# altimeter_to_sea_level_pressure¶

metpy.calc.altimeter_to_sea_level_pressure(altimeter_value, height, temperature)

Convert the altimeter setting to sea-level pressure.

This function is useful for working with METARs since most provide altimeter values, but not sea-level pressure, which is often plotted on surface maps. The following definitions of altimeter setting, station pressure, and sea-level pressure are taken from [Smithsonian1951]. Altimeter setting is the pressure value to which an aircraft altimeter scale is set so that it will indicate the altitude above mean sea-level of an aircraft on the ground at the location for which the value is determined. It assumes a standard atmosphere. Station pressure is the atmospheric pressure at the designated station elevation. Sea-level pressure is a pressure value obtained by the theoretical reduction of barometric pressure to sea level. It is assumed that atmosphere extends to sea level below the station and that the properties of the atmosphere are related to conditions observed at the station. This value is recorded by some surface observation stations, but not all. If the value is recorded, it can be found in the remarks section. Finding the sea-level pressure is helpful for plotting purposes and different calculations.

Parameters
• altimeter_value ('pint.Quantity') – The altimeter setting value is defined by the METAR or other observation, with units of inches of mercury (in Hg) or millibars (hPa).

• height ('pint.Quantity') – Elevation of the station measuring pressure. Often times measured in meters

• temperature ('pint.Quantity') – Temperature at the station

Returns

‘pint.Quantity’ – The sea-level pressure in hPa and makes pressure values easier to compare between different stations.

Notes

This function is implemented using the following equations from Wallace and Hobbs (1977).

Equation 2.29:
$\Delta z = Z_{2} - Z_{1} = \frac{R_{d} \bar T_{v}}{g_0}ln\left(\frac{p_{1}}{p_{2}}\right) = \bar H ln \left (\frac {p_{1}}{p_{2}} \right)$
Equation 2.31:
$p_{0} = p_{g}exp \left(\frac{Z_{g}}{\bar H} \right) = p_{g}exp \left(\frac{g_{0}Z_{g}}{R_{d}\bar T_{v}} \right)$
Then by substituting $$\Delta_{Z}$$ for $$Z_{g}$$ in Equation 2.31:
$p_{sealevel} = p_{station} exp\left(\frac{\Delta z}{H}\right)$

where $$\Delta_{Z}$$ is the elevation in meters and $$H = \frac{R_{d}T}{g}$$