Being Weatherwise for Sailing (Part 3)

Reading Between the Lines
You can use maps and charts to help you understand the weather and therefore to plan your sailing. These can be found on the almost endless internet sites providing weather forecasts, charts and observations from around the world. They range from giving a basic picture-based outlook for your area, to the actual current observations, to the model output itself. You are already reading one of the most organised and informative sites!

A good place to start is with the synoptic scale chart, or the pressure chart, which will give you an idea what the broad scale weather is and how it is likely to change over the next few days. Watch the weather on TV and follow the pressure charts through the week to see how fast things are changing; where are the fronts? How fast are they moving?

How strong will the winds be?

A mean sea level pressure chart shows the direct relationship between isobar spacing (pressure gradient) and orientation, and the strength and direction of surface winds. The general rule is that winds are strongest where the isobars are closest together. Thus the strongest winds are usually experienced near cold fronts and low pressure systems. Winds are normally light near high pressure systems where the isobars are widely spaced.

The synoptic chart shows lines of equal pressure to which the surface wind blows about 15 - 30° backed from parallel. To infer a wind speed from these lines, take your dividers or mark off on a piece of paper the distance between the two closet isobars to your sailing area. Then read off the wind speed at your latitude from the scale in the corner of the chart. This is the actual (if taken from an analysis chart) or the predicted wind speed at about 500m, the wind on the water is around 20 or 30% less.

• The Met Office animates the charts but these do not have the geostrophic scale on.
• weather.noaa.gov - then click on SIGWX for the required time period
• westwind.ch - If you click on ALL it will take a while to download but you will be able to animate all the forecast charts.

Warm fronts and cold fronts are marked by red semicircles and blue triangles respectively on the synoptic weather charts. As the cold front catches up with the warm front it pushes it's way underneath the rising warm air and the air masses begin to mix, this is marked by an occluded front on the weather map, a semi circle next to a triangle.

One thing to look for when studying the synoptic chart is not only how close the isobars are which gives an idea of wind strength, but also by how much does the direction of the isobars change around the front. A strong 'V' around the front will indicate an active front, however if the isobars bend little around the front it is likely to be less active with lighter winds, less rain and not large veer behind the front.

One very good site to trawl round is westwind.ch, it's a collection of all the weather sites you could wish for, from written forecasts to the actual model output. The AVN model will give you a more detailed forecast of the wind over the course of the day. Wind speeds are displayed using wind barbs. The angle shows the direction and the speed is shown in 5 or 10 knot barbs, short or long respectively. The synoptic charts can also be found under UKMO and Brack.

As well as searching on the internet and watching the weather on TV, newspapers show the weather charts and radio, especially BBC Radio 4, give regular weather reports. Local radio stations often give the sea forecast and sea conditions particularly over the weekend or holiday periods. Once out on the water the coastguard relays the Met Office coastal forecasts, with an announcement on Channel 16 that will advise you which channel to tune into. All times and other sources of national and local forecasts and tides can be found in your almanac.

• For actual observations, on xcweather.co.uk you can look up current winds to see what's coming your way.
• wendywindblows.com is a subscription site but you can access observations online or by phone.

There are far too many sites to list them all, but here are a few I suggest you visit:

• ecmwf.int


• westwind.ch


• greatweather.co.uk

Sailing sites with many links to weather include:

• madforsailing.com


• yachtsandyachting.com


• uksail.com


• ukdinghyracing.com

Plus the many others already linked to via this BBC site.

Next week we will take all these forecasts and knowledge and begin to condense it down to what we will see on our local waters, and introduce some local effects, which you can forecast yourself.