Temperature
Green tea wants cold. Of all the categories it is the one that genuinely benefits from refrigeration, and a fridge held between 0 and 5 degrees Celsius will roughly double the time a green tea stays fresh compared with a cupboard.
Stability matters more than hitting a particular number. A spot that swings from a warm afternoon to a cold night does more damage than a steady, mildly warm room, because every swing pulls the leaf through a humidity cycle. Condensation forms, and then dries, and then forms again.
If you will not refrigerate, the next best thing is the coolest cupboard in the house, on an internal wall, away from the oven and away from any radiator. A wine fridge set to about 12 degrees is a good compromise if you have one.
Humidity
Green tea wants dry. Below about 50 percent relative humidity is the safe zone, and lower is fine, because green tea is not supposed to be transforming into anything.
The risk above roughly 60 percent is mould. Dried leaf picks up water quickly, and once water activity rises enough the leaf can support fungal growth. The first sign is almost never visible fuzz. It is a change in smell: sweet-damp, or cellar-like, instead of clean and vegetal.
This is also why the fridge needs care. A refrigerator is a humid place, so the container has to be genuinely moisture-proof, not merely closed. A tin with a gasket lid inside a sealed bag is the usual solution.
Light and air
Keep it dark. Light bleaches both the colour and the flavour of the leaf, and a clear jar on a sunny worktop will flatten a green tea in weeks. An opaque tin costs nothing and solves it.
Keep it away from air, and away from smells. Dried leaf is an excellent odour sponge, and the same porous structure that lets it absorb humidity lets it pick up whatever is in the cupboard. Coffee, spices, and onions are the usual culprits.
And never store green tea in a container that previously held something aromatic. The residue outlasts a wash, and you will taste it for as long as the tea lasts.
The target environment
Put together, the target is simple: cold, dark, dry, sealed, and boring. A gasket-sealed opaque tin, kept in the fridge if you drink it slowly or in a cool cupboard if you drink it fast, opened once rather than twenty times.
Split a large purchase into two tins on the day you get it. Open one and leave the other untouched. The reserve tin is then exposed to air, humidity, and kitchen odours a handful of times instead of daily, which matters more than any of the other variables.
Hit these and nearly every storage problem with green tea disappears. The tea still will not improve, but it will taste like itself for a year instead of three months.
Adjust through the year
Cool and stable is the goal, but seasons move. A cupboard that is genuinely cool in January can sit above 25 degrees in July, and the leaf notices even if you do not.
Recheck the spot twice a year, in late spring and again in early autumn, and move the tea if the reading has changed. A cheap hygrometer makes this visible instead of guessed, and it is the single most useful storage purchase you can make.
If you keep green tea in the fridge, the one rule that overrides everything else is this: let the container come back to room temperature before you open it. Open it cold and condensation forms on the leaf, not on the tin.
One practical trick for summer is to move green tea into the fridge only for the hot months and take it out again in autumn. The tea does not mind moving once or twice a year. It minds being opened cold, and it minds sitting in a room that never cools down.
You can also tell when an environment has changed without any instrument. If the leaf snaps cleanly, the tea is dry enough. If it bends rather than breaks, moisture has got in, and the tin needs a better seal or a better location.