def calc_actual_vapor_pressure(es_tmin, es_tmax, hmin, hmax):
    """
    Calculate actual vapor pressure from saturation vapor pressure
    and relative humidity. Equation 17 of Allen (1998).

    Parameters
    ----------
    es_tmin : numpy ndarray
        Saturation vapor pressure at minimum air temperature.
    es_tmax : numpy ndarray
        Saturation vapor pressure at maximum air temperature.
    hmin : numpy ndarray
        Minimum relative humidity, in percent
    hmax : numpy ndarray
        Maximum relative humidity, in percent

    """
    return ((es_tmax * hmin / 100.) + (es_tmin * hmax / 100.)) / 2.