Measuring Heat
It’s not just the air temperature that impacts the amount of heat players feel during matches. The lack of airflow in the outdoor stadiums and heat produced by spectators both contribute to increases in temperatures.
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The US Open and Australian Open use Wet Bulb Globe Temperature to determine if conditions are too dangerous for competitors to continue playing.
The Australian Open introduced an Extreme Heat Policy in 1998. It states if daytime temperatures hit 35°C (95°F) all sets in progress must be completed and no new sets or matches will be started until conditions are safer. In addition, longer breaks are given to players between sets and matches.