In order to understand properly and to make use of weather forecasts, it is necessary to have some knowledge of simple meteorology. As the yachtsman is essentially interested only in the direction and strength of the wind and the possibility of fog or poor visibility it does not require an expert knowledge, but merely an understanding of certain basic principles.
The weather in the sea areas of the Western Baltic is considerably influenced by the depressions which form on the Polar front and move across the area in an easterly or north-easterly direction. These depressions usually have ridges of high pressure between them. The low pressure areas are caused by warm air rising and cooling, bringing with them rain and strong winds, while high pressure areas are caused by air from the upper regions dropping towards the earth, the associated weather normally being dry with light winds.
The air movement round a low pressure area travels in an anti-clockwise direction (in the Northern Hemisphere) and increases in strength towards the centre, while round a high pressure area it travels in a clockwise direction and decreases in strength towards the centre. From this knowledge an important rule, known as Buys Ballot Law, may be learned and understood. This states that in the Northern Hemisphere an observer facing the wind have low pressure on his right and high pressure on his left.
Another important and helpful rule is called Cross Winds Rule. This rule lays down that if an observer with his back to the surface wind has the upper wind, indicated by the movement and direction of high cloud, coming from his left, the weather can expect to deteriorate, and if coming from his right it will normally improve.
Due to gravity the air exerts a force called atmospheric pressure, expressed in units of measurements called millibars. The average atmospheric pressure at sea level is about 1000 millibars, written as 1000 mb. On a weather or synoptic chart, the line joining the points of equal atmospheric pressure at any given time is called an isobar, and as the wind is caused by air moving due to changes of pressure, it follows that a stronger wind will be experienced where the isobars are closer together, while light winds can be expected where they are widely spaced.
The function of the barometer is to indicate visually changes of atmospheric pressure as they occur, while the barograph will also record these changes of pressure. Changes of pressure should be recorded at regular intervals so that a barometric gradient may be built up. The greater the gradient, the closer will be the isobars and thus the stronger the winds. When the barometer remains steady more settled conditions can be expected, although there will always be some slight variation in pressure during the day even during long settled periods. This is known as Diurnal Wave. Gale warnings are normally issued when a rise or fall of 10 millibars in 3 hours has been recorded, but even a change of 5 millibars in the same time will usually herald a strong blow.