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"Hydraulic" redirects here. Template:Continuum mechanics Atmospheric pressure, sometimes also called barometric pressure, is the pressure exerted by the weight of air in the atmosphere of Earth (or that of another planet). In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point. Low-pressure areas have less atmospheric mass above their location, whereas high-pressure areas have more atmospheric mass above their location. Likewise, as elevation increases, there is less overlying atmospheric mass, so that atmospheric pressure decreases with increasing elevation. On average, a column of air Template:Convert in cross-section, measured from sea level to the top of the atmosphere, has a mass of about Template:Convert and weight of about Template:Convert. That force (across one square centimeter) is a pressure of 10.1 N/cm2 or 101,000 N/m2. A column Template:Convert in cross-section would have a weight of about Template:Cvt or about 65.4 N.

Standard atmospheric

The standard atmosphere (symbol: atm) is a unit of pressure defined as Template:Convert, equivalent to 760 mmHg (torr), 29.92 inHg and 14.696 psi.[1]

Mean sea level pressure

15 year average mean sea level pressure for June, July, and August (top) and December, January, and February (bottom). ERA-15 re-analysis.
Kollsman-type barometric aircraft altimeter (as used in North America) displaying an altitude of Template:Convert.

The mean sea level pressure (MSLP) is the atmospheric pressure at sea level. This is the atmospheric pressure normally given in weather reports on radio, television, and newspapers or on the Internet. When barometers in the home are set to match the local weather reports, they measure pressure adjusted to sea level, not the actual local atmospheric pressure.

The altimeter setting in aviation, is an atmospheric pressure adjustment.

Average sea-level pressure is Template:Convert. In aviation weather reports (METAR), QNH is transmitted around the world in millibars or hectopascals (1 hectopascal = 1 millibar), except in the United States, Canada, and Colombia where it is reported in inches (to two decimal places) of mercury. The United States and Canada also report sea level pressure SLP, which is adjusted to sea level by a different method, in the remarks section, not in the internationally transmitted part of the code, in hectopascals or millibars.[2] However, in Canada's public weather reports, sea level pressure is instead reported in kilopascals.[3]

In the US weather code remarks, three digits are all that are transmitted; decimal points and the one or two most significant digits are omitted: Template:Convert is transmitted as 132; Template:Convert is transmitted as 000; 998.7 mbar is transmitted as 987; etc. The highest sea-level pressure on Earth occurs in Siberia, where the Siberian High often attains a sea-level pressure above Template:Convert, with record highs close to Template:Convert. The lowest measurable sea-level pressure is found at the centers of tropical cyclones and tornadoes, with a record low of Template:Convert (see Atmospheric pressure records).

Altitude variation

A very local storm above Snæfellsjökull, showing clouds formed on the mountain by orographic lift
Variation in atmospheric pressure with altitude, computed for 15 °C and 0% relative humidity.
This plastic bottle was sealed at approximately Template:Convert altitude, and was crushed by the increase in atmospheric pressure —at Template:Convert and Template:Convert— as it was brought down towards sea level.

Pressure varies smoothly from the Earth's surface to the top of the mesosphere. Although the pressure changes with the weather, NASA has averaged the conditions for all parts of the earth year-round. As altitude increases, atmospheric pressure decreases. One can calculate the atmospheric pressure at a given altitude.[4] Temperature and humidity also affect the atmospheric pressure, and it is necessary to know these to compute an accurate figure. The graph at right was developed for a temperature of 15 °C and a relative humidity of 0%.

At low altitudes above the sea level, the pressure decreases by about 1.2 kPa for every 100 meters. For higher altitudes within the troposphere, the following equation (the barometric formula) relates atmospheric pressure p to altitude h

p = p_0 \cdot \left(1 - \frac{L \cdot h}{T_0} \right)^\frac{g \cdot M}{R_0 \cdot L} \approx p_0 \cdot \left(1 - \frac{g \cdot h}{c_p \cdot T_0} \right)^{\frac{c_p \cdot M}{R_0}},
p  \approx p_0 \cdot \exp \left(- \frac{g \cdot M \cdot h}{R_0 \cdot T_0} \right)

where the constant parameters are as described below:

Parameter Description Value
p0 sea level standard atmospheric pressure 101325 Pa
L temperature lapse rate, = g/cp for dry air 0.0065 K/m
cp constant pressure specific heat ~ 1007 J/(kg•K)
T0 sea level standard temperature 288.15 K
g Earth-surface gravitational acceleration 9.80665 m/s2
M molar mass of dry air 0.0289644 kg/mol
R0 universal gas constant 8.31447 J/(mol•K)

Local variation

Hurricane Wilma on 19 October 2005; Template:Convert in the storm's eye

Atmospheric pressure varies widely on Earth, and these changes are important in studying weather and climate. See pressure system for the effects of air pressure variations on weather.

Atmospheric pressure shows a diurnal or semidiurnal (twice-daily) cycle caused by global atmospheric tides. This effect is strongest in tropical zones, with an amplitude of a few millibars, and almost zero in polar areas. These variations have two superimposed cycles, a circadian (24 h) cycle and semi-circadian (12 h) cycle.

Records

The highest adjusted-to-sea level barometric pressure ever recorded on Earth (above 750 meters) was Template:Convert measured in Tosontsengel, Mongolia on 19 December 2001.[5] The highest adjusted-to-sea level barometric pressure ever recorded (below 750 meters) was at Agata, Evenkiyskiy, Russia [66°53’N, 93°28’E, elevation: 261 m (856.3 ft)] on 31 December 1968 of Template:Convert.[6] The discrimination is due to the problematic assumptions (assuming a standard lapse rate) associated with reduction of sea level from high elevations.[5]

The Dead Sea, the lowest place on Earth at 425 metres (1400 feet) below sea level, has a correspondingly high typical atmospheric pressure of 1065 hPa.[7]

The lowest non-tornadic atmospheric pressure ever measured was 0.858 atm (25.69 inHg), 870 hPa, set on 12 October 1979, during Typhoon Tip in the western Pacific Ocean. The measurement was based on an instrumental observation made from a reconnaissance aircraft.[8]

Measurement based on depth of water

One atmosphere (101 kPa or 14.7 psi) is the pressure caused by the weight of a column of fresh water of approximately 10.3 m (33.8 ft). Thus, a diver 10.3 m underwater experiences a pressure of about 2 atmospheres (1 atm of air plus 1 atm of water). Conversely, 10.3 m is the maximum height to which water can be raised using suction under standard atmospheric conditions.

Low pressures such as natural gas lines are sometimes specified in inches of water, typically written as w.c. (water column) or w.g. (inches water gauge). A typical gas-using residential appliance in the US is rated for a maximum of 14 w.c., which is approximately 35 hPa. Similar metric units with a wide variety of names and notation based on millimetres, centimetres or metres are now less commonly used.

Boiling point of water

Pure water boils at Template:Convert at earth's standard atmospheric pressure. The boiling point is the temperature at which the vapor pressure is equal to the atmospheric pressure around the water.[9] Because of this, the boiling point of water is lower at lower pressure and higher at higher pressure. Cooking at high elevations, therefore, requires adjustments to recipes.[10] A rough approximation of elevation can be obtained by measuring the temperature at which water boils; in the mid-19th century, this method was used by explorers.[11]

Measurement and maps

An important application of the knowledge that atmospheric pressure varies directly with altitude was in determining the height of hills and mountains thanks to the availability of reliable pressure measurement devices. While in 1774 Maskelyne was confirming Newton's theory of gravitation at and on Schiehallion in Scotland (using plumb bob deviation to show the effect of "gravity") and accurately measure elevation, William Roy using barometric pressure was able to confirm his height determinations, the agreement being to within one meter (3.28 feet). This was then a useful tool for survey work and map making and long has continued to be useful. It was part of the "application of science" which gave practical people the insight that applied science could easily and relatively cheaply be "useful".[12]

See also

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References

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External links

Experiments

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  1. International Civil Aviation Organization. Manual of the ICAO Standard Atmosphere, Doc 7488-CD, Third Edition, 1993. ISBN 92-9194-004-6.
  2. Sample METAR of CYVR Nav Canada
  3. Template:Citation
  4. A quick derivation relating altitude to air pressure by Portland State Aerospace Society, 2004, accessed 05032011
  5. 5.0 5.1 Template:Citation
  6. Template:Citation
  7. Template:Cite journal
  8. Template:Cite web
  9. Template:Citation
  10. Template:Citation
  11. Template:Cite journal
  12. Hewitt, Rachel, Map of a Nation – a Biography of the Ordnance Survey ISBN 1-84708-098-7