Atmospheric pressure

The atmospheric pressure is the Pression of the Air in an unspecified point of a Atmosphère.

On the Earth, the average atmospheric pressure with the sea level depends primarily on the mass of the atmosphere, this one being able to evolve/move with the average mass of gases with variable concentration like the steam. It remains close to the standard atmosphere, being worth 101 325 Pa.

The atmospheric pressure is measured using a Baromètre, of a Hypsomètre or a Altimètre. It was measured a long time in mm Hg (then in Torr) because of the current use of barometer with column of mercury. Since the adoption of the Pascal like unit of pressure, the meteorologists use a multiple of this unit, the hectopascal (1 hPa = 100 Pa), new denomination of the millibar (1 bar = 100.000 Pa).

Vertical variation

The atmospheric pressure decreases with the Altitude: it decreases, exponentially, of a factor 10 each time one rises of 16 km. It is thus possible to use the pressure to measure the height, which is the basic principle of the Altimètre used in aeronautics.

In meteorology applied, the pressure is often used directly like vertical Coordonnée. One will speak for example about the temperature with 700 hPa. This approach has technical advantages and it simplifies certain equations used in meteorology.

Horizontal variation

The meteorologists analyze the horizontal variations of the atmospheric pressure to locate and follow the weather systems. In particular, the depressions and the hollows in the atmospheric pressure are generally associated with the bad time.

The difference in pressure between two of the same points altitude, or horizontal Gradient of pressure , is the most important driving force of the Vent.

In order to use the pressure to follow the systems weather and to estimate the force of the wind, it is necessary to make agree measures of pressure which were taken at various altitudes: at sea, in the valleys, in mountain, on the shores of the Dead Sea.

With this intention, one subjects rough measurements of pressure to a standardized adjustment. The value resulting from this adjustment is called pressure with the sea level , or PNM . If one takes for example the case of a station located at 100 meters above the sea level, the adjustment will be carried out by estimating the pressure with the bottom of a fictitious hole, 100 depth meters, that one would have dug at the station. More precisely, the value of the PNM is function of the pressure measured at the station and the temperature assigned with the fictitious airstream. For the latter one uses the current that and average temperature at the station measured twelve hours before.

The PNM is an approximation of a great utility, but it is necessary to take care not to give him all the value of an exact physical measurement, particularly in mountainous ground.

The pressure measured on the ground is used for the calibration and the validation of the data coming from weather instruments of remote measurement. Precise measurements of pressure are thus a base necessary for important progress which takes place in this moment in the field of the observation of the Earth.

The atmospheric pressure measured with the sea level varies around a median value of 1.013 hPa.

Values records

PNM: 1086 hPa, with Tosontsengel (Mongolia), the 19 December 2001.
PNM: 870 hPa, with broad of the Filipino , close to the center of the Typhoon Tip, the October 12th 1979 **.

** Of the lower pressures still were recorded within violent Tornade S, but these measurements remain controversées.
** One allotted a minimal pressure of 868,5 hPa, on April 23rd, 2006 with 7:15 UTC, the center of the , when it struck in the north of Maningrida, in Australia. This measurement is not-official and controversée.

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