All competent sailors need to have a good basic understanding of how to interpret a weather forecast. They also need to be able to interpret the actual conditions they are experiencing.
Most weather forecasts present a general picture of what to expect in a given area over a period of time. We rely on such forecasts to provide basic information, but the actual weather and sea conditions we experience don’t always tally with the general view.
This is not necessarily the forecasters’ fault, as it is the local tides, topography and sea breezes that interact with this basic picture and give rise to the actual conditions we experience out on the water. To make more accurate predictions of the weather in your specific area requires more in depth knowledge and skill.
This section through a depression has wind moving from left to right. High cirrus is followed by layers of cloud which gradually become lower as the warm front passes, bringing rain. Thick stratus cloud in the warm sector is followed by heavy rain and eventually cumulus cloud appears, eventually giving way to sunshine:
A typical depression in the northern hemisphere moves in an anti-clockwise direction with low pressure in the centre:
A cold front is approaching with towering cumulus and heavy rain. Squalls are common accompanied by high winds exceeding 20 knots:
It is up to the skipper to make a calculated interpretation of the available forecasts and to decide whether it is safe for their vessel and crew to go afloat. At the same time by studying the weather conditions they can check:
- Where in the forecast weather pattern they actually lie?
- Has the predicted front passed through yet?
- The current wind strength. Has it changed through the last few hours?
- The atmospheric pressure. Is it rising or falling?
Sources of weather forecasts
If you are planning a trip, it is best to gather as much reliable information as you can. Sources of weather forecasts include:
- Weather apps – I use PredictWind, Windfinder and Windy. There are many others.
- National meteorological offices.
- Internet – good source for GRIB forecasts (Gridded Information in Binary files),eg UGrib, weather charts, web services eg Windguru also back-up to VHF, NAVTEX, INMARSAT-C and SSB radio.
- MSI (Marine Safety Information) broadcasts on VHF and SSB radio.
- Public service broadcasts on radio and tv.
- NAVTEX, Weatherfax and INMARSAT-C.
- Harbour and marina offices.
Caution: The internet is not part of the MSI system and should never be relied upon as the only means to obtain the latest forecast and warning information. Access to the service may be interrupted or delayed from time to time, updates may also be delayed. Refer to GMDSS services, INMARSAT SafetyNET or NAVTEX for the latest information. When using these web pages, check that the page on your screen is not from your cache. Use the Refresh button if in doubt.
Compare a number of pressure charts to see how the weather patterns have been forecast to evolve in your area over a number of days. This will help you to judge when fronts will pass through, what local conditions will be as a result and how this tallies with your planned departure and route plan.
Before you go
Study your charts and estimate when and where you can expect wind against tide. Are the elements going to clash severely and if so when is this going to happen? Will you need to avoid being in that area or will the conditions be manageable?
Always study the weather forecast thoroughly before going afloat in order to remain safe and to make an efficient passage plan. Before departure, download forecasts to cover the anticipated length of your passage. Be prepared to delay your departure or change your destination if the weather forecast is unfavourable.
Once committed to going, ensure that you have the means to get regular, reliable weather forecasts if you are at sea for any length of time. This will allow you to change your plans and head for a safe haven if the forecast is for stormy weather, and to take advantage of detailed weather information when planning your route.
- Study the Beaufort scale and use it to judge wind strengths.
- Practice how to interpret barometric pressure charts.
- The barometer is arguably the most useful forecasting tool. Keep a note in the log to monitor change in barometric pressure.
- Learn how to observe cloud formations to forecast the weather.