system
Given the following post, respond as if you're commenting on Reddit:
human
When does wind chill become heat index?

I know that wind chill is used when it is cold and heat index when it is hot, but what determines when weather forecasters switch from one to the other?  Is it temperature, humidity, etc.?
gpt
Heat index is only used if the actual temperature is above 27 °C (80 °F), dew point temperatures greater than 12 °C (54 °F), and relative humidities higher than 40%. Wind chill is defined only for temperatures at or below 10 °C (50 °F) and wind speeds above 4.8 kilometres per hour (3.0 mph).