Dry tea is not dry
Finished tea is not a dehydrated material in the sense that a desiccant is. It has been fired down to a low residual water content and then left there, and what remains is enough to matter. More importantly, that water is not fixed. Dry leaf is hygroscopic: it takes up water vapour from the air when the air is damper than the leaf and gives water back when the air is drier, and it keeps doing so until the two are in balance. A tin opened in a kitchen and closed again has admitted a volume of air whose moisture the leaf will now equilibrate with, and a cake sitting unwrapped in a room is in continuous exchange with that room. This is the single fact that makes storage advice cohere. A container is not a wall around a fixed object; it is a boundary between the leaf and an atmosphere, and its job is to decide which atmosphere the leaf gets to talk to. A sealed pouch gives the leaf a very small atmosphere that it quickly comes into balance with and then stops changing. An open jar in a room gives it the room. A cake in a cotton wrapper inside a carton inside a cupboard gives it a slow, buffered version of the room. Every storage decision in the catalogue is a version of that choice.
Relative humidity is about the air; the leaf has its own number
The reading on a hygrometer describes how full of water vapour the air is, relative to how much it could hold at that temperature. It says nothing directly about the leaf. What matters to the leaf is the point at which the two stop exchanging — the humidity the leaf would produce if you sealed it in a small container and waited. Storage people talk about ambient humidity because it is the thing you can measure, and the thing that actually governs the leaf is where the leaf settles against it. Two consequences follow that are worth holding. First, a leaf moved from one environment to another does not change instantly; it drifts towards the new balance over days or weeks depending on how much air reaches it, which is why a brief exposure costs less than the panic it usually causes and why a slow leak costs more than it looks. Second, sealing a tea does not fix its moisture at a good level, it fixes it at whatever level it had when the seal closed. A green tea sealed on a humid August afternoon is sealed with that afternoon inside it. This is the real argument for opening fragile teas briefly and in a dry room, and it is a better argument than the usual one about air.
Why warmth and moisture cannot be separated
Warm air holds far more water vapour than cool air, so the same quantity of water in a sealed container reads as a lower humidity when warm and a higher one when cool. A tin that heats through the afternoon and cools overnight is therefore not a stable environment even if nothing enters or leaves it: the leaf is being pushed towards giving water up and then towards taking it back, once a day, every day. Over months that cycling does more than a single steady reading at either end would. This is the mechanism behind the advice that stability matters more than any particular temperature, and it is why that advice is so often given without a reason attached. It also explains why the coldest place in a house is frequently a poor store while a boring, dull, interior cupboard is a good one. A cold outside wall, a garage, a loft and a windowsill all swing; a cupboard in the middle of a heated flat barely moves. When choosing between two locations, the question worth asking is not which is cooler but which changes less across a day and across a season, and the answer is usually the one with the least outside wall and the fewest appliances near it.
The threshold that separates staling from spoilage
Below a certain water level almost nothing biological can establish itself on dried plant material, which is the whole reason tea is fired down to where it is. Above it, the material becomes hospitable, and what follows is not a faster version of ordinary ageing but a different event — the musty, sour, cellar character that no brewing adjustment addresses. TeaHQ does not state a figure for that threshold, because published values depend on temperature and on the organism and because a number invites a household to think it can be measured at home, which it cannot. What can be stated is the shape of the relationship: the risk is negligible across the range dry tea normally occupies, rises steeply over a fairly narrow band, and is a categorical change rather than a matter of degree once crossed. That shape explains why the ageing categories are managed rather than merely stored. A pu-erh store is being held deliberately in the region where change proceeds and the band is still some distance away, which is a narrower target than it sounds and is why anyone doing it seriously owns an instrument. It also explains why the same humidity that is correct for a cake is a straightforward hazard for a sencha, which is being held as far from the interesting region as the household can manage.
What the leaf tells you before an instrument does
Residual moisture is one of the few storage variables with a free domestic test. Well-dried tea is light, brittle and noisy: whole leaf rattles against a tin, a twisted sencha needle snaps rather than bends, a rolled oolong pellet is hard, and a pinch of leaf crushed between finger and thumb crumbles cleanly. Leaf that has taken up water feels softer and slightly cool, sounds dull, bends before it breaks and crushes into flakes rather than powder. Anyone who checks a tea on the day it is opened has a reference, and the same check months later reports a change that no amount of looking at the leaf would show. Two cautions apply. Categories differ in their baseline, so a fluffy white tea and a hard-pressed cake will never feel alike, and the comparison is only meaningful against the same tea. And the test detects a fairly large change rather than a small one, so it is a check on whether something has gone wrong rather than a monitoring instrument. For anything being stored deliberately over years, a hygrometer inside the enclosure remains the only way to know what conditions the leaf is actually in, and the tactile check is what tells you to go and look at it.
Rules that fall out of this without being memorised
Fill a container rather than half-filling it, because a smaller reservoir of air carries less water to give the leaf and reaches balance sooner. Do not wash a tin between teas, because the residue that matters is water rather than aroma. Bring a cold sealed pack to room temperature before opening it, because a leaf below the dew point of the room is a surface for water to collect on. Keep tea out of kitchens, because kitchens cycle between damp and dry several times a day. Open a fragile tea briefly, and preferably not while something is boiling on the hob. Put a desiccant with a preservation tea and never with an ageing one. Buy a hygrometer before buying a humidifier. Each of those appears in the Sprint 31 storage pages as an instruction, and each of them is the same instruction: control what air the leaf is in equilibrium with, and stop that equilibrium from moving. A reader who holds the underlying picture can also derive the answer for a case nobody has written down — a tea arriving by post in winter, an unfamiliar category, a house that has just been repainted — which is the point of putting the mechanism on its own page rather than lengthening the list of rules.