An important characteristic of the Earth's atmosphere is its pressure as it often determines wind and weather patterns across the globe. By definition, atmospheric or air pressure is the force per unit of area exerted on the Earth’s surface by the weight of the air above the surface.
Air pressure is conventionally measured with a mercury or aneroid barometer. A mercury barometer measures the height of a mercury column in a vertical glass tube. As air pressure changes, the height of the mercury column does as well- it drops when pressure falls and rises when it increases. An aneroid barometer uses a coil of tubing with most of the air removed. The coil then bends inward when pressure rises and bows out when pressure drops. Using instruments such as these, scientists have set the standard of normal sea level pressure at about 101.325 kPa.
Air pressure is not uniform across the Earth however. The normal range of the Earth's air pressure is from 98(kPa) to 105(kPa). These differences are the result of low and high air pressure systems which are caused by unequal heating across the Earth's surface and the pressure gradient force.
A low pressure system, or "low," is an area where the atmospheric pressure is lower than that of the area surrounding it. Lows are usually associated with high winds, warm air, and atmospheric lifting. Because of this, lows normally produce clouds, precipitation, and other bad weather such as tropical storms and cyclones.
In addition, areas prone to low pressure do not have extreme diurnal (day vs. night) nor extreme seasonal temperatures because the clouds present over such areas reflect incoming solar radiation back into the atmosphere so they cannot warm as much during the day (or in the summer) and at night they act as a blanket, trapping heat below.
Conversely, a high pressure system, or "high," is an area where the atmospheric pressure is greater than that of the surrounding area. In some places highs are referred to as anticyclones. These move clockwise in the northern hemisphere and counterclockwise in the southern due to the Coriolis Effect.
High pressure areas are normally caused by a phenomenon called subsidence, meaning that as the air in the high cools it becomes denser and moves toward the ground. Pressure increases here because more air fills the space left from the low. Subsidence also evaporates most of the atmosphere's water vapor so high pressure systems are usually associated with clear skies and calm weather.
Unlike areas of low pressure, the absence of clouds means that areas prone to high pressure experience extremes in diurnal and seasonal temperatures since there are no clouds to block incoming solar radiation or trap outgoing longwave radiation at night. Thus such areas have higher high temperatures and lower lows.
In order that atmospheric pressures measured at different locations with different heights above sea level be comparable for meteorological purposes, they are commonly reduced to values corresponding to a single height datum. This datum is normally the mean sea level. For all locations situated above sea level, this sea level value is theoretical: it refers to the atmosperic pressure which would be exerted if that location was at sea level, i.e at a subterranean depth in practice. Despite this, the use of mean sea level pressure is a very useful standardisation.
There are internationally agreed methods of carrying out this standardisation. The World Meteorological Organisation publishes its International Meteorological Tables in which is used the method described in WMO Technical Note 91, "Methods in use for the reduction of atmospheric pressure". In the USA, the Smithsonian Meteorological Tables are used. Although there are slight variations between methods used by different countries, they all are based on the same equation and, in practice, yield comparable values.
The fundamental equation which relates the atmospheric pressure to altitude is that due to Laplace.
The first term, which is a correction for the temperature of the atmosphere is handled in different
ways. In the UK, the average temperature of the air column θm is taken to be the station
temperature. In France, θm is calculated assuming a temperature gradient of 0.5 °C/100
metres. In Italy, 0.6 °C/100m is used. Other countries use other variants, but in all cases the values are
comparable to 0.1 mB.
The second term is a correction for humidity. In the UK it is neglected. In other countries a fixed value is used.
In Spain this is a water vapour pressure of 11 mm Mercury. Again these options affect only very slightly the normalised pressure.
The third term, the correction for the asphericity of the earth is normally neglected or a fixed value of the
lattitude chosen. The fourth term for the variation of gravitational acceleration with height is neglected in the UK and in
most other countries.
A close approximation to the corrections obtained from the Laplace equation is afforded by the formula of Babinet. This gives close correspondence up to 1000 metres and results within 1% to considerably greater heights and is probably all that
is required for home systems.
The MPX4115A/MPXA4115A series piezoresistive transducer from Motorola is a state–of–the–art,
monolithic, signal conditioned, silicon pressure sensor. This sensor provides an accurate, high level
analog output signal (4.590V span) that is proportional to applied pressure (15 to 115 kPa) at an accuracy of
+-1.5%.TheMPX4115A is ratiometric, which requires that both the output voltage representing atmospheric pressure and
the supply voltage across the element be known in order to accurately calculate the barometricpressure.
The small form factor and high reliability of on–chip integration make theMotorola pressure sensor a logical and
economical choice for a home weather system.
The MPX4115A measures the absolute pressure from 15 to 115 kPa but atmospheric pressure at a location seldom varies by more than 20 kPa. To enhance the sensitivity the output of the MPX4115 is first offset by a voltage equivalent to the lowest expected pressure at the altitude at which the device will operate. This offset output is then amplified with a fixed gain of 10. Both these operations are carried out by an INA122P Instrumentation amplifier. Using an offset and a 10X gain allows for a pressure resolution of 0.02 kPA. The schematic of this arrangement with a DS2438 one-wire slave is shown below.
Since there is little to no difference between indoor and outdoor atmospheric pressure the pressure sensor is best mounted indoors.