5.1 Weather Forecasts
What we want to know
Mariners need to know slightly different facts about the weather than, say, farmers. We are not particularly interested in quantities of rainfall, or frost, but the following are very important to us:
Wind - its strength and direction. From this we can work out: how rough it is likely to be; how difficult boat handling will be; and where it is likely to be exposed or sheltered. Sailors can also work out how they can sail from A to B (i.e. what point of sail they will be on) and how easy thay journey is likely to be.
Visibility. Apart from rough seas, poor visibility is probably our greatest fear. It makes it harder to see where we are, and harder to see other vessels in order to avoid hitting them. Dive boats may also have difficulty finding their divers, and fishermen their gear.
Sea state. How rough it is, or how big the waves are. This can largely be deduced from wind information, but it may also be susceptible to other factors: swell (large long waves that originate a long way away) and the wind that has been blowing for the last few days, causing the sea state to build up.
General weather. Less important, but it is nice to know whether it's going to rain, and more importantly whether there will be squally showers which generate strong local wind.
Weather (particularly in the UK) is frequently changeable, so we also want to know when the conditions described are likely to apply. We may be interested in longer range forecasts so that we can plan our sailing for the next few days, and finally, to aid our understanding of what is going on, it is helpful to know what wide area weather features are causing our weather, and how they are going to develop.
Sources of Forecasts
The Almanacs, cruising guides and other publications contain up-to-date information on the weather forecasting services available to mariners. Naturally these vary from country to country.
The media by which forecast information can be received moves ever forward with technological development, and now include local and national radio stations, telephone, fax, VHF radio, NAVTEX, "ordinary" internet access, WAP, SMS messages, fax broadcast by HF radio, internet access by satellite, etc etc. It's a good idea to look at any or all of these, to see which ones suit your location, equipment, type of sailing etc. The type of information you get will be very similar in each case (see above), but the forecasts may have originated from different forecasting organisations and therefore may differ.
For "ordinary" UK coastal sailing I recommend the following (all of which are free):
From home: access the Met Office inshore waters forecast on the internet. This is (currently) a 48 hour forecast updated three times a day, and includes warnings of gales and strong winds.
At the marina or harbour office: a copy of the inshore wates forecast, and/or a dial-up fax from Marinecall (associated with the Met Office), which should give similar information in words, and a chart showing surface pressure and fronts.
At sea: broadcasts from the Coastguard at three-hourly intervals on VHF radio, giving gale and strong wind warnings, inshore waters forecast and (less frequently) the shipping forecast, for the local area. These broadcasts also include new gale warnings which are very important, because they may supersede and invalidate a previous, less severe, weather forecast.
Terms Used & Interpretation
Forecasts need to be clear, concise and precise and therefore certain words that are used in them are given very specific meanings.
Wind
Wind direction is given as the direction the wind is blowing from, which can be a little confusing as other directions we are given - tidal stream for example - are usually the direction something (water, in the tidal stream case) is going to. Also, the directions are usually given in terms of north, south, east and west etc rather than degrees from north (which is the usual modern convention for direction). The reason for this is that forecasts are necessarily approximate, and "north-west" gives a better feel of the approximation than "315". Most people are familiar with this usage, and we are happy to talk about a "north wind" or a "south-east wind".
Changes in wind direction are described using words with specific meanings:
veering means changing in a clockwise direction, e.g. south to west
backing means changing in an anticlockwise direction, e.g. north-west to south-west
cyclonic means considerable change in wind direction as a depression passes through the area (see section 4 on Wide Area Weather for an explanation of this).
Wind strength is generally given in Beaufort Scale. This was developed in 1805 by Admiral Sir Francis Beaufort. Force 0 is calm, and each strength number, increasing from 0 up to 12, corresponds to a range of wind speeds and an observable "look" to the sea. The following table gives the speed (in knots) and term for each force number, together with my own description for sailors.
Force |
Knots |
Term | Description |
|---|---|---|---|
| 0 | less than 1 | calm | glass calm, no sailing! |
| 1 | 1 - 3 | light air | small ripples |
| 2 | 4 - 6 | light breeze | small wavelets |
| 3 | 7 - 10 | gentle breeze | very occasional white wave crest |
| 4 | 11 - 16 | moderate breeze | longer formed waves, some "white horses" |
| 5 | 17 - 21 | fresh breeze | bigger waves, frequent white horses, spray onto boat - reef sails |
| 6 | 22 - 27 | strong breeze | get all reefs in - quite lumpy, especially sailing to windward |
| 7 | 28 - 33 | near gale | uncomfortably large waves, spray begins to be blown in streaks |
| 8 | 34 - 40 | gale | breaking waves blown into well marked streaks |
| 9 | 41 - 47 | strong gale | for yachts, we're now in survival conditions - max wave height 10m |
| 10 | 48 - 55 | storm | max wave height 12.5m |
| 11 | 55 - 63 | violent storm | max wave height 16m |
| 12 | over 64 | hurricane | mean wave heigh exceeds 14m |
Some forecasters use wind speed - in knots, miles per hour of kilometers per hour - instead of Beaufort Scale. You can use the table above to get an idea of the conditions at various windspeeds.
In practice, most people will develop an idea of the wind strengths they would rather avoid. For relatively inexperienced coastal sailing, anything above Force 5 may be difficult to handle, requiring deep reefs and probably involving seasickness. Very few people willingly set out in Gale Force 8 (we may have to deal with a gale at sea, but try to avoid them) and Force 6 to 7 is known (by wise yacht skippers, and rather more disparagingly by professional seamen) as a "yachtsman's gale".
Time
Every forcast has a time when it was issued, which should be noted down. The following terms then relate to that time:
imminent means within 6 hours of the time of issue
soon means between 6 and 12 hours of the time of issue
later means more than 12 hours from the time of issue
These terms are particularly relevant to Gale and Strong Wind warnings.
Visibility
Visibility is also given using words with specific meanings:
good means more than 5 miles
moderate means between 2 and 5 miles
poor means between 1000m and 2 miles
fog means less than 1000m
Sea State
Again, descriptive terms with specific meanings. The following are the wave heights for the various sea state terms (the height in question refers to the average of the highest 1/3 of the waves, NOT the maximum wave height - which could be very much bigger)
smooth is wave height less than 0.5m
slight is wave height of 0.5m to 1.25m
moderate is wave height of 1.25m to 2.5m
rough is wave height of 2.5m to 4m
very rough is wave height of 4m to 6m
That's enough for us, because above this we are in survival conditions for leisure craft. High, very high and phenominal are used to describe wave heights from 6m to over 14m.
Other Terms
Other terms with specific meanings are used to state the speed at which the barometer is rising or falling at a particular location; and the speed at which a pressure system is moving. These are less important for the average yachtsman, and can be found (along with the ones above) on the Met Office website.
Accuracy of forecasts
Weather forecasting is a complex and inexact science (or art?). Forecasters are gathering large amounts of data and trying to forcast what weather patterns will form, how they will move, and what their characteristics will be (we're talking about wide area weather here - see section 4).
Forecasts have improved over recent years, because the necessary data has become easier to gather, and computers for modelling what is going to happen have become more powerful. But fundamentally, weather is a difficult thing to predict - and impossible to predict to a precision which will satisfy everyone (you know the grumble: "they can't even tell us whether it's going to rain or not", on a day when there is a heavy slow-moving shower in one place and beautiful, fair, sunny conditions 2 miles away).
The following considerations may help when you are trying to work with weather forecasts:
start understanding their language and what's going on. A bit of understanding about weather patterns (section 4) will help you to appreciate what is being described and what is likely to occur at each stage of, say, a frontal passage. You will start to recognise the underlying events driving the weather, so you will still be able to use the forecast even if the weather system's actual progress is a bit quicker or slower than forecast.
don't expect micro-forecasts. On a given day, conditions can be quite different from one locality to the next, and you don't get mile-by-mile forecasts! Understand local weather (section 3), and you will understand some of these differences: it's windier round exposed headlands and calm in sheltered bays! (Not really rocket science, is it??)
the longer range the forecast, the less accurate it will be. Five day forecasts are handy, and will often give you enough accuracy for general planning - but expect the forecast for days 3 to 5 to change quite a lot as they approach, and always get the updated forecast as soon as it becomes available.
sometimes the unexpected happens. With the best will in the world, it can sometimes become apparent that things are not developing as forecast (this is not surprising in a complex weather situation: the data is many hours old by the time the forecast is issued). This can sometimes result, for example, in a gale warning being issued at pretty short notice. In my experience the previous forecast may not be explicitly rescinded - and may even be repeated. I happen to think this is poor procedure, but the important lesson is, listen to the issue times: heed the gale warning, and if it is issued after the "current" forecast, disregard the earlier forecast if it was for more benign weather.
If possible, always get the synoptic chart as well as the narrative forecast. This will help in your education (you get an experienced forecaster's view of the actual conditions that will accompany a particular weather pattern) and will help you to understand the "big picture" - enabling you to cope better if the narrative forecast doesn't seem to be quite accurate.
