def molar_concentration_water_vapour(relative_humidity, saturation_pressure, pressure):
    """
    Molar concentration of water vapour :math:`h`.
    
    :param relative_humidity: Relative humidity :math:`h_r`
    :param saturation_pressure: Saturation pressure :math:`p_{sat}`
    :param pressure: Ambient pressure :math:`p`
   
    The molar concentration of water vapour is calculated using
   
    .. math:: h = h_r  \\frac{p_{sat}}{p_a}
    
    """
    return relative_humidity * saturation_pressure / pressure