def p_w_from_rh_p_and_ws(rh, p_ws):
    """
    Calculate water vapor pressure from relative humidity and water vapor saturation pressure
    Eq(6) in "CHAPTER 6 - PSYCHROMETRICS" in "2001 ASHRAE Fundamentals Handbook (SI)"

    :param rh: relative humidity [-]
    :type rh: double
    :param p_ws: water vapor saturation pressure [Pa]
    :type p_ws: double
    :return: water vapor pressure [Pa]
    :rtype: double
    """
    return rh * p_ws
