The Extreme Heat Policy is a rule pertaining to the Australian Open

Australian Open

The Australian Open is the only Grand Slam tennis tournament held in the southern hemisphere. The tournament was held for the first time in 1905 and was last contested on grass in 1987. Since 1972 the Australian Open has been held in Melbourne, Victoria. In 1988, the tournament became a hard court...

 (tennis

Tennis

Tennis is a sport usually played between two players or between two teams of two players each . Each player uses a racket that is strung to strike a hollow rubber ball covered with felt over a net into the opponent's court. Tennis is an Olympic sport and is played at all levels of society at all...

). It was introduced in 1998 after consultation with a number of tennis players. Here is the text of the policy for the 2009 tournament:
The WBGT

Wet Bulb Globe Temperature

The Wet Bulb Globe Temperature is a composite temperature used to estimate the effect of temperature, humidity, wind speed and solar radiation on humans. It is used by industrial hygienists, athletes, and the military to determine appropriate exposure levels to high temperatures...

is used to help decide whether the EHP should be implemented. The WBGT is a measure of heat stress index and takes into account the actual temperature as well as radiation, wind, and humidity. It is a combination of the WBGT with the actual air temperature that is used to decide whether the EHP is activated.

The heat rule was first invoked during the quarterfinal round in 1997.http://www.nytimes.com/1997/01/21/sports/australian-open-hits-the-roof-as-the-temperatures-soar.html

Officials considered closing the roof for the final in 1993 due to a temperature of 104 degrees (40 °C),http://articles.latimes.com/1993-01-31/sports/sp-1174_1_australian-open but Jim Courier threatened to boycott the match unless the roof remained open. http://www.australianopen.com/en_AU/event_guide/history/draws/1993_MS_1.html