It's "or" because it shows either heat index or windchill, depending on temperature. "heat index and windchill" would sound like it includes the effects of both wind and humidity at the same time, and it doesn't.
It's "or" because it shows either heat index or windchill, depending on temperature. "heat index and windchill" would sound like it includes the effects of both wind and humidity at the same time, and it doesn't.