WEATHER BRIEFING

CHRIS TIBBS ON FORECAST ACCURACY

is expensive and there’ll be a cost for the user.

Wind forecasts are for a standard 10m height; at 10m the effect of the surface pays a big part causing drag which slows and backs the wind (veers in the Southern Hemisphere). Any tall mast will read a greater wind speed than that felt at 10m or deck level, but as the relationship between wind speed and height is not fixed – but depends on the stability of the atmosphere – a certain amount of estimation is involved.

The most extreme case I have experienced was sailing on the Baltic in very cold water.

This cooled the air and made it very stable at low levels; it felt calm at deck level yet the anemometer at the masthead at 25m was reading 15 knots.

WHAT FORECASTS DON’T TELL

Forecasts tell us an average not a maximum.

On a ‘normal’ day gusts are likely to be around a one third greater than the average. Some forecasts will warn of gusts half as much again as the average.

A squall is a longer increase in wind and usually associated with precipitation. As these are generally associated with showers and thunderstorms they are unlikely to be picked up in a forecast for a specific area. We sometimes get a percentage chance of showers in a forecast; this helps give a general impression but does not tell you whether you will actually see a squall. Boats sailing just a few miles apart can experience quite different conditions.

GRIB files may indicate 20 knots of wind but in reality we will get some gusts and lulls; this could easily give gusts into the mid-20s and a small squall could increase the top end of the wind range further, while a thunderstorm could produce squalls of gale force intensity.

There has also been a profusion of spot forecasts on various apps. These can be good for general weather but for wind forecasts they’re not so accurate as they will also be greatly affected by land features. While giving a flavour for the weather they are not as useful as a marine forecast as other sources.

To generate an accurate forecast needs time and a certain amount of knowledge. The more time we spend adding value to them the more accurate they become. Most apps are purely computer generated and they are not a substitute to a marine forecast which will cover additional variables, for example visibility which can be hard to forecast from limited information.