Weather Forecasting for Yachtsmen

So on a more optimistic note let us look at a few facts:

Two items of equipment considered vital in weather forecasting are the barometer and the thermometer.

A mercurial barometer in its simplest form is merely a glass tube closed at one end and filled with mercury. It is inverted and its open end placed below the level of the surface of the mercury in a small cistern. The mercury now descends in the tube until the weight of the column is balanced by the pressure of the atmosphere on the mercury in the cistern, thus representing a correct measure of such pressure. By means of a scale of millibars, whose zero is level with the surface of the mercury in the cistern, the exact height of the column can be recorded.

The thermometer of course, is used for measuring temperature of sea water and ambient temperature.

WINDS The primary cause of all winds is a difference of temperature which in turn, owing to hot air rising causes a difference in pressure over two areas, and the air flows from the regions of high pressure to the regions of low pressure.

WIND SYSTEMS ARE EITHER CYCLONIC OR ANTI-CYCLONIC

Cyclonic systems are those which blow around an area of low pressure, and generally travel quickly. They are usually accompanied by bad weather. They blow anti-clockwise in the northern hemisphere and clockwise in the southern hemisphere. Anti-cyclonic winds are those which blow around an area of high pressure; they travel slowly and are generally accompanied by fine and settled weather. They blow clockwise in the northern hemisphere and anti-clockwise in the southern hemisphere.

These days there is an abundance of information available on weather, so it is up to us to make the best of it. Radio; television; websites; it's all there, most of it provided by satellite. But for 'yotties' a lot of the fun of the sailing game is studying the weather and attempting our own forecasts, and sometimes scaring the life out of ourselves while we're at it! I can speak from experience when I say we often get caught out by marine forecasts; staying in harbour through an expected force 8 that doesn't materialise, then being swept by force 8 gusts while nicely cruising in 3 to 4.

	BEAUFORT'S SCALE

Nautical Miles per Hour	Wind Force	Description
0-1	0	Calm
2-6	1-2	Light Airs
7-10	3	Light Breeze
11-16	4	Moderate Breeze
17-21	5	Moderate Breeze
22-27	6	Strong Wind
28-33	7	Yachtsman's Gale
34-40	8	Gale Force
41-47	9	Strong Gale
48-55	10	Storm Force
56-65	11	Severe Storm
Above 65	12	Hurricane

As already stated, there are many sources for forecasts; MARINECALL 0871 200 3985

They provide a comprehensive service including MMS and SMS direct to your mobile phone with:

• Individual coastal locations
• Mean wind speed and direction
• Maximum temp in Celsius
• Likelihood of rainfall and visibility

Then of course we have BBC radio four and their time honoured shipping forecasts.

How many sailors have lain in their bunks and listened to the evocative music of 'Sailing By' that preceded the 0048hrs forecast then dropped off into an easy or uneasy sleep depending upon what they had just heard? Further shipping forecasts on radio four are; 0545 - 1200 - 1754.

So, one wonders why should anyone bother to learn about the subject? Well of course it is individual choice isn't it, but it should be just as much a part of small craft sailing as knots and splices, bends and hitches, or sail handling. After all, we don't always have these modern methods to hand to do it all for us, just as, if suddenly our GPS decides to take a holiday, would we be able to navigate safely back to port? Therefore it would seem to be common sense to be able to read the weather from all/any signs available.

SEA BREEZES are created by the movement of air from sea to land. The land heats up more quickly than the sea therefore causing the air to rise drawing in air from the sea.

LAND BREEZES. By evening the land begins to cool but the sea holds its warmth longer so the air above it rises drawing in air from the land.

There are also certain physical features that are well worth taking into account. High cliffs can create a barrier against the sea breeze causing it to become erratic or 'hit and miss', whilst prominent headlands can cause an increase in wind strength, so it's worth giving them a wide berth if necessary.

You will notice that if the isobars on the weather charts shown on television forecasts are close together then this will indicate strong winds. Widely spaced isobars generally indicate fine weather. If you see a scattering of little puff ball cotton wool clouds across a blue sky you can go sailing because the weather will be fine with light winds. This is usually called a 'trade wind sky.'

If anyone is interested enough to want to take the subject further there are many good manuals on the market, but one of the best is 'Reading the Weather' by Alan Watts, and published by Adlard Coles.

He has thirty-five years sailing experience, most of which were spent cruising the Irish Sea in his Macwester Seaforth, a thirty six foot ketch. He currently lives in the Isle of Man, and owns and maintains a Mirror Offshore, 19'6', his sixth boat.

Email: hcrossley@manx.net