Heat Rules in Sport: Air Temperature Is the Wrong Number

Why the thermometer is misleading

A body sheds heat mainly by evaporating sweat. Whether that works depends far less on air temperature than on humidity.

At 32°C in dry desert air, sweat evaporates freely and an athlete can cope for hours. At 32°C in tropical humidity, sweat runs off without evaporating, the cooling mechanism fails, and core temperature rises with nothing to stop it.

Same number on the thermometer. Completely different physiological situation.

That is why sports rules almost never key off air temperature alone. It is the number the audience knows and the wrong number for the decision.

What WBGT actually combines

Wet bulb globe temperature folds four things into one figure:

Air temperature — the familiar component.

Humidity, which determines whether sweat can evaporate at all.

Radiant heat, measured by a black globe thermometer — the sun’s load, and the heat coming off a hard court, which on a still afternoon can be far hotter than the air above it.

Wind, which carries away the humid layer sitting against the skin.

The output is a single number that describes how hard it is for a body to stay cool, which is the question a heat rule is actually asking.

The ATP’s thresholds, and what the numbers do

From 2026, the ATP uses WBGT as its definitive metric.

At 30.1°C WBGT or above during the first two sets of a best-of-three match, either player may request a 10-minute cooling break after the second set.

Above 32.2°C, play is halted.

Two features are worth noting.

The first threshold produces an entitlement, not an automatic stoppage — either player may request it. That matters because heat tolerance varies enormously, and a rule that forced a break would take a tactical decision away from both players in conditions one of them was managing comfortably.

The second is automatic. Above 32.2°C the sport stops deciding case by case, because at that level the risk is no longer something an individual can reasonably be asked to assess about themselves.

That structure — discretion below a line, no discretion above it — is how most good welfare rules are built. Compare open water swimming’s temperature limits, where wetsuits become mandatory rather than optional below 18°C.

The Australian Open built its own scale

Melbourne uses a tennis-specific heat stress scale, assessed courtside from air temperature, mean radiant temperature, wind speed and humidity, and modelled for tennis players.

The output is a five-point scale: level 1, play as normal; levels 2 to 4, progressively stricter interventions; level 5, suspension.

The reason for building a bespoke model is a real limitation of WBGT: it was originally developed for US military training, and it does not account for what a specific sport actually asks of a body — the clothing, the work-to-rest ratio, the duration, the surface.

A tennis player in white on a blue hard court in a fifth set is in a different situation from a soldier marching, and a scale calibrated for one is an approximation for the other.

A graduated scale is also better than a single cut-off. Between “carry on” and “stop” there is a great deal a tournament can do — extend breaks, close roofs, move matches, add ice, shorten warm-ups — and a five-point scale gives it a defined ladder rather than a cliff.

The harder problem underneath

Heat rules are, at present, mostly about individual matches. They are much less good at the thing that actually harms athletes: cumulative exposure across a tournament, and the interaction of heat with a schedule nobody would design if heat were the priority.

Scheduling a match at 2pm in a heatwave because that is the broadcast slot, and then applying a cooling break rule, treats the symptom of a decision made hours earlier.

That is the same gap identified in open water swimming’s maximum temperature question: the danger is not a property of the environment alone, it is the environment multiplied by effort and duration, and rules keyed to a single reading struggle to capture it.

Sports are moving in the right direction — better metrics, graduated responses, sport-specific modelling. What they have mostly not done is accept that the strongest heat intervention available is not playing at that hour, which costs money in a way a cooling break does not.

For the rest of the sport’s rules, see tennis rules and the grand slam final-set tiebreak.