Being Weatherwise for Sailing (Part 2)

"And now the shipping forecast..."
There are many forecasts available for sailors before they set to sea. All these forecasts are written by the meteorologist using the most recent data available and is the best picture they can give at that time. It is always worth noting that most forecasts are often very general as they have to cover such a wide area, particularly the shipping forecast. Also land-based forecasts will often give wind speeds less than those that you will see on the water, due to many things, including friction as we discussed in the previous article, and will discuss in more depth later.

Reporting Wind Speed
Wind speed in mariners' terms is measured using the Beaufort Scale, each force corresponding to a wind speed range and in turn a water state.

Wind is also reported in miles per hour or kilometres per hour on many land based forecasts. So when looking at television and newspaper forecasts make sure you check the unit of measurement. Miles per hour is similar to knots (1 knot = 1.15 mph), but a forecast of 30 kph may sound strong but it would actually be a nice sail in 16 knots of breeze (1 knot = 1.85 kph). You can use the calculators on the BBC Weather site to convert these.

The BBC also use arrows, which are range from thin to thick as wind speed increases, and they also change in colour depicting warm or cool winds. Green or blue arrows depict cooler air which when coming down from the north flow over increasingly warm sea surface temperatures. The lower level air is heated which rises and is then replaced by cooler air aloft as it sinks which, on the water gives gusty conditions as the air is 'unstable' and 'turbulent'.

Rain later. Good.
In 1853 Captain Robert FitzRoy was put in charge of forming the Met Office to provide meteorological and sea current data to mariners. The Met Office continues to provide this information in the form of the shipping forecast and the inshore waters forecasts that you can listen to on BBC Radio 4 or read on this website. At first these seem quite daunting, especially when heard being read at pace on Radio 4, however once understood and read at one's leisure they can give you a good overall picture of what is currently happening and what to expect.

The shipping forecast gives a general synopsis which explains where the highs and lows are and where they are expected to travel to over the next twenty four hours. It then proceeds to go around British waters and give the wind direction and strength followed by weather and visibility. There are a few things which you need to remember; backing is anticlockwise and veering is clockwise, or left and right looking upwind. When wind directions vary cyclonically it means that the low centre is coming close by and wind directions will vary by a large amount in a cyclonic or anticlockwise direction.

Following the shipping forecast in the radio bulletins is the coastal forecast. This brings the synoptic scale down to a more local level. The areas are much smaller but the format is the same; wind direction and strength, weather and visibility. This forecast will take into account more of the effects of the land, such as where winds funnel and increase through channels, such as Dover in a westerly, and where winds will increase as they are forced around headlands, St Davids Heads in a south to south westerly, or decrease if in the lee of high ground such as Thames Estuary in a light north westerly.

Tides - When sailing in coastal waters, particularly those such as the Solent or Bristol Channel, you should always remember that a tide pushing you into the wind will add up to 3 or 4 knots onto the present wind speed, and subtract it when it is wind with tide. Check UK Tides on this site.

In addition to the shipping forecast there are many other sources of information. Next week we will look at where to find them, which are the most useful and also how to read the synoptic charts in more detail.