Atmospheric pressure is defined as the force per
unit area exerted against a surface by the weight of air above that
surface at any given point in the
Earth's atmosphere. 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. Similarly, as
elevation increases there is less overlying
atmospheric mass, so that pressure decreases with increasing
elevation. A column of air one square inch in cross-section,
measured from
sea level to the top of the
atmosphere, would weigh approximately . The weight of a column of
air would be about .
Standard atmospheric pressure
The
standard atmosphere (symbol:
atm) is a
unit of pressure and is
defined as being equal to 101 325 Pa or 101.325
kPa. The following units are equivalent, but
only to the number of decimal places displayed: 760
mmHg (
torr), 29.92
inHg, 14.696
PSI,
1013.25
millibars. One standard
atmosphere is standard pressure used for pneumatic fluid power (ISO
R554), and in the aerospace (ISO 2533) and petroleum (ISO 5024)
industries.
In 1999, the
International
Union of Pure and Applied Chemistry (IUPAC) recommended that
for the purposes of specifying the properties of substances,
“
the standard pressure” should be defined as precisely
100 kPa (≈750.01
torr) or 29.53
inHg rather than the 101.325 kPa value of
“one standard atmosphere”. This value is used as the standard
pressure for the compressor and the pneumatic tool industries (ISO
2787).
(See also Standard temperature and
pressure.) In the United States, compressed air flow is often measured in "standard
cubic feet" per unit of time, where the "standard" means the
equivalent quantity of moisture at standard temperature and
pressure. For every 1,000 feet you ascend the atmospheric
pressure decreases 4%. However, this standard atmosphere is defined
slightly differently: temperature = , air density =
1.225 kg/m³ (0.0765 lb/cu ft), altitude = sea level, and
relative humidity = 20%. In the air conditioning industry, the
standard is often temperature = instead. For natural gas, the
petroleum industry uses a standard temperature of , pressure . (air
pressure)
Mean sea level pressure
Mean sea level pressure (MSLP) is the pressure at sea level or
(when measured at a given elevation on land) the station pressure
reduced to sea level assuming an
isothermal layer at the station
temperature.
This is the 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 reduced to sea level, not the actual local atmospheric
pressure. See
Altimeter .
The reduction to sea level means that the
normal range of
fluctuations in pressure is the same for everyone. The
pressures which are considered
high pressure or
low
pressure do not depend on geographical location. This makes
isobars on a weather map meaningful and
useful tools.
The
altimeter setting in
aviation, set either
QNH or QFE, is another
atmospheric pressure reduced to sea level, but the method of making
this reduction differs slightly.
- QNH: The barometric altimeter setting which will cause the
altimeter to read airfield elevation when on the airfield. In ISA
temperature conditions the altimeter will read altitude above mean
sea level in the vicinity of the airfield
- QFE: The barometric altimeter setting which will cause an
altimeter to read zero when at the reference datum of a particular
airfield (generally a runway threshold). In ISA temperature
conditions the altimeter will read height above the datum in the
vicinity of the airfield.
QFE and QNH are arbitrary
Q codes rather
than abbreviations, but the
mnemonics
"Nautical Height" (for QNH) and "Field Elevation" (for QFE) are
often used by pilots to distinguish them.
Average
sea-level pressure is
101.325 kPa
(1013.25 mbar, or hPa) or
29.921 inches of mercury
(inHg) or
760 millimeters (mmHg). In aviation
weather reports (
METAR), QNH is transmitted
around the world in millibars or hectopascals (1 millibar = 1
hectopascal), except in the United States and in Canada where it is
reported in inches (or hundredths of inches) of mercury. (The
United States and Canada also report
sea level pressure
SLP, which is reduced to sea level by a different method, in the
remarks section, not an internationally transmitted part of the
code, in hectopascals or millibars.
However, in Canada's public weather
reports, sea level pressure is instead reported in kilopascals
[7673], while Environment Canada's standard unit of
pressure is the same [7674] [7675].) In the weather code, three digits are
all that is needed; decimal points and the one or two most
significant digits are omitted: 1013.2 mbar or 101.32 kPa is
transmitted as 132; 1000.0 mbar or 100.00 kPa is transmitted as
000; 998.7 mbar or 99.87 kPa 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 1087.0 mbar. The lowest
measurable
sea-level pressure is found at the centers of
tropical cyclones.
Altitude atmospheric pressure variation
This plastic bottle, sealed at
approximately altitude, was crushed by the increase in atmospheric
pressure when brought to 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. The following is a list of air pressures (as
a fraction of one atmosphere) with the corresponding average
altitudes. The table gives a rough idea of air pressure at various
altitudes.
fraction of 1 atm |
average altitude |
(m) |
(ft) |
1 |
0 |
0 |
1/2 |
5,486 |
18,000 |
1/e |
7,915 |
25,970 |
1/3 |
8,376 |
27,480 |
1/10 |
16,132 |
52,926 |
1/100 |
30,901 |
101,381 |
1/1000 |
48,467 |
159,013 |
1/10000 |
69,464 |
227,899 |
1/100000 |
86,282 |
283,076 |
Calculating variation with altitude
There are two different equations for computing the average
pressure at various height regimes below . Equation 1 is used when
the value of standard temperature
lapse
rate is not equal to zero and equation 2 is used when standard
temperature lapse rate equals zero.
Equation 1:
- {P}=P_b \cdot \left[\frac{T_b}{T_b +
L_b\cdot(h-h_b)}\right]^{\textstyle \frac{g_0 \cdot M}{R^* \cdot
L_b}}
Equation 2:
- {P}=P_b \cdot \exp \left[\frac{-g_0 \cdot M \cdot (h-h_b)}{R^*
\cdot T_b}\right]
where
- P_b = Static pressure (pascals, Pa)
- T_b = Standard temperature (kelvin,
K)
- L_b = Standard temperature lapse rate (kelvin per meter,
K/m)
- h = Height above sea level (meters, m)
- h_b = Height at bottom of layer b (meters; e.g., h_1 =
11,000 m)
- R^* = Universal gas
constant: 8.31432 Nm/(K·mol)
- g_0 = Standard gravity
(9.80665 m/s^{2})
- M = Molar mass of Earth's air (0.0289644 kg/mol)
Or converted to Imperial units:
where
- P_b = Static pressure (inches of mercury, inHg)
- T_b = Standard temperature ([[kelvin]s, K)
- L_b = Standard temperature lapse rate (kelvin per foot,
K/ft)
- h = Height above sea level (feet, ft)
- h_b = Height at bottom of layer b (feet; e.g., h_1 =
36,089 ft)
- R^* = Universal gas
constant; using feet, kelvin, and (SI) moles:
8.9494596×10^{4} gft^{2}/(mol·Ks^{2})
- g_0 = Standard gravity
(32.17405 ft/s^{2})
- M = Molar mass of Earth's air (0.0289644 kg/mol)
The value of subscript
b ranges from 0 to 6 in accordance
with each of seven successive layers of the atmosphere shown in the
table below. In these equations,
g_{0},
M
and
R^{*} are each single-valued constants, while
P, L, T, and
h are multivalued
constants in accordance with the table below. (Note that according
to the convention in this equation,
L_{0}, the
tropospheric lapse rate, is negative.) It should be noted that the
values used for
M, g_{0}, and R^* are in
accordance with the
U.S.
Standard Atmosphere, 1976,
and that the value for R^* in particular does not agree with
standard values for this constant. The reference value for
P_{b} for
b = 0 is the defined sea level
value,
P_{0} = 101325
pascals or 29.92126
inHg.
Values of
P_{b} of
b = 1 through
b = 6 are obtained from the application of the appropriate
member of the pair equations 1 and 2 for the case when h =
h_{b+1}.:
Subscript b |
Height Above Sea Level |
Static Pressure |
Standard Temperature
(K) |
Temperature Lapse Rate |
(m) |
(ft) |
(pascals) |
(inHg) |
(K/m) |
(K/ft) |
0 |
0 |
0 |
101325 |
29.92126 |
288.15 |
-0.00649 |
-0.0019812 |
1 |
11,000 |
36,089 |
22632 |
6.683245 |
216.65 |
0.0 |
0.0 |
2 |
20,000 |
65,617 |
5474 |
1.616734 |
216.65 |
0.001 |
0.0003048 |
3 |
32,000 |
104,987 |
868 |
0.2563258 |
228.65 |
0.0028 |
0.00085344 |
4 |
47,000 |
154,199 |
110 |
0.0327506 |
270.65 |
0.0 |
0.0 |
5 |
51,000 |
167,323 |
66 |
0.01976704 |
270.65 |
-0.0028 |
-0.00085344 |
6 |
71,000 |
232,940 |
4 |
0.00116833 |
214.65 |
-0.002 |
-0.0006097 |
Local atmospheric pressure variation
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 (twice-daily) cycle caused by
global
atmospheric tides. This
effect is strongest in tropical zones, with 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.
Atmospheric pressure based on height of water
Atmospheric pressure is often measured with a mercury
barometer, and a height of approximately of
mercury is often used to illustrate (and measure) atmospheric
pressure. However, since mercury is not a substance that humans
commonly come in contact with,
water often
provides a more intuitive way to visualize the pressure of one
atmosphere.
One atmosphere (101.325 kPa or 14.7 psi) is the amount of
pressure that can lift water approximately . Thus, a diver
10.3 m underwater experiences a pressure of about 2
atmospheres (1 atm of air plus 1 atm of water). This is
also the maximum height to which a column of water can be drawn up
by
suction.
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 is rated for a maximum of 14 w.c. which is
approximately 0.034 atmosphere.
Non-professional
barometers are generally
aneroid barometers or
strain gauge based. See
pressure measurement for a
description of barometers.
Water's boiling point
Boiling water
By definition water
boils at at one
atmosphere. The boiling point is the temperature at which the vapor
pressure is equal to the atmospheric pressure around the water.
Because of this, the boiling point of water is decreased in lower
pressure and raised at higher pressure. This is why baking at
elevations more than above
sea level
requires adjustments to recipes. 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.
See also
Notes
- International Civil Aviation Organization, Manual of the ICAO
Standard Atmosphere, Doc 7488-CD, Third Edition, 1993, ISBN
92-9194-004-6.
- OPCIT
http://en.wikipedia.org/wiki/ICAO_Standard_Atmosphere
- IUPAC.org, Publications, Standard Pressure (
20 kB PDF)
- Compressor.co.za, May 2003 Newsletter
- Sample METAR of CYVR Nav Canada
- Mechtly, E. A., 1973: The International System of Units,
Physical Constants and Conversion Factors. NASA SP-7012,
Second Revision, National Aeronautics and Space Administration,
Washington, D.C.
- U.S. Standard Atmosphere, 1976, U.S. Government
Printing Office, Washington, D.C., 1976. (Linked file is very
large.)
- Vapor Pressure
- Crisco - Articles & Tips - Cooking Tips - High
Altitude Cooking
- [M.N. Berberan-Santos, E.N. Bodunov, L. Pogliani, On the
barometric formula. Am. J. Phys. 65 (5), 404-412 (1997)]
References
- * US Department of Defense Military Standard 810E
- * Burt, Christopher C., (2004). Extreme Weather, A Guide
& Record Book. W. W. Norton & Company ISBN
0-393-32658-6
- * U.S. Standard Atmosphere, 1962, U.S.
Government Printing Office, Washington, D.C., 1962.
External links
Experiments