Ha, I realize I have not written about this subject before, it is like air just there and essential to tea as air to life, quietly and permanently. Thanks for bringing up the subject Truth and Go!
Qi is not a self standing object only appears in tea. We should take a look at nature/universe before getting back to tea and qi in tea.
Before anything else, qi in a broad concept is energy, written in Chinese character as air, not oxygen. Qi is an imperceptible mater by human eyes, it is a force, an energy that keeps perceptible matters circulating. For example, keeps our heart beats going, keeps our blood circulating, keeps our intestines moving, wind sweeping, water floating from underground, tides going up and down, earth spinning. All happenings in the universe requires qi to make it happen, all forces is energy and transfer of energy from one form to another. In the case of tea, the sun, soil and water energy being processed and transformed into a tea plant, that energy is being transferred into human body, effecting our physical health and mental health.
Energy exist in every form, not only in food, energy drinks or excise. It existed in the big bang, it exists in air, plant, rocks, minerals, water, wind, words, conversation, music, friendship, love relationship and everything you name it. There are positive energies, there are negative energies.
Human body is affected by external energy while maintaining internal energy. We produce energy and consume energy. Food, beverage are consumable energies, words of kindness, loving gestures are out put energies can be either internal or external.
Cha Qi appears in different strength and effects different parts of a human body according to the type, region, quality and age of tea. The universe has everything plan out already, in a simple matrix view, tea color of light to dark coordinating with seasons from Spring to winter, from dawn to dark nights, from young to matured human age, from head to toe human body, all mapped out. Other factors of cha qi influence are sex related, mainly due to the difference of our own body composition, particularly our sensitivity which greatly effected by genetic, daily activities, food intakes, and stress level. Clean and sensitive bodies respond to cha qi or any form of energy more effectively, in comparison to bodies are constantly under stress by work, relationships and bad diet. Our body is constantly in work of detoxing and replenishing, certain tea has more detox effects, and certain teas has more nourishing effects. Our body needs both, and it is smart enough to crave the type of tea that is more needed by our body. A stock broker would want a more detoxing tea, while a musician may want a more nurturing tea, a college student may want a more alerting mind focusing tea.
How is cha qi different by region, type and age? Regional soil is the basic composition of substance in a plant eventually presents in the tea leaves we consume. Each location has its own energy, some harsh, some delicate, some refine, some coarse, the climate of the location dictates the energy of anything grown at that location, food, plant, to human characteristics both physical and spiritual. I declare this does not involve any location/racial discrimination! It is what it is.
An old Chinese saying: a land of water and soil raise men of that land. The western old saying: we are what we eat. Both world agrees upon the same principle, and that principle is nothing but law of nature. With this in mind, one can identify the type of tea with associated energy by learning history and geography of the world. Ah, that sounds like a lot of studies to do right? Yes, it is. Pieces of puzzles can be linking together once you understand the law of nature. It's vague and it is vast and it here as long as the universe has been existed. The energy of cha qi differs by type, how? The process of tea based on fermentation and roasting lines them up on a scale from light to dark, such oxidation and roasting produces the kind of chemical substance that effect the human body medicinally. The flow of cha qi is also dictated by this process, light effects upper part of body, medium processed effects the center part of body, and dark carries a down ward traveling energy. Age of a tree matter in strength of energy, more powerful than younger ones, simply because it absorbs and accumulates more nutrients through the roots. We also take a lot about high elevation produces good tea, but how so? Cloud layers provides moisture while blocking direct sunlight is an ideal environment for tea growth, sunlight is the main element in how a tea leaf balances the substances inside, more sunlight produces more polyphenol, less direct sunlight produces more amino acid. The proportion of polyphenol and amino acid dictates the taste of tea, and the energy. Again, our body needs to detox and replenish, body cells are made up of amino acid. A clean body respond to a tea with more amino acid more, while a stressed body full of free radical responds to a tea with more polyphenol. Male and female choice of tea can be reflected base on this energy type. Sensitive heart warming teas such as oolong is preferred by female and sensitive males, by this I mean it in a positive constructive way! Buddha is a combination of male and female in wisdom, both male and female are never strictly one sex by nature, we posses energy of both more or less. The perfect balance should be equally present. A woman can be both feminine while strong, a man can be macho while sensitive, there shouldn't be any conflict within the same body.
To put it in a Daoism term, energy is the black and white fishes in the Ba Gua diagram, it's forever changing around the other force or demand, strengthen or weaken accordingly. Tea and human body evolves based on the need of the body, change of environment effecting both tea and human. We shall observe without a set rules.