Although all types of tea come from the same plant, they have very different thermal energies. The thermal energy of tea is influenced by either the teas inherent properties, production, and/or how the tea is prepared.
1- As the growing time of tea leaves increase, its thermal nature becomes warmer. This factor explains how first flushes or shincha, which grow in an explosion of spring growth, is cooler in nature than a second flush, wherein the tea plant begins a short period of dormancy and grows much slower. This factor also explains how teas grown at higher altitudes have slightly warmer thermal energy because the tea grows much slower at a higher altitude.
2- Chemically fertilized tea plants generally have a cooler nature than wild or organic tea plants. This is true because, generally, fertilizers stimulate the tea plant to grow quickly.
3- The older the tea tree, the more warmth the tea has. This explains how old tree puerh is much warmer than puerh from younger trees.
4- The part of the tea leaf used influences the teas thermal nature. A bud is cooler than a leaf. So more tippy puerh is cooler than puerh with mostly leaves. It also explains how earlier flushes that contain more buds are cooler than later flushes. Twigs and stems are warmer than leaves. So the more stems in a puerh cake, or the inclusion of stems in some Japanese teas, makes them warmer.
5- The higher level of oxidization during production, the more warmth a tea has. This factor explains how teas that aren't oxidized, such as green teas, have cooler thermal nature than higher oxidized, teas such as black tea. This also explains how oolongs, yellow teas, and black teas that are highly oxidizated are warmer than oolongs, yellow teas, and black tea that go through lighter oxidization.
6- Steaming creates a cooler thermal energy than pan frying during green tea production. This explains how Japanese teas, which use a steaming method, are cooler than Chinese and Korean green teas, which use a heated iron caldron during production.
7- The thermal nature is related to the colour of the tea. This is probably the most obvious indicator of a teas' thermal nature. Tea with blue, green, or purple colours and hues are cooler than tea with red, orange, or yellow hues or colours. The following is generally accepted- from coldest to warmest- blue, purple, green, yellow, white, orange, black, red. The use of colour as an indication of thermal energy can be used to assess the dry leaf, the colour of the liquid, and the thermal nature of the types of tea (ex. green tea vs yellow tea vs black tea).
8- The longer a tea is aged, the warmer the tea becomes. The older the puerh, oolong, or ddokcha, the more enzymes and micobacteria act on the tea, and the more heat it generates.
9- Roasting the tea before preparation, increases its warmth.
10- The hotter the water temperature used to prepare tea, the warmer the thermal nature of the tea. This explains how subtle green and white tea, which are optimally prepared with cooler water, have cooler thermal nature.
Understanding the thermal nature of tea is paramount to using tea as a medicine, balancing the energies of the body, and thereby insuring that the tea you are drinking is helping and not hindering your health.