I found this on wikipedia when I was brewing some Kukicha and wanting to know how to brew at a lower temp visually by seeing an open pot of filtered water get hot via flame. I know this is not really gong fu as I don't own all of the tea accessories; but I do utilize or at least estimate how hot the water is for different types of teas such as green, black, oolong, or white tea.
Chemistry and physics
In essence, what is desired in Gongfu Cha is a brew that tastes good and is satisfying to the soul. Tea masters in China and other Asian tea cultures study for years to perfect this method. However, method alone will not determine whether a great cup of tea will be produced. It has been suggested that the chemistry and physics behind Gongfu Cha are what make this method far superior to any other when brewing Chinese teas. Essentially, two things have to be taken into consideration: chemistry and temperature.
Water should be given careful consideration when conducting Gongfu Cha. Water which tastes or smells bad will adversely affect the brewed tea. However, distilled or extremely soft water should never be utilized as this form of water lacks minerals, which will negatively affect the flavor of the tea and so can result in a "flat" brew. For these reasons, most tea masters will use a good clean local source of spring water. If this natural spring water is not available, bottled spring water will suffice. Yet high content mineral water also needs to be avoided. Hard water needs to be filtered.
During the process of Gongfucha, the tea master will first determine what is the appropriate temperature for the tea being used, in order to extract the essential oils of the tea. An optimal temperature must be reached and maintained. The water temperature depends on the type of tea used.
95 °C for Oolong (Chinese: traditional 烏龍; simplified 乌龙; pinyin: wūlóng) tea
100 °C (boiling) for compressed teas, such as Pu-erh tea (Chinese: 普洱; pinyin: pǔ'ěr)
Note: Green tea is usually not used for a Gongfu tea ceremony.
The temperature of the water can be determined by timing, as well as the size and the sizzling sound made by the air bubbles in the kettle.
At 75–85 °C, the bubbles formed are known as "crab eyes" and are about 3 mm in diameter. They are accompanied by loud, rapid sizzling sounds.
At 90–95 °C, the bubbles, which are now around 8 mm in diameter and accompanied by less frequent sizzling sounds and a lower sizzling pitch, are dubbed "fish eyes".
When the water is boiling, neither the formation of air bubbles nor sizzling sounds occurs.
At high altitudes water boils at lower temperatures, so the above rules cannot be applied.