Weather: A Mix of Pattern and Chaos

Jessica Watson
Posted February 22 2017

Favourable weather is a key ingredient to a good day on the water. In fact, I’d go so far as to say that a sailor’s life revolves around the weather. But favourable weather can be elusive and tricky to predict, so I thought I’d ask an expert to explain the mystery that is weather.

And Kiwi Bob McDavitt is a very well credentialed weather expert. He qualified as a meteorologist in 1975 and has been forecasting ever since. He’s a veteran of two America’s Cup campaigns, helped a certain girl in a pink boat sail around the world and was New Zealand’s MetService ambassador for 20 years.

The Art of Forecasting

Bob explains that forecasters firstly gather observations of what’s currently happening. Then he explains, ‘We transfer these into a matrix of dots in a computer model, then use a mathematical model to run an experiment to push the data stored in this matrix one time-step into the future, and again another time step and so forth.

‘A marine forecast is framed in terms of winds, waves, and weather. But these all come from isobars on a weather map, and these isobars are what really capture the pattern of the weather and its changes.

‘Marine forecast come in two timeframes,’ says Bob. ‘One covers today/tomorrow, and the other is an outlook for next few days.’ Because ‘real weather’ deviates from the ‘captured pattern of the observations, the outlook period is always less reliable than the short-term forecast.

‘Thanks to advances in computer processing and mathematical modelling, the accuracy of the forecasts has increased a lot over the last few decades, but this pace of improvement is easing off now,’ he says. ‘Weather forecasting will never reach perfection.

‘The winning formula is to take the forecast and then tweak it – using your own observations to ascertain possible alternatives,’ an approach that Bob tells me is not only common sense but also consistent with scientific principles.

An Inexact Science

However, Bob stresses that weather forecasting is an inexact science and explains that there are two reasons for this. Firstly, ‘the matrix of dots averages out the weather. Any extremes that occur between the dots is lost. And so, as the time-steps get further into the future, this averaging loses the pattern.’

And, secondly, ‘weather is a mixture of pattern and chaos. In chaos theory, slight changes in initial conditions can give result to an ensemble of different outcomes. Each run of a global computer model can only come out with one outcome. We produce an ensemble of outcomes by varying the initial conditions slightly, and these ensemble forecasts explain more of the possible future than does one computer run.’

Bob asks that sailors treat weather forecasts as an ‘idea’ and remember that ‘in the real world the weather will unravel away for the forecast, and after a while, the forecast is no longer what we get’.

Keeping a Constant Watch

Bob suggests that sailors should also establish a routine of watching the environment and can ‘use technology to arrange alarms for any measurable external changes’. The main things he suggests we should watch for are changes in the barometer and the clouds, which he suggests are ‘useful signs as they start changing within hours of wind changes and act as a herald’.

He explains that it’s important for sailors to understand and anticipate ‘events that may do damage’. For example, the conditions that breed squalls are in the forecast. ‘The actual timing or intensity or duration of a particular damaging squall will not be mentioned in any routine marine forecasts.

‘I’m still waiting for someone to invent a barometer with which the user can get target alarms. Meanwhile, we should learn to read the clouds and the colour of the sunrise/sunset,’ says Bob.

Bob offers a weather information service for cruising sailors and is currently helping Lisa Blair navigate the Southern Ocean’s ferocious weather as she circumnavigates Antarctica.  

20 people found this helpful. Do you?Thank Jessica Watson


Write your reply...
Jack and Jude
Deckee Pro  Posted February 23 2017
Thank you Jess and Bob for that helpful explanation.

In the early days we could only watch the barometer, the speed that it moves both up or down indicating strength of wind, and the swell for an increase or change in direction as it's the harbinger of storms.

Today we add weather forecast images from three sources, captured and saved for the expected duration of the voyage plus a couple of extra days. And then we compare what were getting to evaluate whether to trust the remainder of the forecast and see which source is closest.

Sometimes systems move slower or faster than expected and this can be judged by comparing the data with what we're getting.

The bureau's MetEye is fairly good short term, WindTV is also and of course we download grib files from ZyGrib to overlay on our OpenCPN charts. A very useful tool for route planning.

The rest is like Bob says, watch the skies, know what each cloud type means.

5 people found this helpful. Do you?Thank Jack and Jude
Posted February 24 2017
Great Article, I am new to boating and not sailing at this point. Weather is crucial for a good day on the water. What is the consensus on Weather app for the closest or best modelling for weather. Something like PredictWind?
3 people found this helpful. Do you?Thank Jacques
Captain Crayfish
Posted March 10 2017

You ask for an opinion. I have just submitted the following to Deckee:

What has been a handy planning tool for me and an essential app for many has just hiccuped.

My Nexus 7 running Android 5.1.1 got an "update" Version of Predictwind thru the Google store and now my tablet is unable to run wind maps on Predictwind.

Their support team have tried to assist but gave up saying I had to revert to the old Version (they supplied the download).

I guess this is an example (caveat) of using technology as a tool. Sextants, barometers, and other hardware went on forever, well almost anyway, whilst we have become the test benches for, and the victims of the rapidly diminishing useful life of tech hardware and associated software (apps).

Constantly we are commanded to login, logout, update, purchase up too date hardware and software always on the presumption that we are in a position to do so - whether by virtue of internet access or endless cash resources. Software seemingly developed by none real people, aficionados of minecraft and facebook and other toys as their CIC's battle for supremacy in the "latest release of partially working releases fraught with bugs and faults requiring frequent and untested "new releases".

Quote from direct Predictwind support "It is the technology we use which can go across all platforms that doesnt like the older:) android os. If people are on a satellite of limited connection it wont update any way."

Well that's comforting......
2 people found this helpful. Do you?Thank Captain Crayfish
Jessica Watson
Posted February 28 2017
Thanks for adding your experience Jack and Jude. I'm willing to bet you guys would be pretty good at reading the sky by now!

Jacques, I'm glad you enjoyed the post. I've used and found PredictWind to be good, their models have a good reputation among many racing sailors. And as Jack and Jude said for short trip's MetEye is also hard to fault and is free.

I see there's a review of the Seabreeze weather app over on Deckee's product reviews - Please write a review and share your experiences with any other weather app's you've used, I'm sure there's plenty of other sailors who will appreciate your thoughts.

Best wishes for your sailing adventures!
2 people found this helpful. Do you?Thank Jessica Watson
Learning Curve
Posted October 28 2017
Hi Jess,

Bob mentions ‘I’m still waiting for someone to invent a barometer with which the user can get target alarm". Good news. My wife bought me this Sunnto core watch that has this an altimeter, barometer, compass , sunrise/sunset and calibration features. It was only a few hundred dollars. My only gripe is the backlight isn't very bright but it has so many features and works. Cheers
1 person found this helpful. Do you?Thank Learning Curve

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