The All Blacks are set to face a challenge tonight, no, probably not Canada but the heat and humidity of playing indoors.

At kick-off, the temperature in Oita is due to be around 25C with humidity at a whopping 89 per cent.

And while the roof of the 40,000 capacity Oita Stadium will be closed, with the threat of Typhoon Mitag looming, a sweaty encounter looms.

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"I've never experienced humidity like it. It makes Auckland's feel like a cool summer breeze," Herald senior rugby writer Patrick McKendry said.

"Here it feels like you have to part the air before you walk through it."

Jack Goodhue, Sonny Bill Williams and Kieran Read during All Blacks training. Photo / Getty Images
Jack Goodhue, Sonny Bill Williams and Kieran Read during All Blacks training. Photo / Getty Images

Humidity refers to the concentration of water vapour present in the air.

Simply put, a day with high humidity would feel sticky due to a high concentration of water vapour in the air, Niwa meteorologist Maria Augutis said.

Asked what conditions would make humidity stronger, Augutis said it was very dependant on where the air was coming from.

"Wind travelling over large bodies of water are generally more humid than air moving over big and dry land masses.

"The [Japan] summer weather is often influenced by southerly winds associated with the 'Pacific high', an area of high pressure that forms over the warm and humid Pacific Ocean."

Under the laws of the game, when weather conditions were "exceptionally hot and/or humid", the referee could allow for a water break.

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The one-minute break should be taken midway through the half, following a score or when the ball was dead near the halfway line.

During exercise, our bodies heat up and we start sweating. Sweat would then evaporate from the skin which helps us cool off.

All Blacks hooker Codie Taylor throwing a pass. Photo / Mark Mitchell
All Blacks hooker Codie Taylor throwing a pass. Photo / Mark Mitchell

However, on a hot and humid day, it was harder for sweat to evaporate, making it difficult to cool down, Augutis said.

"Hard physical work in extreme heat and humidity may cause our bodies to overheat and exhaust more easily."

Walking across the pitch at Oita Stadium yesterday, an All Blacks management member told McKendry his shoes were soaked.

"The stadium is covered but the ball will be very wet due to the players' sweat but also the moisture on the pitch caused by the humidity," McKendry said.

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"Meanwhile, there are a lot of Kiwis arriving in Beppu, a 12-minute train journey from Oita, and the beer was flowing last night as they refreshed themselves."