def vapour_pressure(temperature_C):
    """Tetens' formula for calculating saturation pressure of water vapor,
    return value is in pascals.

    https://wahiduddin.net/calc/density_altitude.htm

    NOTE: 1 Pascal == 100 mb

        >>> vapour_pressure(22) / 100   # in mb
        26.43707387256724
    """
    return 100 * 6.1078 * 10**(
        (7.5 * temperature_C) / (237.3 + temperature_C))