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.