THE DIVING IS BOOKED and the weekend is fast approaching. The one big doubt is this: Will the weather hold out
Both divers and the skipper follow weather forecasts through the week and, depending on how far the divers have to travel, and how clear-cut the decision is, the skipper gives a go or no-go decision any time from a couple of days before to the morning of the dive.
The traditional source of marine weather is the shipping forecast. Over marine VHF radio, every few hours,
the local Coastguard gives both this and the inshore-waters forecast for the immediate shipping areas and coastline.
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...or will the sea look more like this There are ways of finding out
With a weekend dive in mind, the 5:54 Radio 4 forecast on Friday afternoon gives a pretty good idea as to whether Saturdays dive will go ahead.
In general, force 5 is the limit for diving. All dive-boats are capable of handling bigger winds and waves, but the problem is kitting up on a rocking boat and then, more importantly, being able to see and safely recover divers after the dive. This can vary greatly from one boat to another.
Some boats give a more comfortable journey in a big sea, while others may be more stable at low speed or at rest.
The type and position of a ladder or diver-lift can also make a big difference to recovering divers. The height of the helm above the sea affects the spotting of divers or the following of delayed SMBs.
Charter skippers know their boat and their area and, even without the many other forecasts available on the Internet, can use the Friday-evening forecasts to make an informed decision on the weekends diving.
They have a good idea which sites can be dived from their boats and, in the event of higher wind speed, which they can push a bit for diving to go ahead.
Both the shipping forecast and the inshore-waters forecast have a couple of limitations from the point of view of planning a dive trip. They generalise weather to quite large areas or regions of coastline, and they look up to only two days ahead.
To get round this, numerous websites forecast up to a week ahead, and in far more detail than the shipping forecasts. Favourites are reviewed here.
But beware, lots of pretty graphics and maps does not make the forecast any more accurate. The more detailed a forecast, the more likely that detail is to be slightly wrong. The further ahead a forecast looks, the more likely it is that it will change before the day arrives.
There used to be a saying along the lines of the weather forecast could be 12 hours or 100 miles out.

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WE CAN APPLY A SIMILAR STRATEGY to any weather forecast. Rather than just the day in which were interested, look at the day before and the day after to see what we could be in for if the forecast is wrong by a day in either direction.
Similarly, by looking at the forecast for surrounding areas, we can see what we could be in for if the forecast is out geographically.
We can also get a bit more technical, and guess at what lies beyond the forecast. Weather predominantly arises from low- and high-pressure systems in the atmosphere. These systems grow, move and fade away.
Around Britain and anywhere in the northern hemisphere, winds circle anti-clockwise about low-pressure systems, and clockwise about high-pressure systems. The strength of the wind depends on the pressure gradient.
All this is summarised on synoptic charts, the maps with rings of isobars plotted about low- and high-pressure areas, and fronts marked as radial lines spreading from low-pressure areas.
Such charts are available from websites up to a week ahead, and can be interpreted to give a forecast of the weather and wind to come.
Rather than get buried in the technicalities of interpreting synoptic charts, we can use a few rules of thumb.
Weather patterns follow the movement of low- and high-pressure areas. If a low-pressure system in the North Atlantic is moving east towards Norway, a reasonable prediction is to look at the weather forecast for areas west of where well be diving, and anticipate that this weather will move towards us at a similar rate to the movement of the low-pressure system.
We see an animated form of this on TV weather forecasts every evening, but unfortunately TV weather is more specific about temperature, sun and rain than wind speed and sea state.
If a low-pressure system is described as deepening, the pressure gradients about it are increasing, as is the wind.
If it is described as filling, the pressure gradients are decreasing, like the wind.

VARIOUS FORECASTING SERVICES around the world share their basic data, measurements of wind and pressure, humidity and temperature. They then put all this into big computer models of the atmosphere, to derive what will be happening in the future.
The data may be much the same but computer models may not be, and the weather services then put a certain amount of human interpretation on top of that. So forecasts for the same region can vary between services.
The answer is to look at several. If they are the same, perhaps we can have greater confidence; if they differ, less so.
On Monday morning, we can use a variety of forecasts to work out if we can dive the following weekend, and where it could be best to go.
As the week progresses, the forecasts usually change slightly, and should be more accurate. Come Friday, the go or no-go decision should be left to skippers, who generally have a better feel for the local forecast than we do, and know the capabilities of their boats.

THE SHIPPING FORECAST
hspace=4 Updated by the Meteorological Office four times each day, the shipping forecast gives wind speed on the Beaufort Scale, and other weather information for regions of sea around Britain, from Iceland to Spain.
Critically for those at sea, it is broadcast on Radio 4 at 12:48am, 5:20am, 12:01pm and 5:54pm on 198m long wave, with some of the transmissions also on VHF.
With a good antenna, long wave can be received over all the shipping regions, whereas VHF doesnt reach much further than the horizon from the transmitter.
As well as using named sea regions and the Beaufort Scale of wind speed (below), some other words in the shipping forecast have special meaning.
When the wind is veering, it is changing direction clockwise, while backing is changing direction
anti-clockwise.
Anything described as imminent is expected to happen within six hours, if not already.
Soon means from 6-12 hours,and later means more than 12 hours away.
Other terminology is defined for visibility, movement of weather systems and wave height.
See www.metoffice.gov.uk or www.bbc.co.uk/weather/coast/ shipping

THE BEAUFORT SCALE
Created by Admiral Sir Francis Beaufort in 1806, the Beaufort Scale has become the standard wind scale for marine use in Britain and many other countries.
At the time, there was no easy way to measure wind speed accurately in knots or other precise units, so for at least a century previously, navigators
had adopted the practice of assigning a simple scale to observable effects of the wind, such as the shape of waves.
What Beaufort achieved was to define a single standard for such scales, ranging from a perfectly calm 0 to a hurricane-force 12.

Beaufort
number
Wind speed
(knots)
Wave height
(metres)
Sea conditions
0 (Calm)Up to 10Flat
11-20.2Ripples without crests
23-60.5Small wavelets. Crests of glassy appearance, not breaking
37-101Large wavelets. Crests begin to break. Scattered whitecaps
411-152Small waves with breaking crests. Fairly frequent white horses
516-203Moderate waves of some length. Many white horses. Small amounts of spray
621-264Long waves begin to form. White foam crests very frequent. Some airborne spray
727-335.5Sea heaps up. Some foam from breaking waves is blown into streaks along wind direction. Moderate amounts of airborne spray
8
(Gale)
34-407.5Moderately high waves with breaking crests forming spindrift. Well-marked streaks of foam blown along wind direction. Considerable airborne spray.
9
(Severe Gale)
41-4710High waves, crests sometimes roll over. Dense foam blown along wind direction. Large amounts of airborne spray may begin to reduce visibility.
10
(Storm)
48-5512.5Very high waves with long overhanging crests. Large patches of foam from wave crests give sea a white appearance. Considerable tumbling of waves with heavy impact. Large amounts of airborne spray reduce visibility.
11
(Violent Storm)
56-6316 
12
(Hurricane)
More than 64  


INSHORE-WATERS FORECAST

Both wind speed and direction can be changed close to land by the degree of shelter from that land; by friction between the wind and the land; and by air rising over the land.
Therefore the inshore-waters forecast is a second forecast applying up to 12 miles offshore. This is updated twice each day, and is broadcast only on Radio 4, after the 12:48 shipping forecast.
The inshore-waters forecast is given in terms of stretches of coastline, with specific headlands separating them. The 24-hour forecast is followed by an outlook for the following 24 hours. See www.metoffice.gov.uk or www.bbc.co.uk/weather/coast/inshore

USEFUL WEBSITES
hspace=4 XC WEATHER - www.xcweather.co.uk
XC Weather is a site originally created for paragliders that has become a favourite with many sea-users because of the clean simplicity of its interface. Just look at the map, pick an arrow close to where you will be diving, hover the cursor over it and you get the forecast for the next five days.
You can set preferences to show wind speed on the Beaufort scale. Wind data is colour-coded from blue for calm through green, yellow, orange and red for storms.
Options swap from the default display of wind speed to temperature or weather displays. Tabs switch to
forecast maps, sequenced overview maps and animations.
There is a version of the site optimised for mobile phones, and an iPhone application. Applications for other phones are promised.
hspace=4 WINDFINDER - www.windfinder.com
A very comprehensive site for windsurfers, with map-based indices, forecast maps and tabular forecasts for the next seven days. Forecasts are colour-coded with wind speed, but the speeds are in knots and there is no Beaufort information.
There is also a mobile phone version of the site, and applications for Android and iPhone.
hspace=4/WIND GURU - www.windguru.cz
Another site developed for windsurfers and kitesurfers, while other sites show the data from a single weather model, Wind Guru presents forecasts from a host of models.
This, the sites worldwide coverage, and emphasis of the names of windsurfing locations, can make it difficult to get to grips with, so having found your location, it is best set as a browser favourite.
Wind Guru gives a seven-day forecast in tabular format, with an option for the Beaufort scale.
It doesnt have the easy map overview of other sites.
A map interface based on Google maps is available only with the paid-subscription Wind Guru Pro package.
hspace=4 METCHECK - www.metcheck.com
Metcheck is a very comprehensive site with a seven-day forecast, just one of the forecasts available as a free taster for the subscription services offered.
The seven-day forecast is a general one presented in tabular format with wind speed in mph and a colour-coded background, though it is not clear what the colours mean. For the big picture, synoptic charts are also free for the next five days, but you need to be able to read them.