def _air_pressure(elev, method='asce'):
    """Mean atmospheric pressure at station elevation (Eqs. 3 & 34)

    Parameters
    ----------
    elev : ee.Image or ee.Number
        Elevation [m].
    method : {'asce' (default), 'refet'}, optional
        Calculation method:
        * 'asce' -- Calculations will follow ASCE-EWRI 2005 [1] equations.
        * 'refet' -- Calculations will follow RefET software.

    Returns
    -------
    pair : ee.Image or ee.Number
        Air pressure [kPa].

    Notes
    -----
    The current calculation in Ref-ET:
        101.3 * (((293 - 0.0065 * elev) / 293) ** (9.8 / (0.0065 * 286.9)))
    Equation 3 in ASCE-EWRI 2005:
        101.3 * (((293 - 0.0065 * elev) / 293) ** 5.26)
    Per Dr. Allen, the calculation with full precision:
        101.3 * (((293.15 - 0.0065 * elev) / 293.15) ** (9.80665 / (0.0065 * 286.9)))

    """
    if method == 'asce':
        return elev.multiply(-0.0065).add(293).divide(293)\
            .pow(5.26).multiply(101.3)
    elif method == 'refet':
        return elev.multiply(-0.0065).add(293).divide(293)\
            .pow(9.8 / (0.0065 * 286.9)).multiply(101.3)