Navigation briefing: How to use weather observations to improve your forecasts

Yachting World

All sailors use weather forecasts (and probably endlessly debate their accuracy), but weather observations are ‘ground truth’ and don’t lie, writes Mike Broughton

I have some tips for maximising the real-time information you can see, both online and using the naked eye.

For instance, if sailing west across the Solent you can utilise wind observations by checking the data from the Bramble Bank beacon or the starting platform at Lymington and have a really good idea of what wind speed and direction to expect. If racing, these observations can be ‘gold dust’ information to help gain from a shift just a mile upwind.

Superforecasts

Forecasts have improved immensely over the last few years and the UK Meteorological Office has recently announced a project to build a £1.2 billion supercomputer for 2022. The present one cost £97 million and takes 200 billion weather observations every single day. Across the UK, forecasts are currently broken down to squares of 1,500m (300m over London). The ambition for the new computer is to have a nationwide resolution of just 100m.

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Weather observation stations are a critical part of this improved forecasting accuracy, and there are currently 270 Met Office synoptic stations dotted across the UK. But there are also many stand-alone observation stations such as airports and marine weather stations, which tend to transmit data that is accessible for mariners.

To be a recognised meteorological observation station there are strict criteria to be fulfilled, such as clear area around the station and wind readings are taken at 10m above ground level (about the height of the mast for a 24-footer). Due to friction effects most surface winds and GRIB forecasts are also for 10m above ground level.

If your mast is at 30m then you should expect to add about 5-10% to the 10m height forecasts, but there are many factors involved here depending on the stability, density, temperature and local topography.

The difference in wind speed with height is known as the wind gradient (not to be mixed up with gradient wind), while the accompanying change in direction is wind shear. Sail and mast design take into account wind gradient and shear. The wind speed reduces nearer to the surface due to land friction and increased density.

Also helpful for sailors are the ever-increasing number of extra wind stations that have been put online for public access. Some can be viewed at weatherfile.com, a site created by Lymington dinghy sailor Richard Paul Russell. He sells solid-state anemometers that connect online and have been used by Olympic sailing teams, as well as building contractors and the military.

Modify and adapt

Due to topography, you’ll often need to modify wind observations. For example, the height of the anemometer at Hurst Castle is 20m, while close by the Lymington Starting platform reading is taken at about 8m, and therefore the recorded wind speeds are less.

Wind observations at Bournemouth airport ten miles to the north-west are also lower than Hurst Castle, mainly due to being further inland, while the airfield also has high fir trees surrounding it.

At St Catherine’s Point on the south of the Isle of Wight, cliffs north of the anemometer tend to funnel and accelerate the winds to mostly show as coming from east or west, so we need to mentally modify the readings we see.

I find it easiest to see wind flow when observations are displayed on a map with arrows, rather than just a list of weather stations.

I remember a well-established, slow moving high-pressure system over the UK for the Round the Island Race. I kept a check on how the winds changed at St Catherine’s through the preceding two days. On race day, the sea breeze filled in bang on my forecast time allowing us to make a great gain on our rivals.

Tidal height also has an effect on the wind at the Bramble Bank beacon: we see slightly lower readings at high water compared to low water when the top of the beacon is about 4m higher above sea level.

Similar tools can often be accessed at other destinations, for example in the Bay of Palma the airport wind station is a useful tool to monitor the wind, as the airport is just inland from the coast.

But traditional observations still have their part to play: one of the early indications of a summer sea breeze for sailors in Palma is when the planes change landing direction – often around 1230.

In the 1997 Volvo Ocean Race the entire fleet was becalmed on day two of the second leg close to South Africa, when one keen-eyed crewman on Swedish Match observed horizontal smoke from a distant ship to the south. After heading that way Swedish Match pulled out a huge lead, going on to take their only leg win in the race.

Wind observation resources

Mike-Broughton-Headshot-400x400About the author

Mike Broughton is a pro race navigator who has won many titles including World and European championships. He is a qualified MCA Master to captain superyachts and previously had a successful career in the Fleet Air Arm flying Sea King and Lynx helicopters.

First published in the August 2020 issue of Yachting World.