Air temperature is a poor description of how hard it is to play in summer heat. Staff therefore measure a combined index built from several readings rather than a thermometer.
The body cools by evaporating sweat
Working muscles generate far more heat than they convert into movement, and that heat has to leave the body or core temperature rises.
Evaporation from the skin is the main route once the air is warm, because losing heat by simple contact with the air requires the air to be cooler than the body.
Humidity determines whether evaporation can happen. In damp air sweat runs off rather than evaporating, so the cooling mechanism is producing fluid loss without producing much cooling.
A single index combines the relevant readings
Measurement uses several thermometers at once: one shaded, one wetted to capture evaporative potential, and one blackened to capture radiant heat from sun and surfaces.
Those readings are weighted into one number, with the humidity-sensitive reading carrying the most weight because it reflects the constraint that actually binds.
The result frequently indicates severe conditions on a day the air temperature alone would call unremarkable, which is the entire reason the index exists.
Where the sensor sits changes the answer
Readings taken in a shaded office describe a place nobody is playing. The measurement has to be made on the field, in the sun, at roughly playing height.
Artificial surfaces complicate this considerably, because they absorb and re-radiate heat, producing conditions above the field that differ from conditions a few feet away on grass.
Stadium geometry matters too. A bowl that blocks wind removes the air movement that would otherwise assist evaporation, so an enclosed venue can be harsher than an open one at the same temperature.
Thresholds trigger protocols, not opinions
Competitions set index values at which cooling breaks are added, and the point of a measured threshold is that the decision does not depend on how anyone feels.
Breaks work by giving the body a period where heat production drops sharply while cooling continues, which slows the rise in core temperature rather than reversing it.
Shade, cold fluids and cooling applied to the skin during those breaks all contribute, and their value is measurable in how quickly core temperature stabilizes.
Why this shapes American scheduling
Much of the country plays through a summer with high humidity, so index-driven protocols are routine rather than exceptional across the domestic season.
Kickoff times, roof operation in covered venues and even acclimatization schedules for visiting squads are planned against expected index values rather than forecast temperature.
Treating heat as a measured environmental input, on the same footing as field condition, is what turns it from a talking point into something operations can plan around.