Why Does Heating Make a Room Feel So Dry?
― Not a Single Drop of Water Has Gone. The Air's "Container" Just Got Bigger
Many people think a heater sucks the water out of the air. In fact, the amount of water in the room hardly changes. What changes is the upper limit of what the air can hold. The warmer the air, the higher that limit, so the same water starts to look like "not enough".
On a winter morning, you switch on the air conditioner's heating and the room turns nice and warm. By noon, though, your throat feels scratchy and your lips are chapped.
You check the hygrometer. Outside it read 70%, but the room is only about 25%. Laundry hung indoors dries surprisingly fast.
No water drained out of the air conditioner. So where did the room's water go?
There are only two reasons it dries
Air can hold only so much water, and that limit rises steeply with temperature. With the same amount of water, a container three times larger is only one third full.
Through gaps and ventilation, the room's air is slowly swapped with outside air. Any water you add leaves with the air, and dry air comes in again.
"Humidity" is not the amount of water itself. It is a proportion of the container. Once you grasp this, what happens in a winter room suddenly makes sense.
Humidity is "how full", not "how much water"
One cubic metre of air can hold only so much water vapour, and the limit depends on temperature. At 5℃ it is about 6.8 grams. At 22℃ it is about 19.4 grams. A difference of just 17℃ makes the container roughly three times bigger.
Outside air at 5℃ and 70% humidity holds about 4.8 grams of water per cubic metre. Warm that air to 22℃ and it still holds 4.8 grams. But the container is now 19.4 grams, so humidity falls to about 25%. Use the slider in Figure 1 to change the room temperature and see for yourself.
The water in a room is only about half a glass
So how much water would it take to bring humidity back to 50%? The air in a 6-tatami room is about 24 cubic metres. To reach 50% at 22℃, you only need to add about 120 grams of water to what is there now. That is about half a glass.
Yet a humidifier can empty its tank in a day. The cause is air exchange. Modern homes are required to be ventilated so that the room's air is replaced once every two hours. The incoming air is dry outside air, so the water you add leaves along with the air. The calculation shows that holding 50% takes about 1.4 litres of water a day.
In other words, the water stored in the room is small. The big quantity is the water that flows out. A winter room is like a bucket with a hole in it, and you keep pouring water in.
In cooling mode, water condenses on the cold parts and is drained outside through a hose. In heating mode, those parts are hot, so almost no water is removed. The dryness comes from the bigger container and from outside air entering through ventilation.
In 25% air, three quarters of the container is empty. The more empty space there is, the more easily water leaves a wet cloth. In a heated winter room, indoor drying also works as a decent humidifier.
Summary
Heating dries a room not because water was taken away. Warming air raises the most water it can hold, so the same amount of water gives a lower humidity figure. Ventilation also keeps bringing in dry outside air, so any water you add slowly escapes. Japan's Ministry of Health, Labour and Welfare recommends keeping indoor humidity at 50–60% as a winter measure against infection. On the other hand, humidifying so much that droplets form on the windows can cause mould.
Humidity is not the amount of water, but how full the container is.
The warmer it gets, the more only the container grows.
Related winter phenomena are also explained in why your breath looks white, why static shocks you in winter and how morning dew forms. All of them can be explained by the container size changing with temperature.
- Put a hygrometer in the room and write down the temperature and humidity before you turn on the heating and one hour after.
- Check whether humidity fell as much as the temperature rise suggests, by comparing with the slider values in Figure 1.
- Next, hang one wet towel to dry and see how much humidity recovers after one hour.
Weigh the towel before and after hanging it to find how much water went into the air. If it lost about 100 grams, that is close to the amount needed to moisten one room.
For those who want more ― terms, formulas and links to textbooksWe state clearly which level each part belongs to, from junior high science to university courses
- Junior highCovered in junior high school science
- High schoolCovered in high school physics or earth science
- High school+Advanced high school material, or textbook sidebar content
- UniversityUniversity-level courses not taught in high school (thermodynamics, meteorology, building environmental engineering)
- ResearchTopics not yet taught as settled fact, still being studied by researchers
Junior highTerms: this phenomenon has names
- Saturation vapour content: the most water that 1 cubic metre of air can hold as vapour. This is the "container" in the article. It grows as temperature rises.
- Relative humidity: what we usually call "humidity". It shows what percentage of the saturation vapour content the actual vapour amount is.
- Absolute humidity (volumetric): the amount of water vapour itself in 1 cubic metre of air. Warming with a heater hardly changes this value.
Junior highHigh schoolChecking with a formula: humidity of the warmed room, and the water needed to hold it
We calculate the case where outside air at 5℃ and 70% humidity comes in and is warmed to 22℃. The room is 6 tatami (about 24 cubic metres).
| In symbols | H ＝ ( a ÷ as(T) ) × 100 |
| In words | Humidity (%) ＝ water vapour in 1 m³ of air ÷ saturation vapour content at that temperature × 100 |
| Where it comes from | It is simply the definition of humidity. The saturation vapour content in the denominator is the amount at which the rate of water evaporating balances the rate of vapour returning to water, and it depends only on temperature. Warming conserves the water in the numerator, so humidity falls by as much as the denominator grows. |
| Symbol | Meaning and unit |
|---|---|
| H | Relative humidity (%) |
| a | Water vapour in 1 m³ of air (g/m³) |
| as(T) | Saturation vapour content at temperature T (g/m³) |
| T | Air temperature (℃) |
| Saturation vapour content at 5℃ | about 6.8 g/m³ |
| Saturation vapour content at 22℃ | about 19.4 g/m³ |
| Humidity of outside air | 70% |
| Air volume of a 6-tatami room | about 24 m³ |
| Air changes (guideline for homes) | said to be 0.5 per hour |
| Water in 1 m³ of outside air | 6.8 × 0.7 ＝ 4.76 g |
| How full it is when warmed to 22℃ | 4.76 ÷ 19.4 ≒ 0.245 |
| Humidity as a percentage | 0.245 × 100 ＝ 24.5% |
| Water at 22℃ and 50% humidity | 19.4 × 0.5 ＝ 9.7 g/m³ |
| Water missing per 1 m³ | 9.7 − 4.76 ＝ 4.94 g |
| Water to moisten the whole room once | 4.94 × 24 ≒ 119 g |
| Air replaced in 1 hour | 24 × 0.5 ＝ 12 m³ |
| Water escaping in 1 hour | 4.94 × 12 ≒ 59 g |
| Water needed in 1 day | 59 × 24 ＝ 1416 g |
The room holds only about 120 grams at a time, yet keeping it there takes about 1.4 litres a day. A humidifier tank emptying quickly is not a fault; it is just what the calculation predicts. In real life, breathing and cooking also release water, so the amount needed is somewhat smaller.
High schoolHigh school+Why does the container grow so steeply with temperature?
High schoolAt a water surface, water molecules fly out (evaporation) while water vapour molecules in the air return to the water. The higher the temperature, the more vigorously molecules move, and the stronger the push to fly out. The vapour pressure at which the two balance is called the saturation vapour pressure, and converting it into an amount gives the saturation vapour content.
High school+Saturation vapour pressure rises almost exponentially with temperature, nearly doubling for every 10℃ near room temperature. In practice, approximate formulas (such as the Tetens equation) are used, where you enter a temperature and get a value. The interactive figure in Figure 1 also uses this approximation to compute the container size.
UniversitySaturation vapour pressure in thermodynamics, and a home's moisture balance
How saturation vapour pressure changes with temperature is described by the Clausius–Clapeyron equation of thermodynamics. It shows that the larger the heat needed to evaporate water (the latent heat of vaporisation), the more steeply the pressure rises with temperature. In building environmental engineering, the flow of water vapour in and out of a room is treated as a moisture balance, and the indoor absolute humidity is worked out from the ventilation rate, the moisture generated, and the moisture absorbed and released by walls and furniture. Under Japan's Building Standards Act, in force since 2003 as a measure against sick building syndrome, habitable rooms in homes must have at least 0.5 air changes per hour.
📖 Derivations and further reading: Clausius–Clapeyron equation (Wikipedia) ／ Saturation vapour content and approximations (Wikipedia)
ResearchWhat is still not clear
- Humidity and virus survival Many studies show that influenza viruses spread more easily in dry air. But how much each of these matters is still not sorted out: how long viruses survive in the air, how the throat lining works, and how people behave.
- Water that walls and furniture absorb and release Wood, paper and cloth take in and give off moisture, softening changes in indoor humidity. The amount varies greatly with material and thickness, and estimating it accurately is still a hard problem.
- Individual differences in "feeling dry" At the same humidity, some people feel their eyes or skin drying strongly and others do not. Where this difference in feeling comes from is not well understood.
So the content of this article, too, is "an explanation within what is known today". Because humidity figures change a lot with temperature, it matters to look at temperature as well when comparing them.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Junior high | Science, year 2: "Weather and its changes" (saturation vapour content and humidity) | The container and how full it is; humidity calculation |
| High school | Physics "Heat and gases", Earth science "Water in the atmosphere" | Balance of evaporation and condensation; saturation vapour pressure |
| High school+ | Approximate formulas for saturation vapour pressure | Nearly doubling per 10℃; the calculation in Figure 1 |
| University | Thermodynamics, building environmental engineering (ventilation, moisture balance) | Clausius–Clapeyron equation; air change rate |
| Research | Environmental health, moisture absorption and release of building materials | Humidity and infection; humidity buffering by walls and furniture |
| ― | Links to daily life | Humidifier water volume, indoor drying, window droplets and mould |
- Humidity (relative and absolute) ― Wikipedia
- Saturation vapour content ― Wikipedia
- Ventilation ― Wikipedia
- Ministry of Land, Infrastructure, Transport and Tourism (国土交通省), "Sick building syndrome measures under the Building Standards Act" (建築基準法に基づくシックハウス対策について) (air changes in habitable rooms: 0.5 per hour)
- Ministry of Health, Labour and Welfare (厚生労働省), "Influenza Q&A" (インフルエンザQ&A) (maintaining appropriate indoor humidity)
※This article is a popular-science explanation for general readers. The figures given are rough guides to help you understand the mechanism. Actual values vary with room size, insulation and ventilation method. Please use humidifiers according to their instruction manuals.