Why one temperature does not fit every tea
Different compounds leave the leaf at different rates, and those rates are strongly temperature-dependent. Catechins — the main contributors to bitterness and astringency — extract far more readily in hot water than the amino acids that give a green tea its sweetness and savoury depth. Pour boiling water on a delicate green tea and you extract the harsh compounds efficiently and the pleasant ones inefficiently, which is why the result tastes thin and bitter at the same time. A robust roasted oolong or a compressed dark tea, by contrast, needs full boiling water to open up at all: at 80 °C it will taste of very little.
Rough starting points
Shaded Japanese greens such as gyokuro sit lowest, around 50-60 °C. Ordinary Japanese greens sit around 70-80 °C. Chinese greens and lighter oolongs sit around 80-90 °C. White teas tolerate a wide range, 80-95 °C. Roasted oolongs, black teas and dark teas want 95-100 °C. These are starting points to adjust from, not rules — the specific tea’s own parameters are always the better guide, and they are recorded on each tea’s page.
Getting to temperature without a variable kettle
You do not need special equipment. Boiled water left standing in an open vessel drops fairly quickly at first and then more slowly — a few minutes gets you into green-tea territory in most kitchens. Pouring boiled water between two vessels drops it faster. Adding a measured splash of room-temperature water works too, and is the most controllable of the three. What matters is that you are deliberately below boiling for delicate teas; precision to the degree matters far less than people fear.
Temperature is not the only lever, and bitterness is not astringency
If a tea tastes harsh, temperature is the first thing to check, but not the only one. Bitterness and astringency are different sensations with different causes: bitterness is a taste, driven largely by over-extraction and heat; astringency is a tactile drying sensation, driven by polyphenols binding to proteins in saliva, and it is affected by leaf grade and agitation as well as by heat. Advice that runs them together will fix one and leave the other.