A little knowledge goes a long way in understanding the nuance and detail contained in a weather broadcast, says Chris Tibbs.
When ashore we get our forecasts from a variety of sources, nowadays largely through the internet. However, unless we have the long-range communications of a bluewater cruiser, once our link to the land is cut and mobile phones are out of range we still largely rely on the shipping and inshore waters forecasts.
In UK waters, these forecasts are generally delivered via BBC Radio 4. The same forecast, reduced in size to cover just the relevant sea areas, is repeated by the Coastguard on VHF and through Navtex. Many countries follow a similar format broadcasting coastal as well as offshore forecasts.
We know that particular words in maritime forecasts have particular definitions and these can be found in the almanac or online. Particularly relevant are the definitions about when the forecast conditions will arrive: ‘imminent’ means within six hours, ‘soon’ 6-12 hours, and ‘later’ after 12 hours. Bear in mind that this means six hours from the time of issue; we may be listening to the forecast a fair few hours later.
The list of definitions is quite long, but there are a number of points that are not covered in the standard list.
Wind forecasts are for a standard ten-metre height. Wind strength generally increases with height, because the surface friction effect becomes less significant, so measuring the wind speed at the top of a high mast will therefore give a slightly higher reading than a low mast.
It is also worth thinking about averages. Wind speed is generally averaged over a ten-minute period, while gusts are over a three-second period. Onboard wind instruments typically sample over a much shorter period, giving more variable readings.
Some instruments allow us to change the damping, which is useful. Some also allow speed calibration, but anemometers are factory set and generally very accurate, so I think best left alone.
Understanding atmospheric stability
Even in a generally stable atmosphere there is nothing unusual about seeing gusts 30 per cent above the average speed, so take note if a forecast specifically warns of gusts beyond these parameters. An unstable atmosphere that features rain squalls can give much higher gusts.
Showers come from cumulonimbus clouds and will be accompanied by increased winds which, in the case of thunderstorms or tropical squalls, can easily be gale force. The clues are all there in the forecast: in the event of showers and thunderstorms, there is no need to add that it will be squally, because it will be by definition.
If the forecast is for rain, however, you can infer that it is coming from layer clouds. With less vertical movement in the atmosphere, rain will produce fewer gusts of wind than showery conditions.
This is where weather charts and the general synopsis are useful: knowing where we are in relation to the sectors of a weather system helps to add depth to any forecast. A little knowledge goes a long way in this respect.
For coastal sailing, the inshore waters or coastal forecast is the primary source. The heating and cooling of land greatly affects the weather all around the coast but, on a more local level, different landmasses create different effects by blocking, funnelling and generally disturbing the wind flow.
A forecast covering long stretches of coast cannot take into account all the variables and will only ever be an estimate. The weather will be different in various places along the coast with the strongest wind in the forecasts generally around the headlands and where funnelled. When interpreting a forecast, look at your passage plan alongside the coastal geography to help anticipate where the strongest winds are likely to be.
Offshore, things do get somewhat easier as your timing and positioning within the various weather systems is what will make the biggest difference to your sailing conditions.
Top tip: visibility matters
A marine forecast is particularly useful for predicting visibility, which usually extends over the whole of the sea area it covers.