RH and Temperature Calculator
Relative humidity is relative to temperature. Warm air holds more water before saturating, so the same quantity of moisture reads as a lower percentage when it warms and a higher percentage when it cools. Nothing has to enter or leave the container for the number on the display to change.
This is why a box that read 66% in August reads 61% in January and is not leaking, and why moving a container to a cooler room appears to add humidity. Both are the same effect in opposite directions.
Same moisture, different temperature — air alone would read
85.8% RH
A shift of +20.8 points with no water added or removed — so a reading that moves when the room temperature moves is not evidence of a leak.
Treat that as a ceiling, not a prediction. It is the unbuffered figure for air on its own. Cedar, tobacco and two-way packs are all hygroscopic and push back, so a real container moves considerably less — and one running two-way packs may hardly move at all. The direction is what to trust here; the magnitude is the worst case.
Above roughly 72% RH mould becomes a live risk — worth acting on rather than noting.
What the number does and doesn’t tell you
The figure above is the unbuffered case: what air on its own would do. It is a ceiling, and it is worth being clear that a real humidor will not move that far. Cedar, tobacco and two-way packs are all hygroscopic; as the air tries to change, they absorb or release moisture to oppose it. A container running two-way packs — which are engineered to hold one fixed value — may barely move at all.
So use it for the direction and the reason, not as a prediction. If your reading dropped when the room warmed, this is why, and the appropriate response is usually none. If it dropped by considerably more than the buffered fraction of this figure, or dropped without any temperature change, then something else is happening and the troubleshooting order applies.
The formula, in the open
Saturation vapour pressure comes from the Magnus-Tetens approximation, and with absolute moisture held constant the conversion is simply the inverse ratio of the two saturation pressures:
RH₂ = RH₁ × Psat(T₁) ÷ Psat(T₂)
The implementation was checked against reference saturation pressures from 0–30°C and agrees to within 0.1%. There is no fitting or fudge factor — unlike the “135 rule”, which is a useful mnemonic with no physics behind it at all.
The practical upshot
- —Read temperature and humidity together, from the same position. Either number alone is ambiguous.
- —Before adjusting a device because RH moved, check whether the temperature moved first.
- —Store at the drier end in warm weather — heat raises both the mould and beetle risks at once.
- —A cold container brought into a warm room can condense moisture onto surfaces, which is a mould event rather than a reading problem.
Sizing packs for a container instead? The pack calculator works from interior volume. And if the reading itself is in doubt, calibrate the hygrometer before trusting any of this.