The mechanism, which is real
TeaHQ's own reading of well-attested general knowledge, with no single source behind it.
Temperature falls with altitude, and cooler conditions slow the growth of a tea shoot. A shoot that takes longer to reach picking size accumulates differently: the balance shifts toward the compounds associated with sweetness, aroma complexity and body, and away from the rapid vegetative growth that gives thin, brisk material. This is the same principle that operates seasonally. A first flush after winter dormancy is slow-grown for the same reason a high-mountain tea is, and both are priced accordingly. Altitude is a way of getting a cool-season effect all year. Secondary effects follow: greater day-night temperature swing, more persistent mist and cloud reducing direct light, and in some districts a shorter and more sharply defined season.
Where the market has formalised it
SRI LANKA divides its production into elevation bands — high-grown, mid-grown and low-grown — as an official classification, and the bands describe genuinely different teas. High-grown Ceylon is lighter and brisker; low-grown is darker and stronger, and is what much of the Middle Eastern market prefers. Note carefully that the market preference does not follow the price ladder in every destination. TAIWAN sells gaoshan — high mountain — as a category, with the best-known districts well above a thousand metres, and altitude is stated on packaging as a quality claim. DARJEELING and NILGIRI both trade partly on hill-grown character.
Where it stops holding, and this is the important half
SEVERAL OF THE WORLD'S MOST PRIZED TEAS ARE NOT HIGH-GROWN. Longjing's core area around West Lake is a few hundred metres. Uji's tea fields are low. Japanese gyokuro and tencha, among the most expensive tea in the world, come from ordinary lowland fields — their quality comes from shading, cultivar and manufacture, not from altitude. Wuyi's cliffs are not high mountains in the Taiwanese sense. LATITUDE SUBSTITUTES FOR ALTITUDE. Cool conditions are what matters and there is more than one way to get them. Japan and Georgia are cool because they are north; a Sri Lankan hillside is cool because it is high. The mechanism is temperature, and altitude is a proxy for it. ALTITUDE IS ALSO A PROXY FOR OTHER THINGS. High districts are frequently harder to work, so they are picked by hand rather than machine, at a finer standard, in smaller lots, by producers charging enough to justify it. Some of what the premium buys is care rather than height. AND HIGH IS NOT ALWAYS WHAT YOU WANT. If you want a strong tea that stands up to milk, low-grown is the correct answer and high-grown is the wrong one.
How to read an altitude claim
A stated figure in metres from a producer who also names the district is a real piece of information. "High mountain" with no figure is a category word, and in some markets it is used loosely. The more useful question is what the altitude is being used as a proxy for. If the seller can also tell you the cultivar, the harvest month and the manufacture, the altitude is one variable among several you can now weigh. If altitude is the only thing on the label, it is carrying more weight than any single variable should.