Relative vs Absolute Humidity, Explained
Here is a sentence that sounds impossible: afternoon air at 40% humidity can carry more water than morning air at 90%. It’s true, and misunderstanding it is the single most common way people get home ventilation wrong. The culprit is the difference between relative and absolute humidity.
Two different questions
Relative and absolute humidity answer different questions about the same air.
Absolute humidity answers: how much water is actually in this air? It is a real quantity — grams of water vapour per cubic metre of air. Take a cubic metre, weigh the water in it, done.
Relative humidity — the % your weather app shows — answers: how full is this air, compared to the most it could hold at its current temperature? It is a percentage of capacity, and that capacity changes dramatically with temperature.
One is an amount of water. The other is a fill level of a container whose size keeps changing.
Warm air is a bigger sponge
The capacity of air for water vapour grows steeply with temperature — roughly doubling for every 10 °C or so of warming. Air at 30 °C can hold about twice the water of air at 20 °C, and around four times that of air at 10 °C.
So the same “50% RH” means completely different amounts of water depending on how warm the air is. A half-full bucket tells you nothing until you know the size of the bucket.
Some rounded numbers, computed from the standard vapour-pressure formula (the Magnus formula):
| Air | Relative humidity | Actual water content |
|---|---|---|
| 30 °C, hot afternoon | 40% (“dry”) | ~12 g/m³ |
| 15 °C, cool morning | 90% (“damp”) | ~11.5 g/m³ |
The “dry” afternoon air carries more water than the “damp” morning air. The percentage pointed you exactly the wrong way.
Why this wrecks ventilation advice
The classic trap goes like this: your weather app shows 35% RH at 3 pm, so you open up to “dry out the flat”. You are actually importing hot air loaded with water. As that air cools inside your home overnight, its relative humidity climbs right back up — and the water is now your water, ready to condense on the first cold surface it finds. The trap is common enough that it has its own article.
Meanwhile, the truly drying air — cool early-morning air whose scary “90%” hides a modest water content — gets skipped because the percentage looks bad.
The rule that actually works compares absolute humidity: bring air in only when it holds less water than the air you already have (and is cooler, if cooling is the goal — see the heatwave rule). Grams versus grams, never percent versus percent.
Where relative humidity still matters
RH isn’t useless — it just answers a different question. Comfort and mold care about RH: keeping indoor air roughly between 40 and 60% RH is the commonly cited comfort band, and mold thrives when humid air meets surfaces cold enough to push local RH toward saturation — which is really a story about the dew point.
So: use relative humidity to judge how a room feels and whether surfaces are at risk. Use absolute humidity to decide whether outdoor air will make things better or worse. Ventilation decisions belong to the second number, always.
Let the app watch for you
Weather apps hand you the misleading number; doing the grams-per-cubic-metre conversion in your head at 3 pm is nobody’s hobby. Open/Shut computes absolute humidity inside and out, compares it with temperature, and simply tells you: open, or shut. The right physics, none of the arithmetic.