What are the heat rules at Australian Open?

"I don't know what is going on with Melbourne, it's like four seasons in a day."

World number 13 Casper Ruud was quick to state his desire to once again play in a night match at the Australian Open, despite the cooler conditions which that involved on Thursday.

And with good reason.

The forecasts predicting temperatures approaching 40C by the time of his next match on Saturday will not have escaped the players' attentions.

Melbourne is set for a particularly hot spell over the coming days, with the latest forecasts predicting highs of 37C on Saturday and 38C on Tuesday.

So, at what point do the conditions become unplayable and what measures will be taken to reduce the risk to players' health?

The Australian Open monitors the severity of the conditions using its extreme heat policy (EHP), which is backed by research from 2019 into the effects of heat stress on players.

This research led to the development of the heat stress scale (HSS), used at the tournament to decide when the conditions have become too challenging - and when player health could be at risk.

The scale measures four factors: external radiant heat (or the strength of the sun), air temperature in the shade, relative humidity and wind speed.

These factors are measured at various points around Melbourne Park, where the Australian Open is played.

Should the scale hit level five, play will be suspended.

In that instance, the tournament referee will consult the HSS and decide when to award extra breaks, or even suspend matches.

If matches are already in play, the referee can suspend them once an even number of games have been played in a set, or after a tie-break has finished.

There are air conditioning systems in the Rod Laver Arena, Margaret Court Arena and the John Cain Arena, each of which have retractable roofs that can be closed if necessary.

The tournament referee can take the decision to pause the match and close the roof, allowing play to continue in extreme heat or wet weather.

Beyond a certain threshold players are allowed additional breaks.

When the HSS reaches level four - before or during the first two sets of a match - women and junior players will be given an additional 10-minute break, and wheelchair singles players a 15-minute break, between the second and third sets.

For men's singles matches, which are best-of-five sets, a 10-minute break is added after the third set.

Conditions in Melbourne are notoriously changeable and the 2025 tournament witnessed shifts from sunny skies to torrential rain and thunderstorms.

Temperatures can often fluctuate between 30C and 40C during the day, but can drop into the mid-to-low teens for the evening sessions.

In 2014, Melbourne had three consecutive days with temperatures above 40C.

The shifting weather patterns are largely due to Melbourne's position on the southeastern coast of Australia, at the meeting point of contrasting hot and cold air masses.

Analysis by Matt Taylor, BBC Lead Weather Presenter

Melbourne is set to experience some wild swings in temperatures. That is all down to the city's location on the south-east coast of Australia.

With the desert interior to the north, and the chilly Southern Ocean and Antarctica to the south, a quick change in wind direction can have you reaching for a fan one day and grabbing your jumper the next.

During the summer the interior of Australia heats up rapidly, and any wind that blows in from a northerly direction will bring that searing heat with it.

However, any switch to a southerly wind will blow in air off a sea that sits at about 16-18C this time of year - the city's own version of natural air conditioning.

If the origin of this wind comes a long way from the south, then you are tapping into even colder conditions and, of course, grabbing those warmer clothes.

Each Grand Slam has a slightly different way of deciding when to enforce any heat policies.

The heat rule used at Wimbledon is based on the 'wet bulb global temperature' - a measure of heat stress.

When this exceeds 30.1C, a 10-minute break will then be added between certain sets, which can be requested by the players.

The US Open has a similar strategy, allowing 10-minute breaks between sets three and four for the men's singles, and between sets two and three for the women's and juniors.

The roof of the Arthur Ashe Stadium and the Louis Armstrong Stadium will also be partially shut in extreme conditions, as part of new policies adopted last year.

The French Open was the last Grand Slam to add a retractable roof to one of its courts, the Philippe Chatrier, in 2020.

This article is the latest from BBC Sport's Ask Me Anything team.