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Understanding the weather

British Sailing Team meteorologist, Simon Rowell, explains how understanding the weather can help out on the water

One of the good things about sailing, especially if you race, is that the field of play changes every day, every beat, every run as the weather changes. This means that getting a good forecast for the day can really help. However, much as I’d love to think otherwise the best forecast in the world is never going to be able to tell you in advance which way to go on the second beat. Why do we bother then?

By understanding what’s happening with the weather and understanding the basic mechanisms which drive each day you can make much better decisions on the water. Once you get reasonably good at sailing the mechanics of tacking, for example, get to be second nature, almost a background task – you don’t have to think through the various steps involved, it just happens. Likewise if you put effort into understanding how the weather is going each day then this will help make your tactical decisions quicker and, hopefully, consistently better.

Let’s look at a large scale example. If you know a warm front’s coming through in the afternoon, forecast for around 1500, and you’re sailing an offshore race then don’t plan everything to happen just before 1500, put a reef in, change headsails, and then when it doesn’t come through till 1700 sit there and bob along slowly. A much better way is to understand what the oncoming front will bring (thickening, lowering clouds, with rain usually just ahead of the front itself) and then keep a weather eye out for these changes. This way if the system does slow down or speed up you’re on it anyway.

On a much more local scale, let’s say you’ve got an offshore wind with a windward/leeward course set up just downwind of a shore with several valleys leading down to the sea. If you look at the forecast and compare the wind direction with where it’s going to come off the land you may well find that two adjacent valleys will have the effect of funnelling wind down the course in two slightly different directions, depending how the wind starts to flow down them. This will lead to seemingly random shifts left and right over the course. With this knowledge though, you may be able to say that the wind on a particular side of the course will come mostly from a particular direction, and that could well be advantageous. With an offshore wind a few minutes looking at a topographical map or overhead photography can really help understand how the wind flows over the inshore sea areas.

Lastly, don’t forget that computer model forecasts are just that – forecasts. Modern ones tend to be very good, but they’re still not great at detail, and the smallest thing that can be forecast is about 4-5km across. This means that a sea breeze cell may just get forecast, but may not – this is the scale you need to remember. Sugar Loaf in Rio was the single most important feature – but it was not big enough to show up on even the highest resolution forecasts.

The mark one eyeball is still the most useful immediate forecasting tool! The sailor who reads and understands the forecast then keeps their head out of the boat to understand and interpret what’s going on has already given themselves an advantage for the day.

Find out more about The British Sailing Team at https://britishsailingteam.rya.org.uk/

Want to learn more about the weather? Pick up a copy of the RYA Weather Handbook available in print and eBook formats - www.rya.org.uk/go/shop


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