import math


def atmospheric_pressure(elev):
    """
    Approximation of the atmospheric pressure
    as a function of elevation (Allen et al. 1998).

    NOTE: This equation is a simplification of the ideal gas law,
    assuming 20 deg C and a standard atmosphere.

    Parameters
    ----------
    elev: float
        Elevation (m a.s.l.).

    Returns
    -------
    float
        Atmospheric pressure (kPa).

    """
    return 101.3 * math.pow((293 - 0.0065 * elev) / 293, 5.26)