unusually large number of unsuccessful crossings.  People look on Windfinder, and yearn for the Beaufort F1/F2 winds which herald ideal conditions for solo crossings.  There have not been many of these this year.  Here is the Windfinder forecast for the next 3 days.

Dover Forecast for the Next 3 Days - A Little Windy

Dover Forecast for the Next 3 Days – A Little Windy

Lots of F3/F4/F5 here.  Not what you want to see.  Also note that the wind and wave directions are also not favouring the crossing direction from England to France, which is NW to SE.

Yesterday was an interesting case in point.  On paper, it didn’t look like an auspicious day, other than being on the bottom of a neap tide.  Winds were forecast at F3/F4 all day, yet 12 Channel Boats were out, nearly all of which escorted their swimmers successfully to France.  While it was windy, the wind and swell were both at the swimmers’ feet, helping them to whizz over to France in fantastic times.  One of these was Jim Clifford, who became the Oldest Triple Crown Swimmer (Triple Crown is the English Channel, Catalina Channel, and MIMS), who bolted over in a magnificent 10 hours 3 minutes!

Jim's Track

Jim’s Track

In terms of weather, Helen didn’t have it easy on her recent crossing of the Menorca Channel Swim (Link).  The first half of the swim there was some persistent and annoying chop/swell coming at her over her right shoulder, disrupting her natural rythmn.  Only after 5 hours or so did the wind settle down, and she could relax more in her stroke.

Pressure to Swim

For some swims, the swimmer can find themselves under pressure to go under less than optimal conditions.  If you are a Brit who has a 1 week slot booked for Gibraltar, and arrive to find a week of adverse winds, you either agree to go under adverse weather conditions, or you get weathered out, shrug your shoulders and go home.  Similar dilemnas face non-UK nationals over to swim the English Channel, who can find themselves sitting in Dover looking over at France, as the wind blows day after day.  As your tide ‘ends’, you then lose your position in the pecking order of swimmers, and throw yourself on the mercy of your pilot and the weather.  Under these circumstances you might accept the opportunity to go on a marginal weather day, and take your chances.

I was in that position last year, having lost my pilot due to ill health.  I chose to go on a day when only a few boats out of the possible 12 went out (the sure sign of an iffy day), and on a spring tide, which some people try and avoid.  As it happened, I had a nice day, not flat by any means, but perfectly swimmable with the training I had done.  That’s the maxim, hope for the best, train for the worst!

The Catalina Channel

….or more correctly the San Pedro Channel between LA and Catalina is generally a different story.  The weather patterns are generally predictable.  You book a certain day many months ahead, and you nearly always get to go on that very day. There is generally very little wind during the night and morning, before the Santa Anna wind gets up in the afternoon, nearly always blowing onto the mainland between F3 and F4 for a few hours, before dropping away in the early evening.

Taking account of this, swims generally start around midnight, so most swimmers can get done before lunch the following day.  Here is the forecast for that part of the world for today.  Textbook stuff.

Classic Catalina Wind Pattern

Classic Catalina Wind Pattern

On top of the wind, the other factor that the swimmer has to contend with is the current.  In the English Channel this manifests itself in the form of a periodic tide change.  The swimmer gets washed up and down the Straits of Dover, the direction changing every 6 hours or so.  They will always be facing in the same direction (France), which is where the pilot hopes to land them after a certain number of hours based on an estimate of their swimming speed.  This is why nearly all English Channel swim tracks describe a serpentine path from England to France.  The speed of the swimmer, and the precise timings and magnitudes of the tides can only be estimated at however, which is why most swims don’t land up at Cap Gris Nez, the closest point of France to Dover.

There are tides in Catalina, but they are of smaller magnitude.  On a neap tide, they are so small that they can be effectively ignored; one of the pilots John Pittman, steers an arrow straight path between the Island and the Mainland, asking the swimmer to swim against any side currents or tides that might occur.  In the English Channel this approach just doesn’t work; even the fastest swimmer will have some curve on their path to France.

There is another opportunity for the weather-obsessive Catalina aspirant to scare themselves.  There is another resource at sccoos.org which allows you to observe surface currents in Southern California.  Here is a screenshot taken just now, with the swim path superimposed, showing cross currents of about 1 kph in the middle of the Channel.

San Pedro Channel currents on September 8th

San Pedro Channel currents on September 8th

Some elementary maths tells you that a cross current of 1 kph will reduce an unaffected swim speed of 3 kph (about my pace) down to about 2.8 kph.  As the current get stronger, and/or the swimmer gets slower, it gets worse, much worse.  A tiring swimmer travelling at 2.4 kph with a cross current of 1.5 kph (just under 1 knot), will have their forward progress cut down to a paltry 1.9 kph.

So can currents be actually useful?  Well certainly they can if they are from behind, more precisely in the quadrant centred on the swimmer’s axis, the closer to that axis the better.  The pilots take currents into account, and try to choose a swim direction where the currents are from behind, more often than not Catalina to Mainland.

However, if you are doing a 2-way crossing, there is even more reason to be obsessive about the currents, because there are even fewer ways to be a winner.

Let us imagine a 3 kph swimmer swimming a 33 km channel.  For simplicity of maths, let’s imagine he doesn’t slow down or feed.  It takes 11 hours to complete the crossing in still water, and 22 hours for a 2-way.

With a 1 kph cross current, the 2-way takes 23.3 hours.

With a 1 kph current directly from behind in one direction, and on the nose in the other, the 2-way now takes 24.75 hours.

Our strategy will be to make the first leg the harder one, while I will be relatively fresh.  I will aim to swim against any head current that might exist, making it easier on the way back, because currents become more and more of a hindrance the slower you swim, as many a swimmer trying to get in to France will tell you!

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