It's essential to foresee the weather framework, and to locate yourself depending on LPS or an another system(anticyclones, LPS). For a full overview's analysis you must mix informations given by this tool and sky's shape(clouds type, visibility), synoptic wind's direction and sea state, you may also add changes in temperature or humidity. But even alone, it is a needful tool to predict weather and for safety.
1)Atmospheric pressure
The barometer indicates atmospheric pressure in hPa. This pressure depending on air weight on a given point has been discovered by a Galilée's apprentice, Evangelista Torricelli, with his invention of the mercury barometer and inspired by Benedetto Castelli, an another student of Galilée specialized in hydrology. To sum up Torricelli realized that the atmosphere's weight created an almost constant pressure on the surface's earth, and this pressure was in fact quantifiable with fluids, malleable under a force.
we almost find the same thing in scubadiving where pressure increases of one atmosphere's earth every 10 meters roughly(the sea surface pressure is 1 bar, in fact about 1013 hpa, and at 20 meters it is 3 bar, in fact about 3040 hpa) the more we go deep the more water's weight above our heads is strong. Fortunately, air's weight is much less heavy!
Knowing that a mass of warm air is less dense so lighter compared to a cold air mass, less light so “heavy”, you soon realize the barometer's usefulness to anticipate LPS, fronts or anticyclones(decreasing pressure may indicate a warm front coming, increasing pressure, a cold front). In fact if you see the barometer falling it will usually indicate the arrival of a LPS, if it rises it will indicate the arrival of anticyclonic conditions.
Atmospheric pressure can be illustrated with the ideal gas equation:
Pressure(pascals)*Volume(in m3)= Mass(in Kg)*R * Temperature(in Kelvin)
R is the gas constant, R value for dry air is 288 and for water vapor 460 roughly. 0 Kelvin = -273.15 ° C.
2)Barometers types
There are mercury barometers, water barometers, gas barometers(they use a fluid's deformation under pressure to find data). On board we mainly find brass barometers called “aneroid barometers” and electronic instruments that operate with a vacuum capsule's deformation.
Aneroid barometers use a hermetically closed box, empty of air. A membrane fixed on this box moves under pressure and a mechanism transcribes data obtained on a dial with a needle.
3)On board
To adjust a barometer with accuracy, you may catch atmospheric pressure data at the closest harbour's office, and add the following correction:
Correction(hpa)= 0.12 * height difference(in meters)
The height difference is between our barometer and an another used as a reference. If our barometer is lower than the reference barometer correction is added.
Pressure changes more or less fast are very important, on each line written in the logbook we have to write those barometer changes too. The more pressure changes quickly , the more wind may rise in strengh(this is true both for a decreasing or an increasing pressure, main idea is the speed of change).
Under our midlatitudes(about 45 ° of latitude), we may say roughly:
.A change of 5 hpa in 1 hour announces at least 50 knots(well then it's better to put away fishing rods and take a drink to the next pub…).
.A change of 3 hpa in 1 time announces about 40 knots.
.A change of 3 hpa in 3 hours announces about 25 knots.
If we are lower in latitude, for the same rate of pressure changes, there will be more wind; if we are higher in latitude, for the same change there will be less wind. I guess that this is brought by Coriolis which is less powerful when you approach equator.