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## Pressure (Pa) to Altitude (meters)
def pres2alt(pressure):
    '''
    Determine altitude from site pressure.

    Parameters
    ----------
    pressure : numeric
        Atmospheric pressure (Pascals)

    Returns
    -------
    altitude : numeric
        Altitude in meters above sea level

    Notes
    ------
    The following assumptions are made

    ============================   ================
    Parameter                      Value
    ============================   ================
    Base pressure                  101325 Pa
    Temperature at zero altitude   288.15 K
    Gravitational acceleration     9.80665 m/s^2
    Lapse rate                     -6.5E-3 K/m
    Gas constant for air           287.053 J/(kgK)
    Relative Humidity              0%
    ============================   ================

    References
    -----------
    [1] "A Quick Derivation relating altitude to air pressure" from
    Portland State Aerospace Society, Version 1.03, 12/22/2004.
    '''

    alt = 44331.5 - 4946.62 * pressure ** (0.190263)

    return alt


## Altitude (meters) to Pressure (Pa)
def alt2pres(altitude):
    '''
    Determine site pressure from altitude.

    Parameters
    ----------
    altitude : numeric
        Altitude in meters above sea level

    Returns
    -------
    pressure : numeric
        Atmospheric pressure (Pascals)

    Notes
    ------
    The following assumptions are made

    ============================   ================
    Parameter                      Value
    ============================   ================
    Base pressure                  101325 Pa
    Temperature at zero altitude   288.15 K
    Gravitational acceleration     9.80665 m/s^2
    Lapse rate                     -6.5E-3 K/m
    Gas constant for air           287.053 J/(kgK)
    Relative Humidity              0%
    ============================   ================

    References
    -----------
    [1] "A Quick Derivation relating altitude to air pressure" from
    Portland State Aerospace Society, Version 1.03, 12/22/2004.
    '''

    press = 100 * ((44331.514 - altitude) / 11880.516) ** (1 / 0.1902632)

    return press


""" MAIN """
if __name__ == "__main__":
    # pressure to altitude
    ipress = 100000 # Pa
    ialt = pres2alt(ipress)
    print('Pressure = %s Pa --> Altitude = %.5f meters'%(ipress,ialt))

    # altitude to pressure
    ialt = 1000 # meters
    ipress = alt2pres(ialt)
    print('Altitude = %s meters  --> Pressure = %.5f Pa'%(ialt,ipress))