The

*mean sea-level pressure* (MSLP) is the atmospheric pressure at

mean sea level (PMSL). 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 1013.25 mbar (101.325 kPa; 29.921 inHg; 760.00 mmHg). 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 of mercury (to two decimal places). 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.^{[5]} However, in Canada's public weather reports, sea level pressure is instead reported in kilopascals.^{[6]}

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: 1013.2 mbar (101.32 kPa) is transmitted as 132; 1000.0 mbar (100.00 kPa) 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 1050 mbar (105 kPa; 31 inHg), with record highs close to 1085 mbar (108.5 kPa; 32.0 inHg). The lowest measurable *sea-level pressure* is found at the centers of tropical cyclones and tornadoes, with a record low of 870 mbar (87 kPa; 26 inHg).

## Surface pressure

"Surface pressure" redirects here. For surface pressure in physical chemistry, see

Pressure § Surface pThe *altimeter setting* in aviation is an atmospheric pressure adjustment.

Average *sea-level pressure* is 1013.25 mbar (101.325 kPa; 29.921 inHg; 760.00 mmHg). 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 of mercury (to two decimal places). 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.^{[5]} However, in Canada's public weather reports, sea level pressure is instead reported in kilopascals.^{[6]}

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: 1013.2 mbar (101.32 kPa) is transmitted as 132; 1000.0 mbar (100.00 kPa) 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 1050 mbar (105 kPa; 31 inHg), with record highs close to 1085 mbar (108.5 kPa; 32.0 inHg). The lowest measurable *sea-level pressure* is found at the centers of tropical cyclones and tornadoes, with a record low of 870 mbar (87 kPa; 26 inHg).

*Surface pressure* is the atmospheric pressure at a location on Earth's surface (terrain and oceans). It is directly proportional to the mass of air over that location.

For numerical reasons, atmospheric models such as general circulation models (GCMs) usually predict the nondimensional *logarithm of surface pressure*.

The average value of surface pressure on Earth is 985 hPa.^{[7]} This is in contrast to mean sea-level pressure, which involves the extrapolation of pressure to sea-level for locations above or below sea-level. The average pressure at mean sea-level (general circulation models (GCMs) usually predict the nondimensional *logarithm of surface pressure*.

The average value of surface pressure on Earth is 985 hPa.^{[7]} This is in contrast to mean sea-level pressure, which involves the extrapolation of pressure to sea-level for locations above or below sea-level. The average pressure at mean sea-level (MSL) in the International Standard Atmosphere (ISA) is 1013.25 hPa, or 1 atmosphere (atm), or 29.92 inches of mercury.

Pressure (p), mass (m), and the acceleration due to gravity (g), are related by P = F/A = (m*g)/A, where A is surface area. Atmospheric pressure is thus proportional to the weight per unit area of the atmospheric mass above that location.

Pressure on Earth varies with the altitude of the surface; so air pressure on mountains is usually lower than air pressure at 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.^{[8]} Temperature and humidity also affect the atmospheric pressure, and it is necessary to know these to compute an accurate figure. The graph at rightabove was developed for a temperature of 15 °C and a relative humidity of 0%.

At low altitudes above sea level, the pressure decreases by about 1.2 kPa (12 hPa) for every 100 metres. For higher altitudes within the troposphere, the following equation (the barometric formula) relates atmospheric pressure *p* to altitude *h*:
${\begin{aligned}p&=p_{0}\cdot \left(1-{\frac {L\cdot h}{T_{0}}}\right)^{\frac {g\cdot M}{R_{0}\cdot L}}\\&=p_{0}\cdot \left(1-{\frac {g\cdot h}{c_{\text{p}}\cdot T_{0}}}\right)^{\frac {c_{\text{p}}\cdot M}{R_{0}}}\approx p_{0}\cdot \exp \left(-{\frac {g\cdot h\cdot M}{T_{0}\cdot R_{0}}}\right)\end{aligned}}$