The evolution of atmospheric pressure is one of the deciding factors in weather forecasting. Below 1.015 hPa, we talk of depression... Above this “magic line”, anticyclones and nice weather.

For the majority of stations which include the "barometric pressure" function, the pressure display is based on the mean value of at least 10 atmospheric pressure readings. The calculation of this mean is carried out after every reading.
If a new pressure represents a difference of 1 hPa or more, compared to the previous mean, this new pressure serves as the new reference value.

To ensure an accurate reading of the barometric changes, any weather station must always remain at a constant altitude- if not at a fixed point.

For example, it should not be moved from the ground floor to the upper floors of a building. If such a change turns out to be necessary, you must restart or configure the settings and ignore the readings of the following 12 to 24 hours. This gives the station time to function at a constant altitude and thus produce accurate readings.

This function, unique to certain weather stations only, allows you to obtain a greater barometric accuracy by adjusting the hPa sensitivity to the user’s outdoor environment, such as marine or high altitude areas in which atmospheric pressures are very different from one another.

In areas experiencing frequent and sometimes considerable changes in pressure (which doesn’t necessarily reflect a change in the weather), the level of sensitivity may be lower than in an area where atmospheric pressure is stagnant... And where there is rather more the need for a very subtle sensitivity.

The standard displays of atmospheric pressure on the weather stations are in hPa (Hecto-Pascal). We can of course on some stations display this in in/mg but this is essentially for North America only

On the majority of appliances, it is possible to choose between absolute pressure display or relative pressure display.

Absolute atmospheric pressure is the pressure in the strictest sense at the time and place of the reading and cannot be graded.

Relative pressurerequires manual setting. Atmospheric relative pressure is a value calculated at sea level using the local absolute atmospheric pressure and can therefore be taken as a reference for the atmospheric conditions and the evolution of weather in the whole country (thus, an absolute atmospheric pressure of 961 hPa in Munich at an approximate altitude of 600 metres above sea level corresponds with a relative atmospheric pressure of 1021 hPa at sea level).

Relative pressure is also the value indicated by the different radio and television stations during their daily weather forecasts. If a weather station has to be graded, it is recommended that you obtain the relative atmospheric pressure at the time of setting, from your local radio station or local weather service.

The trend is calculated using the record of atmospheric pressure recorded by the barometric sensors. Trends are visually translated by the displaying of arrows which combined with traditional weather symbols, allow for a forecast to be made.

Some Weather Stations have a « Rain Meter » function; which allows for the creation of a record function for the “non-cable” rain meters. This function depends on an EEPROM memory.

The EEPROM memory does not become empty in the absence of a power supply, which means it can store data over a long period of time.
Please remember: the reset of records must be done manually, even when changing batteries.

Depending on the appliances, the atmospheric pressure records displayed on the LCD can take into account the last 24, 30, 48 and even 72 hours. The calibration of these records is done in hPa and only absolute pressure is taken into account.

The geographical curve of the barometer indicates the record of atmospheric pressure for the period chosen.

For example, for a station having a record of the last 30 hours, the display is in 9 stages: 0,-1,-3,-6,-12,-18,-24 and -30 hours. The bars correspond to each of the 9 stages and indicate the trend for the period recorded. The curve gives a very visual and telling comparison of the results.
The "0" in the middle of the curve determines the surrounding atmospheric pressure. Each change (±1,±3,±5 and ±7) indicates in hPa the increase or the decrease in past pressure in relation to current pressure..

Bars which rise indicate an improvement in the weather given an increase in atmospheric pressure. Bars which go down indicate a decrease in pressure and a worsening weather situation as of "0" from the moment.

At precise times, the atmospheric pressure of the moment is used as a basis for the display of a new curve. The curve then moves by one bar to the left. This explains the permanent succession of record curves.