def vapor_pressure(df, dt):
    """
    Vapor pressure dervied from dewpoint temp. Formula from NWS:
    https://www.weather.gov/media/epz/wxcalc/vaporPressure.pdf

    :param df: dataframe with dewpoint temp
    :param dt: column with dewpoint temp (in Deg C).
    :return: vapor pressure col in dataframe (output is in Pa).
    """
    df['vp'] = 6.11 * (10 ** ((7.5 * df[dt]) / (237.3 + df[dt])))
    # hPa to Pa
    df['vp'] = df['vp'] * 100
    return df