Because conditions change throughout the day, the Ride Leader is responsible to make the adjustments or cancel the ride, just as they would for other adverse/dangerous conditions or events. The Heat Index can be obtained on most weather apps, Siri, Alexa, or Google. It can also be quickly calculated using the current temperature and humidity on the National Weather Service Heat Index Chart (linked above)
With weather extremes becoming more frequent and severe, it is important that we act with our health and safety in mind.