There is much debate over how to brew tea and what the "best" water temperature is for each tea. Ultimately, it's a matter of personal taste and the specifics of how you brew (including factors like whether you pre-warm your teapot and what ratio of water to leaves you use). However, if you're new to tea (or just not getting the flavor you want from your tea), these tips will help you get the most out of your brew.
General Notes on How to Brew Tea
First, try following the instructions your supplier gave you. If you're not fully satisfied, try using a lower temperature, brewing for more/less time or using more tea leaves.
When in doubt, use cooler water. Whereas teas can be ruined by water that's too hot, they are rarely hurt much by water a few degrees too cold. (Many British tea drinkers will likely disagree with me on that point. For the record, I'm referring to loose-leaf specialty tea that is to be consumed without milk or sugar.)
Boiling water and then letting it cool removes oxygen from the water and decreases the flavor of the tea. It's better to bring water up to (rather than down to) the appropriate temperature.
How to Brew White Tea
Most people agree that white tea should be brewed with water that is well below boiling and that higher temperatures will scald the tea. I generally use water that is around 160 degrees Fahrenheit (about 75 degrees Celsius), which is when tiny bubbles (about 3 mm each) form on the bottom of a pot of water on the stove.
How to Brew Green Tea
It's better to err on the side of lower temperatures with green teas. If your green tea tastes bitter or overly grassy, try brewing it at a lower temperature. Most green teas are best when brewed well below boiling temp (212 degrees Fahrenheit), at somewhere between 150 and 180 degrees Fahrenheit (about 65 to 80 degrees Celsius).
If you're warming water in a pot, this means it will have either tiny bubbles forming in the bottom of the pot or tiny bubbles (about 3 mm) rapidly rising to the surface of the pot.
Steamed Japanese green teas tend to require lower temperatures than other green teas. Some scented or shaped green teas (like Jasmine Pearls) may be brewed at slightly higher temperatures. Yellow tea should be brewed much like green tea.
How to Brew Oolong Tea
The best temperature for brewing oolong tea depends on how you're preparing it. Gong fu brewing usually requires higher brew temperatures (as well as more leaves and very short brew times) compared to Western-style brewing.
For Western-style brewing, oolong tea is generally best when brewed between 190 to 200 degrees Fahrenheit (88 to 93 degrees Celsius). If you're looking at a pot of water in this temperature range, it will have bubbles that are about 5 mm in size and a moderate amount of steam.
How to Brew Black Tea
Some delicate black teas (like First Flush Darjeelings) require lower brewing temperatures of around 180 to 190 degree Fahrenheit (80 to 93 degrees Celsius). However, most black teas can be brewed between 200 and 212 degrees Fahrenheit (88 to 100 degrees Celsius).
I usually use around 205 degrees Fahrenheit (which is just below boiling), whereas in the U.K. they tend to use 100 degrees Celsius (which is a full boil, as well as a small science joke).
When warming water in a pot, it will have small bubbles (between 4 mm and 8 mm) and a moderate to a large amount of steam when it is between 190 and 200 degrees Fahrenheit. By the time it has reached 212 degrees Fahrenheit, it will have very large bubbles, with no small bubbles remaining.
Some say that pu-erh tea should be brewed with fully boiling water (which is, as you may remember, 212 degrees Fahrenheit, 100 degrees Celsius, and a presence of very large bubbles and no small bubbles in the pot). I find that brewing most pu-erhs with temperatures around 205 degrees Fahrenheit (96 degrees Celsius) is optimal.
Whereas fully boiling water has no small bubbles, water at 205 degrees Fahrenheit has a mix of smaller and larger bubbles.
How to Brew Herbal Teas / Tisanes
Tisanes (a.k.a. 'herbal teas') come from many different plants, so their brewing instructions vary widely. A few (like catnip and yerba mate) should not be steeped in water that is fully boiling. Others (like fennel seed) should be boiled to release their full flavor. (Actually boiling teas or tisanes creates a decoction rather than an infusion.) However, generally speaking, water at a full boil (212 degrees Fahrenheit or 100 degrees Celsius) will work.