a measurement of how hot the air feels and how uncomfortable people are likely to be based on the temperature combined with the humidity (= the amount of water in the air)

The weather has been horrible, with temperatures approaching 34 C and the humidex making it feel like it's over 40 C outside. 天气一直很糟糕，气温接近34摄氏度，而湿热指数却让人感觉室外温度超过了40摄氏度。