The Australian Open Heat Stress Scale Explained

The australian open heat policy is the only rule at a Grand Slam that can empty twenty-odd courts inside ten minutes and leave three matches running. It exists because Melbourne in late January is not merely warm. It is a city where a northerly wind off the interior can push the temperature past 40 degrees for three or four days running, and where the tennis is played on a dark, sealed hard court that radiates most of that heat straight back at the people standing on it.

Since 2019 the mechanism has been the AO Heat Stress Scale: a single number between 1.0 and 5.0, updated live, that decides whether the match you are watching gets an extra ten-minute break, gets moved under a roof, or stops entirely. It replaced two decades of fixed temperature triggers that were revised four times and satisfied almost nobody.

This page sets out what the scale measures, what happens at each band, what "suspension" actually means in practice, how the roofs fit in, and how the tournament arrived here. The wider tournament structure sits on the Australian Open format guide, the arena itself is covered on the Rod Laver Arena guide, and the equivalent rules in the country's other summer sport are on the Australian cricket heat policy page.

Why Melbourne needed a rule the other majors did not

Three of the four Grand Slams have a heat problem. Only one of them has Melbourne's.

The US Open in New York is humid rather than extreme, and its worst days sit in the mid-thirties. Roland-Garros and Wimbledon are played in a European summer that occasionally spikes and generally does not. The Australian Open is played in the middle of the Australian summer in a city whose weather is governed by which direction the wind is coming from. A southerly change drops the temperature fifteen degrees in twenty minutes. A northerly brings desert air across four hundred kilometres of dry country and dumps it on a stadium precinct built on reclaimed land beside the Yarra.

The surface makes it worse. Melbourne Park's GreenSet hard courts are a cushioned acrylic laid over asphalt or concrete. Dark, sealed and non-porous, they absorb solar radiation through the morning and release it through the afternoon, so the air at ankle height on an outside court in the middle of the day can be ten degrees or more above the shade temperature recorded a hundred metres away. A player is standing in that layer for four hours. The distinction between the two courts on the same site is real: a match on a shaded outside court in a breeze is a different physiological event from a match on an exposed one in still air, and any policy driven by a single citywide number cannot see the difference.

There is a scheduling factor as well. The tournament runs fifteen days and has to fit more than six hundred matches across singles, doubles, mixed, juniors, wheelchair and legends events into that window. Suspending play is expensive in a way it is not at a tournament with slack in the calendar. That tension — genuine physiological risk against a schedule with almost no give in it — is the thing every version of the policy has had to arbitrate.

What the Heat Stress Scale actually measures

The scale takes four inputs, live, from five monitoring positions spread across Melbourne Park.

The first is air temperature in the shade. This is the number that appears on a weather app and the least useful of the four on its own.

The second is radiant heat, which is the strength of the sun landing on the court and on the player. Two courts at identical air temperatures produce very different loads depending on whether one is in full sun and the other in stand shadow.

The third is relative humidity. This is the input that decides whether the body's main cooling mechanism works. Sweat cools by evaporating. In dry air it evaporates freely and a player at 40 degrees can hold core temperature. In humid air it runs off without evaporating, the cooling stops, and core temperature climbs at 35 degrees in a way it would not at 40 in the dry.

The fourth is wind speed, which accelerates evaporation and shifts the hot boundary layer sitting on the court surface. A ten-knot breeze is worth several degrees of apparent heat.

Those four are combined into one index calibrated against the physiological limits of the athletes actually on site. The scale was built by Tennis Australia's medical staff with the Thermal Ergonomics Laboratory at the University of Sydney, and it incorporates research on sweat rates, core temperature response and the differences between adult, junior and wheelchair athletes. That last point is the reason the scale can trigger different responses for different events on the same afternoon rather than applying one blanket rule.

Five measuring stations matter more than they sound. A single reading taken at the Bureau of Meteorology's Melbourne site — which is what the old policy relied on — describes a location that is not a tennis court. Instrumenting the precinct itself was the single largest technical change from the old system to the new one.

The five bands and what each one triggers

The scale runs from 1.0, which is temperate playing weather, to 5.0, which is suspension. The formal interventions attach to the top two bands.

Reading	Conditions	What it changes
1.0	Temperate	Nothing. Normal play, normal breaks.
2.0	Warm	Nothing mandatory. Cooling equipment and on-court ice available as usual.
3.0	Hot	No mandated break. Medical staff monitoring, increased cooling provision, players advised on hydration.
4.0	Extreme	Extended break added: ten minutes in singles, fifteen in wheelchair singles, taken between sets two and three for women and juniors and after set three for men.
5.0	Beyond safe limits	Play suspended on outside courts and in practice. Roofs closed on the three arenas so those matches continue indoors.

The middle bands do real work even though they trigger nothing formal. A reading of 3.0 changes how the medical team is deployed, how much ice is on court, and how closely a player showing signs of distress is watched. It is also the point at which the tournament's own communications start telling people in the crowd to move into shade and drink water.

The thing worth internalising is that the scale is not a temperature converted into a number. Two afternoons with identical maximum temperatures can sit two full bands apart. That is a feature. It is also the reason people watching from home sometimes see play continue at 41 degrees and stop at 37, and conclude the policy is arbitrary when it is doing exactly what it was designed to do.

Level 4: the extended break, and who gets which one

At 4.0 the tournament inserts a break that does not otherwise exist in tennis.

For women's singles and junior singles it comes between the second and third sets. For men's singles it comes after the third set. The asymmetry is a function of format: a women's match is best of three, so the break falls before the deciding set, while a men's match is best of five and the equivalent point in the match is later.

40%60%

• Singles10
• Wheelchair singles15

Minutes of scheduled break added at a reading of 4.0. Singles covers men's, women's and junior singles. The longer wheelchair break reflects the reduced heat-shedding capacity documented in the research the scale was built on. Neither break is a medical timeout and neither requires a player to request it.

Show the numbers

How long the extended break lasts at 4.0

Item	Value
Singles	10
Wheelchair singles	15

Three things about that break are frequently misread. It is not a medical timeout: nobody has to be injured, nobody has to ask, and it does not consume any of the treatment entitlement a player has for the rest of the match. It is not optional in the sense of a player being able to decline it and keep playing. And it is not the same as the ten-minute break some tours allow between sets in ordinary conditions — this one is triggered by the environment, not by the scoreline.

Players use it the way an athlete uses any recovery window. Ice vests, cold towels, a change of clothing, fluid and electrolytes, and time out of the sun. Ten minutes is not enough to reverse a serious core temperature rise, which is the honest limitation of the intervention, but it is enough to arrest the climb and let a player restart from a lower baseline than the one they would otherwise have carried into the next set.

Wheelchair athletes get fifteen. That is not a courtesy. Many wheelchair athletes have reduced sweating surface, impaired thermoregulation below the level of injury, and additional heat generated by the propulsion itself. The research underpinning the scale treated them as a separate population rather than as adults with a modifier applied, which is one of the more defensible design choices in the whole system.

Level 5: suspension, and what suspension actually means

A reading above 5.0 is the top of the scale and the point at which the tournament stops playing tennis outdoors.

1. 1Reading crosses 5.0The live index from the five monitoring stations goes above 5.0 and the tournament referee is notified. The reading is published on the tournament's own channels at the same time.
2. 2Nothing new startsMatches that have not yet gone on court on outside courts do not start. Practice on outside courts stops. Warm-ups are held.
3. 3Matches in progress play to a breakA match already under way continues until the end of an even number of games in the current set, or until a tiebreak in progress is completed, so that neither player gains an advantage from the stoppage point.
4. 4Players come offOnce that natural break is reached the match is suspended where it stands. The score is preserved and the match resumes from that point when conditions allow.
5. 5The roofs closeRod Laver Arena, Margaret Court Arena and John Cain Arena close their roofs and continue. Three matches carry on while the rest of the site waits.
6. 6ResumptionPlay resumes on outside courts when the index has fallen and the referee is satisfied conditions will hold. Nothing obliges that to happen the same afternoon.

The sequence the tournament referee follows. It is a procedural description, not a measurement. Timings depend on where each match happens to be in its set when the reading crosses the threshold, so the site does not go quiet all at once.

The detail people miss is the third step. Suspension is not a whistle. A match that reaches the threshold at 3-2 in a set plays on to 4-2 or, if a tiebreak is under way, plays the tiebreak out. Stopping inside a game would hand a material advantage to whichever player was serving, and the tournament has consistently judged that unacceptable even when it costs several more minutes of exposure.

The other detail is that a suspension is open-ended. There is no fixed two-hour clock as there was under the 2003 version of the policy. Play comes back when the index falls, which in Melbourne usually means when the southerly change arrives, and that can be a five-hour wait. It can also be a wait that runs past the point where restarting the outside courts is worth it, in which case matches are moved to the following day and the whole schedule compresses.

The three roofs and the rule that governs them

Melbourne Park has three closing roofs, which is more than any other Grand Slam venue.

Rod Laver Arena has had one since the site opened as Flinders Park in 1988 — the first retractable roof at any tennis facility anywhere. Margaret Court Arena gained one for the 2015 tournament, in the same round of changes that followed the 2014 heatwave. John Cain Arena, which has been Hisense Arena and Melbourne Arena in previous lives and is covered in detail on the John Cain Arena guide, has one as well. Kia Arena, Show Court Arena and every numbered outside court are open to the sky.

Two rules matter. First, if outside matches have been suspended, the referee may close the roofs on the arena courts or keep them closed for upcoming matches, which is why a scheduled outdoor evening match can become an indoor one without warning. Second, and less well known: if an arena roof is closed before the end of the first set in a women's match or before the end of the second set in a men's match, no extra cooling break is added even if the index outside is at 4.0. The logic is that the players have spent the bulk of the match indoors and do not have the accumulated heat load the break was designed to relieve.

Closing a roof changes the match, not just the temperature. Indoor conditions are slower, the ball travels differently without sun and wind, and serve-dominant players lose some of the advantage that a hot, fast outdoor court gives them. A player who has spent two sets adapting to a crosswind now has none. This is a genuine competitive consequence of a safety decision, and there is no version of the rule that avoids it.

Before 2019: a number that kept moving

The tournament ran on fixed thresholds for two decades, and the sequence of revisions tells its own story.

The headline temperature threshold in the extreme heat policy at each revision, in degrees Celsius. From 2003 the policy also carried a wet bulb globe temperature threshold, initially 28 degrees and from 2015 32.5 degrees, which is not shown here. The scale replaced fixed thresholds entirely from 2019.

Show the numbers

The air temperature trigger the Australian Open used before the scale

Item	Value
1998	40C
2002	38C
2003	35C
2015	40C

A policy adopted in 1998, after consultation with players, stopped play at 40 degrees. In 2002 that came down to 38. In 2003 it came down again to 35, and a wet bulb globe temperature threshold of 28 degrees was added alongside it. Then, after the 2014 tournament, both numbers went back up — to 40 degrees and a wet bulb reading of 32.5 degrees — explicitly to allow more continuous play and fewer stoppages.

Raising the threshold after the most notorious heatwave in the tournament's history looks perverse until you understand what the 2014 problem actually was. The complaint was not that the thresholds were too low. It was that the policy had barely triggered at all during four days of extreme heat, because the wet bulb component stayed below its trigger in very dry air, and because the discretion over whether to invoke it sat entirely with the referee with no published criteria. Players did not know what would stop play or when. Raising the number while making the trigger more mechanical was an attempt to fix the transparency problem rather than the temperature one. It did not work well enough, and four years later the whole apparatus was replaced.

The roof rules of the early period are worth noting as a curiosity. When Rod Laver Arena opened, the roof could be closed only above 39 degrees, or at the referee's discretion above 35, and only for daytime matches, and only once every remaining singles match could be scheduled inside that one arena. In practice that made roof closure available from about the quarter-finals onward. In 1993, with the temperature at 40 degrees for the final, Jim Courier threatened to boycott if the roof was closed. It stayed open.

2014: the tournament that ended the old policy

Four consecutive days of the 2014 Australian Open peaked between roughly 41.5 and 43.9 degrees. What happened on court became the case study that every subsequent version of the policy has been written against.

• 4Consecutive days above 41 degrees
• 43.9Peak temperature in degrees Celsius
• 9First-round withdrawals
• 970Spectators treated for heat exhaustion on day three

Figures as reported at the time. The temperature is the peak daily maximum in the shade, not an on-court surface reading, which runs substantially higher on a sealed acrylic court in full sun. The spectator figure is for the third day alone.

Frank Dancevic hallucinated a Snoopy figure on court, lost consciousness for around a minute, and afterwards called the conditions inhumane. Peng Shuai vomited during her match. Ivan Dodig collapsed and said publicly that he had feared he might die. A ball boy fainted and was carried off by a player. Water bottles softened in the sun. Nine players withdrew in the first round, a record for the tournament.

Play was uninterrupted on the second, third and fifth days of that stretch. There was a stoppage of about four hours on the fourth. Organisers maintained that humidity had stayed low enough that the policy's own criteria were not met, which was probably true and was exactly the problem: a policy whose criteria could not be met at 43.9 degrees was measuring the wrong thing.

There was a legal dimension as well. A spectator sued the trust that runs Melbourne Park over the failure to close the roof at what was then Hisense Arena during a second-round match at 40 degrees, claiming she fell on stairs while trying to reach shade. Whatever the merits, it made explicit that the heat exposure at Melbourne Park was not purely a player-welfare question.

Why a single wet bulb reading was the wrong instrument

Wet bulb globe temperature is a respectable index. It is used by militaries and by industrial safety regulators, and it does combine temperature, humidity, radiation and air movement. The problem at Melbourne Park was not the index. It was where it was measured and how it was used.

The reading came from a Bureau of Meteorology station near the site rather than from the courts. A station sited for meteorological consistency sits over grass, at a standard height, away from radiating surfaces. A tennis court is the opposite of all three. The number was accurate for the location it described and did not describe the location that mattered.

The second problem is the shape of the index in dry air. A wet bulb threshold can fail to trigger at extremely high dry temperatures, because the humidity term stays low and pulls the composite down. Melbourne's worst days are exactly that: hot, dry, northerly air. It is possible to construct conditions in which the temperature is enormous and the wet bulb figure never reaches its trigger. Melbourne does not need to construct them. It gets them most summers.

The third problem was discretion. Even when the numbers were met, invoking the policy was the referee's call, and the criteria were not published in a way players could plan around. The current system publishes the reading. That single change did more for player confidence than any adjustment to the threshold ever did.

24 January 2026: the policy in operation

The clearest recent demonstration came on the middle Saturday of the 2026 tournament.

The forecast maximum was 40 degrees. By around 3pm the air temperature was 38. Shortly after 2.30pm the index went above 5.0, and the tournament posted the reading directly: play suspended for all matches and practice on outside courts, roofs closing on Rod Laver, Margaret Court and John Cain and remaining closed for the rest of those matches, and a public instruction to anyone on site to seek shade and use cooling strategies.

Outside courts did not resume for approximately five hours. Three matches continued indoors throughout. Everything else moved.

Two things are worth taking from that day. The first is that the system worked as designed and was still deeply disruptive: a five-hour hole in the middle of a Saturday at a fifteen-day tournament pushes matches into the evening, extends already long night sessions, and compresses the following day. The relationship between the heat rule and the schedule is direct, and it is covered from the other side on the Australian Open night sessions guide.

The second is that a crowd of tens of thousands does not get a roof. Three matches were protected. Everyone holding a ground pass spent five hours at a venue where the tennis had stopped and the temperature had not.

What players are allowed to do while the index climbs

Between the mandated breaks, the available responses are the ordinary ones, used harder.

At changes of ends players take ice towels around the neck and shoulders, ice vests where the format allows time to put one on, cold fluid, and shade under a courtside umbrella where one is provided. Between sets there is a longer window for the same. Sodium replacement matters more than most spectators realise: a player can lose two litres an hour in Melbourne conditions and replacing the water without the salt causes its own problems.

What a player cannot do is manufacture a break. A medical timeout requires an actual assessable condition. Heat illness qualifies — a physiotherapist or doctor can be called and a player showing signs of heat exhaustion will be treated and, in serious cases, retired from the match by medical advice. Being extremely hot does not qualify. The distinction is policed, and it is one reason the level 4 break exists at all: without a scheduled window, the pressure to use medical timeouts as informal cooling breaks becomes irresistible. The general framework for on-court medical intervention is set out on the tennis medical timeout rules page.

Retirement remains a player's own call at any point. In extreme conditions the tournament will not force a fit player off court, and the number of mid-match retirements during hot spells is a reasonable proxy for how bad a given afternoon actually was.

What the policy does and does not do for the crowd

The scale governs play. It does not govern attendance, and the gap between those two things is the least examined part of the whole subject.

Melbourne Park runs shaded seating areas, free water refill stations, misting points, first aid posts across the precinct, and public messaging that escalates as the index climbs. Grand Slam Oval and the covered concourses absorb a lot of people on a bad afternoon. None of that is a roof, and none of it is a refund. A ground pass does not entitle the holder to tennis on a given court, and a suspension does not close the venue.

The 2014 figure — 970 people treated for heat exhaustion in a single day — is the number that should anchor any discussion of crowd safety here. It is more than the entire playing field of the tournament combined, several times over. The people most at risk at Melbourne Park in a heatwave are not elite athletes with medical teams and ice vests. They are spectators in the third hour of a queue.

Practical advice for anyone attending in January: buy the ground pass expecting the possibility of a long stoppage, treat the covered areas as the plan rather than the fallback, and remember that a level 5 reading means the three arenas are the only tennis on the site and the arena tickets are already sold. The ticketing structure that produces that outcome is set out on the night sessions guide linked above.

How it compares with the rest of Australian sport

Australian sport has had to write heat rules more than most, and the approaches differ in instructive ways.

Cricket's response, covered on the Australian cricket heat policy page, leans on heat stress indices and on match-official discretion, with the wrinkle that cricket has drinks breaks built into its structure already and can add them without altering the format. The AFL uses heat policies that scale from extra interchange rotations and longer quarter breaks up to postponement, and at community level the state bodies publish hard thresholds. Football in Australia runs mandatory drinks breaks at set intervals above defined thresholds.

Tennis is the hardest case of the four, for a structural reason. It has no clock, no substitutes, and no way to shorten a contest without changing the result. A cricket team can bring on a fresh bowler. An AFL side can rotate. A tennis player in a five-set match has to keep standing there. The only levers available are a break, a roof, or a stoppage, which is exactly the three-lever system the scale operates.

It is also the only one of the four where the environment is being measured on the field of play itself, at five points, in real time. That is a higher standard of instrumentation than most Australian sports apply, and it came out of the 2014 failure rather than out of foresight.

The criticisms that have not gone away

The scale is a substantial improvement on what it replaced. It is not beyond argument.

The first criticism is that the thresholds sit high. Play continues in conditions that would stop a community sports fixture anywhere in the country, on the reasoning that professional athletes are conditioned, acclimatised and medically supervised. That reasoning is sound and it still means the top of the scale is calibrated to the limits of elite bodies rather than to comfort.

The second is that a ten-minute break is a modest intervention against a serious heat load. It arrests a climb rather than reversing one. Nobody involved claims otherwise, but it is the only intervention available that does not change the format of the match.

The third is the roof asymmetry. Three matches are protected and the rest of the draw is not. A player scheduled on Rod Laver Arena and a player scheduled on court eight are playing the same tournament under materially different conditions, and the allocation of arena slots is driven by ranking and box office rather than by welfare. On a 5.0 day, the seeded players are inside and the qualifiers are waiting.

The fourth is that publishing the reading has not fully solved the predictability problem. The index moves through the day, players cannot know at 10am what the afternoon will bring, and warm-up and nutrition planning for a match that may or may not include a ten-minute break at a known point remains guesswork.

What to re-check before relying on this page

Three things move. First, Tennis Australia reissues the extreme heat protocol for each tournament and the wording changes between editions — check the document published for the specific year rather than assuming continuity. Second, the referee retains discretion for situations the scale does not cover, including the interaction between heat, rain and the roof, so an individual decision may not follow the pattern described here. Third, the number of covered courts at Melbourne Park is a live question: the precinct has been redeveloped repeatedly since 2015 and a fourth roof would change the arithmetic of a 5.0 day substantially.

Related reading on this site: the tournament structure is on the Australian Open format guide, the evening schedule the heat rule keeps disrupting is on the Australian Open night sessions guide, the money at stake for the players standing in it is on the Australian Open prize money guide, and the surface that stores the heat is explained on the hard court tennis guide. The wider set of Australian sport guides sits under the Australia hub, the tennis archive and the sports index.