Atmospheric pressure

The air pressure at any place of the atmosphere is the hydrostatic pressure of the air prevailing in that place. This pressure caused by the weight of the air column, the one located on the ground surface or is located on her body. The average air pressure of the atmosphere ( atmospheric pressure) at sea level is standardized at 101 325 Pa = 1 hPa = 013,25 101.325 kPa ( mb = millibars ).

• 5.1 Meteorology
• 5.2 Aviation
• 5.3 Marine
• 5.4 Boiling point of water

Properties and effects

The Earth's atmosphere has a mass of about 5.1015 tons on every square meter of the earth's surface (about 5108 km ² or 5.1014 m²) So weigh about 10 tons. The pressure ( force per area ) at sea level is 105 N / m. As you climb to higher altitudes allowed a portion of the column of air between them - even to the one with the greater air density, because the density decreases as the air pressure with increasing height (see barometric formula ). Thus, the weight of a lower air mass acts, so the air pressure decreases.

The man has no sense organ for the perception of air pressure. Only rapid and sufficiently strong fluctuations in air pressure ( lift, cable car, tunnel passage, airplane takeoff / landing ) can be perceived as a feeling of pressure in the ear when the ventilation channel ( Eustachian tube ) of the middle ear will be closed during the pressure change. Is there a difference in pressure between the middle ear and environment, then this is balanced once opens the Eustachian tube (eg can be provoked by chewing and swallowing ).

The weight of the air, which displaces a body, causing the buoyancy, which must be taken into account when determining a precise mass by weighing. The weight of the displaced air, and thus also the buoyancy will depend on the air pressure.

Units

The unit used internationally ( SI unit ) the air pressure is the Pascal ( unit symbol Pa) or the legally permissible type in Germany and Austria Bar unit ( unit symbol bar, 1 bar = 105 Pa). The exact integer decimal values ​​ratio of SI unit Pascal and the non-SI unit bar arises from the fact that both are defined using SI units. Instead of inappropriately small unit Pascal the air pressure often with the SI prefix Hekto in hPa (hPa ) is specified, at sea level around 1000 hPa, or with the same numerical value in millibars ( mbar).

The barometer used to measure the air pressure often still show outdated units. In particular in mercury barometers a millimeter scale is only placed to detect the air pressure in torr ( 1 torr is equivalent to 1 mm of mercury ). The standard pressure of 1013.25 hPa is equal to 760 Torr.

Another unit in the context of air pressure is the atmosphere, although these old units such as Physical atmosphere, Technical atmosphere, atmosphere absolute pressure, atmospheric pressure or sub-atmospheric pressure after the unit shall no longer be permitted.

Variability and extreme values

The average air pressure of Earth's atmosphere at sea level is 1013.25 hPa and is therefore part of the normal conditions as well as many standard conditions.

Decrease with height

The air pressure decreases rapidly with height - at sea level by about 1 hPa per 8 meters. An exact mathematical description of the pressure curve is not possible because of the weather dynamics and other factors. Known as the barometric height formula exponential

Therefore gives only an approximation to the actual air pressure conditions. It is based on the (incorrect) assumption of that the temperature is constant for change in the amount not taken into account, the decrease of the gravitational acceleration with height and placed under a constant composition of the air. Nevertheless, allow barometric altimeter, use this formula when the weather is stable within a few hours and a few hundred meters, a height, with an accuracy of ± 10 m. From this simplified formula, the following air pressure results:

More detailed calculations show half the air pressure in approximately 5.5 km altitude and 10% of the land value about 18.4 km above the ground.

Diurnal cycle

The air pressure is subject to a daily recurring periodicity which has two maximum values ​​and minimum values ​​of two per day. He follows the variations of the air temperature, resulting in a stronger 12-hour rhythm (as semicircadian called ) and a weaker 24 -hour cycle ( circadian ) show. The maxima are found at 10 and 22 clock, the minima against 4 and 16 clock (local time). The amplitudes are latitude dependent. Near the equator, the variation in values ​​are up to 5 hPa in midlatitudes are the fluctuations at about 0.5 to 1 hPa Knowledge of the local daily passage of the air pressure increases the value of an barogram for the estimation of the weather, especially in tropical areas. Jump to observe, however, the diurnal cycle is usually not because it is overshadowed by dynamic air pressure fluctuations. Only with sufficiently precise measuring apparatus and stable high pressure weather conditions, it is possible to observe these fluctuations undisturbed.

A representation of the diurnal cycle, as it was recorded in northern Germany, is seen here on the right.

In the background noise, there are weak superpositions of air pressure, with typically very low frequency of 0.2 Hz and amplitudes less than 1 Pascal. Such Mikrobarome are a result of weather phenomena or stormy seas. Even the bursting of a meteor, the crack of a supersonic flight, (atomic ) bomb explosion cause pressure waves that can be detected very far yet.

Annual cycle

The annual course of air pressure, based on either a daily or monthly averages as a long -term average values ​​, shows a small but also relatively complex variation between months. This reveals a minimum in April, responsible for the term of the April weather, and relatively high values ​​for May and September ( Indian summer ).

Air pressure records

The historical global minimum value of the air pressure at sea level is depending on the source 856 hPa (measured in September 1958 during a typhoon at Okinawa ) and 870 hPa (estimated on 12 October 1979 Typhoon "Tip " near Guam in the Pacific Northwest ). The largest measured to date air pressure drop within 24 hours on the Atlantic was measured in October 2005 Hurricane Wilma at 98 hPa. The central pressure dropped to 882 hPa When Typhoon Forrest in September 1983, a drop in pressure of 100 hPa measured in the northwestern Pacific in just over 23 hours.

The highest measured to date and reduced to sea level air pressure values ​​were 1085.8 hPa in Tosontsengel (Mongolia) on 19 December 2001 for stations above 750 m and 1083.8 hPa December 31, 1968 at Agata Lake ( Siberia 66.88 ° N/93, 47 ° E) for stations below 750 m recorded.

The highest air pressure in Germany was measured with 1060.8 hPa on 23 January 1907 in Greifswald, the lowest with 954.4 hPa on 27 November 1983 in Emden.

Air pressure, weight of the air column

While the average value of the air pressure at sea level means that a column of air of mass loaded 10 tons on each square corresponding to fluctuations (see " Air Pressure Records" ) of a mass change in this column of air of about one ton per square meter.

Experiments and measurement

Otto von Guericke was able to prove in 1663 the air pressure with the Magdeburg hemispheres. These were two closely spaced half hollow spheres. As soon as the air between the hollow spheres had been evacuated, attracting teams of horses were also two opposite they are no longer separate.

Another experiment that can be used to accurately measure, is closed on one side and more than a ten meter long glass tube. It is first laid horizontally in a water vessel so that the air escapes. Is there one with the opening under water and the sealed side facing up, then sets a maximum height to which the water table, push through the wedge on the surrounding water surface air pressure, up leaves. This is about ten meters, less at high air pressure more, at low air pressure. In the cavity then there is almost a vacuum, however, this is " contaminated " by some water vapor. This is known as a liquid barometer. Evangelista Torricelli used mercury instead of water, which breaks off already at a height of 760 mm and also hardly evaporated. Another instrument for measuring air pressure according to this principle is the Goethe barometer.

Today mostly canned barometers are used which contain a so-called Vidie - box or a stack of such doses. This is a can-like hollow bodies made ​​of thin sheet metal, which is connected to a pointer. When atmospheric pressure increases, the box is compressed, the pointer moves. Thus, the measurement is independent of the temperature, is in the can, a vacuum, since air contained in it would expand when heated. Nevertheless, there are temperature-dependent measurement error. In order to keep this small, alloys are used with a low thermal expansion coefficient.

Water column

Principle of doses barometer

Importance

Meteorology

Regional variations in air pressure are significantly involved in the development of the weather, so the air pressure in the form of isobars is also the most important element in weather maps. For the weather forecast of importance of the air pressure at a defined height in the atmosphere, which is chosen so that no disturbance of the pressure can be expected by buildings or small-scale landforms, ie without being affected by the friction of the air stream at the bottom of the so-called is free atmosphere. A measurement that refers to the height of the ground surface above sea level ( so-called absolute pressure ) would be transferred to the surface are believed to reflect the topography of the terrain than the actual fluctuations in air pressure. To compensate for this and to make the values ​​comparable with it, a reduction to sea level one uses (so-called relative pressure ). Are mainly the local differences in air pressure of interest for the weather patterns near the ground. They lead to the formation of high and low pressure areas. Between them, due to the pressure gradient, the wind has worked as a make-up flow.

Aviation

The air pressure plays a major role in aviation, since the usual altimeter barometer are in principle. The altitude of the aircraft is so over the prevailing static air pressure according to the barometric formula determined (see air pressure measurement in aviation ). An unexpectedly low local air pressure can the pilot pretend an excessive height and thus to dangerous low flight tempt (Mnemonic: " From High to Low goes wrong " ), must therefore before and possibly during a flight the altimeter to the current at the bottom of the air pressure prevailing be adjusted.

Above a certain transition altitude or transition altitude (see main article altitude ), all altimeter set to standard pressure of 1013.25 hPa. The display then while minimizing any fixed reference more to the actual altitude, but since the absolute measurement error for all aircraft is the same size, the risk of collisions due to different altitude readings will.

The following tire pressures are in the aviation use:

• QFE: the actual air pressure at the measurement site; a set to QFE altimeter indicates the altitude above the measurement site.
• QNH: the back-calculated air pressure at sea level and the ICAO standard atmosphere (15 ° C, 0.65 degrees temperature gradient / 100 m); a set to QNH altimeter indicates the altitude above sea level.
• QFF: the back-calculated air pressure at sea level, taking account of local altitude, humidity, temperature and other factors.

On airfields QNH value is mainly used while is used in meteorology the QFF value to compare air pressure values ​​at different places and local heights.

Shipping

The air pressure plays in shipping a major role, since rapid changes in air pressure usually cause rapid changes in the weather.

A rapid fall of the air pressure means in the majority of cases of strong wind or storm risk. The importance of air pressure loss per period for a wind forecast is however latitude dependent. If the air pressure in our latitudes by more than 1 to 2 mb in an hour, there is a high probability of strong wind or storm.

Boiling point of water

Since the air pressure decreases with altitude, also decreases the boiling point of water. As a rule of thumb, the boiling point is lowered per 300 m by about one degree. Thus can be explained by the determination of the boiling point of pure water, the respective amount estimated above mean sea level.